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The Linux-USB Host Side API

Introduction to USB on Linux

A Universal Serial Bus (USB) is used to connect a host, such as a PC or workstation, to a number of peripheral devices. USB uses a tree structure, with the host as the root (the system's master), hubs as interior nodes, and peripherals as leaves (and slaves). Modern PCs support several such trees of USB devices, usually one USB 2.0 tree (480 Mbit/sec each) with a few USB 1.1 trees (12 Mbit/sec each) that are used when you connect a USB 1.1 device directly to the machine's “root hub”.

That master/slave asymmetry was designed-in for a number of reasons, one being ease of use. It is not physically possible to assemble (legal) USB cables incorrectly: all upstream “to the host” connectors are the rectangular type (matching the sockets on root hubs), and all downstream connectors are the squarish type (or they are built into the peripheral). Also, the host software doesn't need to deal with distributed auto-configuration since the pre-designated master node manages all that. And finally, at the electrical level, bus protocol overhead is reduced by eliminating arbitration and moving scheduling into the host software.

USB 1.0 was announced in January 1996 and was revised as USB 1.1 (with improvements in hub specification and support for interrupt-out transfers) in September 1998. USB 2.0 was released in April 2000, adding high-speed transfers and transaction-translating hubs (used for USB 1.1 and 1.0 backward compatibility).

Kernel developers added USB support to Linux early in the 2.2 kernel series, shortly before 2.3 development forked. Updates from 2.3 were regularly folded back into 2.2 releases, which improved reliability and brought /sbin/hotplug support as well more drivers. Such improvements were continued in the 2.5 kernel series, where they added USB 2.0 support, improved performance, and made the host controller drivers (HCDs) more consistent. They also simplified the API (to make bugs less likely) and added internal “kerneldoc” documentation.

Linux can run inside USB devices as well as on the hosts that control the devices. But USB device drivers running inside those peripherals don't do the same things as the ones running inside hosts, so they've been given a different name: gadget drivers. This document does not cover gadget drivers.

USB Host-Side API Model

Host-side drivers for USB devices talk to the “usbcore” APIs. There are two. One is intended for general-purpose drivers (exposed through driver frameworks), and the other is for drivers that are part of the core. Such core drivers include the hub driver (which manages trees of USB devices) and several different kinds of host controller drivers, which control individual busses.

The device model seen by USB drivers is relatively complex.

  • USB supports four kinds of data transfers (control, bulk, interrupt, and isochronous). Two of them (control and bulk) use bandwidth as it's available, while the other two (interrupt and isochronous) are scheduled to provide guaranteed bandwidth.
  • The device description model includes one or more “configurations” per device, only one of which is active at a time. Devices that are capable of high-speed operation must also support full-speed configurations, along with a way to ask about the “other speed” configurations which might be used.
  • Configurations have one or more “interfaces”, each of which may have “alternate settings”. Interfaces may be standardized by USB “Class” specifications, or may be specific to a vendor or device.

USB device drivers actually bind to interfaces, not devices. Think of them as “interface drivers”, though you may not see many devices where the distinction is important. Most USB devices are simple, with only one configuration, one interface, and one alternate setting.

  • Interfaces have one or more “endpoints”, each of which supports one type and direction of data transfer such as “bulk out” or “interrupt in”. The entire configuration may have up to sixteen endpoints in each direction, allocated as needed among all the interfaces.
  • Data transfer on USB is packetized; each endpoint has a maximum packet size. Drivers must often be aware of conventions such as flagging the end of bulk transfers using “short” (including zero length) packets.
  • The Linux USB API supports synchronous calls for control and bulk messages. It also supports asynchnous calls for all kinds of data transfer, using request structures called “URBs” (USB Request Blocks).

Accordingly, the USB Core API exposed to device drivers covers quite a lot of territory. You'll probably need to consult the USB 2.0 specification, available online from www.usb.org at no cost, as well as class or device specifications.

The only host-side drivers that actually touch hardware (reading/writing registers, handling IRQs, and so on) are the HCDs. In theory, all HCDs provide the same functionality through the same API. In practice, that's becoming more true on the 2.5 kernels, but there are still differences that crop up especially with fault handling. Different controllers don't necessarily report the same aspects of failures, and recovery from faults (including software-induced ones like unlinking an URB) isn't yet fully consistent. Device driver authors should make a point of doing disconnect testing (while the device is active) with each different host controller driver, to make sure drivers don't have bugs of their own as well as to make sure they aren't relying on some HCD-specific behavior. (You will need external USB 1.1 and/or USB 2.0 hubs to perform all those tests.)

USB-Standard Types

In <linux/usb/ch9.h> you will find the USB data types defined in chapter 9 of the USB specification. These data types are used throughout USB, and in APIs including this host side API, gadget APIs, and usbfs.

  • struct usb_ctrlrequest - SETUP data for a USB device control request

Synopsis:

struct usb_ctrlrequest {
  __u8 bRequestType;
  __u8 bRequest;
  __le16 wValue;
  __le16 wIndex;
  __le16 wLength;
};  

Members:

  • bRequestType - matches the USB bmRequestType field
  • bRequest - matches the USB bRequest field
  • wValue - matches the USB wValue field (le16 byte order)
  • wIndex - matches the USB wIndex field (le16 byte order)
  • wLength - matches the USB wLength field (le16 byte order)

Description:

This structure is used to send control requests to a USB device. It matches the different fields of the USB 2.0 Spec section 9.3, table 9-2. See the USB spec for a fuller description of the different fields, and what they are used for.

Note that the driver for any interface can issue control requests. For most devices, interfaces don't coordinate with each other, so such requests may be made at any time.

  • usb_endpoint_num - get the endpoint's number

Synopsis:

int usb_endpoint_num ( const struct usb_endpoint_descriptor * epd )

Arguments:

  • epd - endpoint to be checked

Description:

Returns epd's number: 0 to 15.

  • usb_endpoint_type - get the endpoint's transfer type

Synopsis:

int usb_endpoint_type ( const struct usb_endpoint_descriptor * epd )

Arguments:

  • epd - endpoint to be checked

Description:

Returns one of USB_ENDPOINT_XFER_{CONTROL, ISOC, BULK, INT} according to epd's transfer type.

  • usb_endpoint_dir_in - check if the endpoint has IN direction

Synopsis:

int usb_endpoint_dir_in ( const struct usb_endpoint_descriptor * epd )

Arguments:

  • epd - endpoint to be checked

Description:

Returns true if the endpoint is of type IN, otherwise it returns false.

  • usb_endpoint_dir_out - check if the endpoint has OUT direction

Synopsis:

int usb_endpoint_dir_out ( const struct usb_endpoint_descriptor * epd )

Arguments:

  • epd - endpoint to be checked

Description:

Returns true if the endpoint is of type OUT, otherwise it returns false.

  • usb_endpoint_xfer_bulk - check if the endpoint has bulk transfer type

Synopsis:

int usb_endpoint_xfer_bulk ( const struct usb_endpoint_descriptor * epd )

Arguments:

  • epd - endpoint to be checked

Description:

Returns true if the endpoint is of type bulk, otherwise it returns false.

  • usb_endpoint_xfer_control - check if the endpoint has control transfer type

Synopsis:

int usb_endpoint_xfer_control ( const struct usb_endpoint_descriptor * epd )

Arguments:

  • epd - endpoint to be checked

Description:

Returns true if the endpoint is of type control, otherwise it returns false.

  • usb_endpoint_xfer_int - check if the endpoint has interrupt transfer type

Synopsis:

int usb_endpoint_xfer_int ( const struct usb_endpoint_descriptor * epd )

Arguments:

  • epd - endpoint to be checked

Description:

Returns true if the endpoint is of type interrupt, otherwise it returns false.

  • usb_endpoint_xfer_isoc - check if the endpoint has isochronous transfer type

Synopsis:

int usb_endpoint_xfer_isoc ( const struct usb_endpoint_descriptor * epd )

Arguments:

  • epd - endpoint to be checked

Description:

Returns true if the endpoint is of type isochronous, otherwise it returns false.

  • usb_endpoint_is_bulk_in - check if the endpoint is bulk IN

Synopsis:

int usb_endpoint_is_bulk_in ( const struct usb_endpoint_descriptor * epd )

Arguments:

  • epd - endpoint to be checked

Description:

Returns true if the endpoint has bulk transfer type and IN direction, otherwise it returns false.

  • usb_endpoint_is_bulk_out - check if the endpoint is bulk OUT

Synopsis:

int usb_endpoint_is_bulk_out ( const struct usb_endpoint_descriptor * epd )

Arguments:

  • epd - endpoint to be checked

Description:

Returns true if the endpoint has bulk transfer type and OUT direction, otherwise it returns false.

  • usb_endpoint_is_int_in - check if the endpoint is interrupt IN

Synopsis:

int usb_endpoint_is_int_in ( const struct usb_endpoint_descriptor * epd )

Arguments:

  • epd - endpoint to be checked

Description:

Returns true if the endpoint has interrupt transfer type and IN direction, otherwise it returns false.

  • usb_endpoint_is_int_out - check if the endpoint is interrupt OUT

Synopsis:

int usb_endpoint_is_int_out ( const struct usb_endpoint_descriptor * epd )

Arguments:

  • epd - endpoint to be checked

Description:

Returns true if the endpoint has interrupt transfer type and OUT direction, otherwise it returns false.

  • usb_endpoint_is_isoc_in - check if the endpoint is isochronous IN

Synopsis:

int usb_endpoint_is_isoc_in ( const struct usb_endpoint_descriptor * epd )

Arguments:

  • epd - endpoint to be checked

Description:

Returns true if the endpoint has isochronous transfer type and IN direction, otherwise it returns false.

  • usb_endpoint_is_isoc_out - check if the endpoint is isochronous OUT

Synopsis:

int usb_endpoint_is_isoc_out ( const struct usb_endpoint_descriptor * epd )

Arguments:

  • epd - endpoint to be checked

Description:

Returns true if the endpoint has isochronous transfer type and OUT direction, otherwise it returns false.

Host-Side Data Types and Macros

The host side API exposes several layers to drivers, some of which are more necessary than others. These support lifecycle models for host side drivers and devices, and support passing buffers through usbcore to some HCD that performs the I/O for the device driver.

  • struct usb_host_ss_ep_comp - Valid for SuperSpeed devices only

Synopsis:

struct usb_host_ss_ep_comp {
  struct usb_ss_ep_comp_descriptor desc;
  unsigned char * extra;
  int extralen;
};  

Members:

  • desc - endpoint companion descriptor, wMaxPacketSize in native byteorder
  • extra - descriptors following this endpoint companion descriptor
  • extralen - how many bytes of extra are valid
  • struct usb_host_endpoint - host-side endpoint descriptor and queue

Synopsis:

struct usb_host_endpoint {
  struct usb_endpoint_descriptor desc;
  struct list_head urb_list;
  void * hcpriv;
  struct ep_device * ep_dev;
  struct usb_host_ss_ep_comp * ss_ep_comp;
  unsigned char * extra;
  int extralen;
  int enabled;
};  

Members:

  • desc - descriptor for this endpoint, wMaxPacketSize in native byteorder
  • urb_list - urbs queued to this endpoint; maintained by usbcore
  • hcpriv - for use by HCD; typically holds hardware dma queue head (QH) with one or more transfer descriptors (TDs) per urb
  • ep_dev - ep_device for sysfs info
  • ss_ep_comp - companion descriptor information for this endpoint
  • extra - descriptors following this endpoint in the configuration
  • extralen - how many bytes of extra are valid
  • enabled - URBs may be submitted to this endpoint

Description:

USB requests are always queued to a given endpoint, identified by a descriptor within an active interface in a given USB configuration.

  • struct usb_interface - what usb device drivers talk to

Synopsis:

struct usb_interface {
  struct usb_host_interface * altsetting;
  struct usb_host_interface * cur_altsetting;
  unsigned num_altsetting;
  struct usb_interface_assoc_descriptor * intf_assoc;
  int minor;
  enum usb_interface_condition condition;
  unsigned is_active:1;
  unsigned sysfs_files_created:1;
  unsigned ep_devs_created:1;
  unsigned unregistering:1;
  unsigned needs_remote_wakeup:1;
  unsigned needs_altsetting0:1;
  unsigned needs_binding:1;
  unsigned reset_running:1;
  struct device dev;
  struct device * usb_dev;
  atomic_t pm_usage_cnt;
  struct work_struct reset_ws;
};  

Members:

  • altsetting - array of interface structures, one for each alternate setting that may be selected. Each one includes a set of endpoint configurations. They will be in no particular order.
  • cur_altsetting - the current altsetting.
  • num_altsetting - number of altsettings defined.
  • intf_assoc - interface association descriptor
  • minor - the minor number assigned to this interface, if this interface is bound to a driver that uses the USB major number. If this interface does not use the USB major, this field should be unused. The driver should set this value in the probe function of the driver, after it has been assigned a minor number from the USB core by calling usb_register_dev.
  • condition - binding state of the interface: not bound, binding (in probe), bound to a driver, or unbinding (in disconnect)
  • is_active - flag set when the interface is bound and not suspended.
  • sysfs_files_created - sysfs attributes exist
  • ep_devs_created - endpoint child pseudo-devices exist
  • unregistering - flag set when the interface is being unregistered
  • needs_remote_wakeup - flag set when the driver requires remote-wakeup capability during autosuspend.
  • needs_altsetting0 - flag set when a set-interface request for altsetting 0 has been deferred.
  • needs_binding - flag set when the driver should be re-probed or unbound following a reset or suspend operation it doesn't support.
  • reset_running - set to 1 if the interface is currently running a queued reset so that usb_cancel_queued_reset doesn't try to remove from the workqueue when running inside the worker thread. See __usb_queue_reset_device.
  • dev - driver model's view of this device
  • usb_dev - if an interface is bound to the USB major, this will point to the sysfs representation for that device.
  • pm_usage_cnt - PM usage counter for this interface; autosuspend is not allowed unless the counter is 0.
  • reset_ws - Used for scheduling resets from atomic context.

Description:

USB device drivers attach to interfaces on a physical device. Each interface encapsulates a single high level function, such as feeding an audio stream to a speaker or reporting a change in a volume control. Many USB devices only have one interface. The protocol used to talk to an interface's endpoints can be defined in a usb class specification, or by a product's vendor. The (default) control endpoint is part of every interface, but is never listed among the interface's descriptors.

The driver that is bound to the interface can use standard driver model calls such as dev_get_drvdata on the dev member of this structure.

Each interface may have alternate settings. The initial configuration of a device sets altsetting 0, but the device driver can change that setting using usb_set_interface. Alternate settings are often used to control the use of periodic endpoints, such as by having different endpoints use different amounts of reserved USB bandwidth. All standards-conformant USB devices that use isochronous endpoints will use them in non-default settings.

The USB specification says that alternate setting numbers must run from 0 to one less than the total number of alternate settings. But some devices manage to mess this up, and the structures aren't necessarily stored in numerical order anyhow. Use usb_altnum_to_altsetting to look up an alternate setting in the altsetting array based on its number.

  • struct usb_interface_cache - long-term representation of a device interface

Synopsis:

struct usb_interface_cache {
  unsigned num_altsetting;
  struct kref ref;
  struct usb_host_interface altsetting[0];
};  

Members:

  • num_altsetting - number of altsettings defined.
  • ref - reference counter.
  • altsetting[0] - variable-length array of interface structures, one for each alternate setting that may be selected. Each one includes a set of endpoint configurations. They will be in no particular order.

Description:

These structures persist for the lifetime of a usb_device, unlike struct usb_interface (which persists only as long as its configuration is installed). The altsetting arrays can be accessed through these structures at any time, permitting comparison of configurations and providing support for the /proc/bus/usb/devices pseudo-file.

  • struct usb_host_config - representation of a device's configuration

Synopsis:

struct usb_host_config {
  struct usb_config_descriptor desc;
  char * string;
  struct usb_interface_assoc_descriptor * intf_assoc[USB_MAXIADS];
  struct usb_interface * interface[USB_MAXINTERFACES];
  struct usb_interface_cache * intf_cache[USB_MAXINTERFACES];
  unsigned char * extra;
  int extralen;
};  

Members:

  • desc - the device's configuration descriptor.
  • string - pointer to the cached version of the iConfiguration string, if present for this configuration.
  • intf_assoc[USB_MAXIADS] - list of any interface association descriptors in this config
  • interface[USB_MAXINTERFACES] - array of pointers to usb_interface structures, one for each interface in the configuration. The number of interfaces is stored in desc.bNumInterfaces. These pointers are valid only while the the configuration is active.
  • intf_cache[USB_MAXINTERFACES] - array of pointers to usb_interface_cache structures, one for each interface in the configuration. These structures exist for the entire life of the device.
  • extra - pointer to buffer containing all extra descriptors associated with this configuration (those preceding the first interface descriptor).
  • extralen - length of the extra descriptors buffer.

Description:

USB devices may have multiple configurations, but only one can be active at any time. Each encapsulates a different operational environment; for example, a dual-speed device would have separate configurations for full-speed and high-speed operation. The number of configurations available is stored in the device descriptor as bNumConfigurations.

A configuration can contain multiple interfaces. Each corresponds to a different function of the USB device, and all are available whenever the configuration is active. The USB standard says that interfaces are supposed to be numbered from 0 to desc.bNumInterfaces-1, but a lot of devices get this wrong. In addition, the interface array is not guaranteed to be sorted in numerical order. Use usb_ifnum_to_if to look up an interface entry based on its number.

Device drivers should not attempt to activate configurations. The choice of which configuration to install is a policy decision based on such considerations as available power, functionality provided, and the user's desires (expressed through userspace tools). However, drivers can call usb_reset_configuration to reinitialize the current configuration and all its interfaces.

  • struct usb_device - kernel's representation of a USB device

Synopsis:

struct usb_device {
  int devnum;
  char devpath[16];
  u32 route;
  enum usb_device_state state;
  enum usb_device_speed speed;
  struct usb_tt * tt;
  int ttport;
  unsigned int toggle[2];
  struct usb_device * parent;
  struct usb_bus * bus;
  struct usb_host_endpoint ep0;
  struct device dev;
  struct usb_device_descriptor descriptor;
  struct usb_host_config * config;
  struct usb_host_config * actconfig;
  struct usb_host_endpoint * ep_in[16];
  struct usb_host_endpoint * ep_out[16];
  char ** rawdescriptors;
  unsigned short bus_mA;
  u8 portnum;
  u8 level;
  unsigned can_submit:1;
  unsigned discon_suspended:1;
  unsigned persist_enabled:1;
  unsigned have_langid:1;
  unsigned authorized:1;
  unsigned authenticated:1;
  unsigned wusb:1;
  int string_langid;
  char * product;
  char * manufacturer;
  char * serial;
  struct list_head filelist;
#ifdef CONFIG_USB_DEVICE_CLASS
  struct device * usb_classdev;
#endif
#ifdef CONFIG_USB_DEVICEFS
  struct dentry * usbfs_dentry;
#endif
  int maxchild;
  struct usb_device * children[USB_MAXCHILDREN];
  int pm_usage_cnt;
  u32 quirks;
  atomic_t urbnum;
  unsigned long active_duration;
#ifdef CONFIG_PM
  struct delayed_work autosuspend;
  struct work_struct autoresume;
  struct mutex pm_mutex;
  unsigned long last_busy;
  int autosuspend_delay;
  unsigned long connect_time;
  unsigned auto_pm:1;
  unsigned do_remote_wakeup:1;
  unsigned reset_resume:1;
  unsigned autosuspend_disabled:1;
  unsigned autoresume_disabled:1;
  unsigned skip_sys_resume:1;
#endif
  struct wusb_dev * wusb_dev;
  int slot_id;
};  

Members:

  • devnum - device number; address on a USB bus
  • devpath[16] - device ID string for use in messages (e.g., /port/…)
  • route - tree topology hex string for use with xHCI
  • state - device state: configured, not attached, etc.
  • speed - device speed: high/full/low (or error)
  • tt - Transaction Translator info; used with low/full speed dev, highspeed hub
  • ttport - device port on that tt hub
  • toggle[2] - one bit for each endpoint, with ([0] = IN, [1] = OUT) endpoints
  • parent - our hub, unless we're the root
  • bus - bus we're part of
  • ep0 - endpoint 0 data (default control pipe)
  • dev - generic device interface
  • descriptor - USB device descriptor
  • config - all of the device's configs
  • actconfig - the active configuration
  • ep_in[16] - array of IN endpoints
  • ep_out[16] - array of OUT endpoints
  • rawdescriptors - raw descriptors for each config
  • bus_mA - Current available from the bus
  • portnum - parent port number (origin 1)
  • level - number of USB hub ancestors
  • can_submit - URBs may be submitted
  • discon_suspended - disconnected while suspended
  • persist_enabled - USB_PERSIST enabled for this device
  • have_langid - whether string_langid is valid
  • authorized - policy has said we can use it; (user space) policy determines if we authorize this device to be used or not. By default, wired USB devices are authorized. WUSB devices are not, until we authorize them from user space. FIXME -- complete doc
  • authenticated - Crypto authentication passed
  • wusb - device is Wireless USB
  • string_langid - language ID for strings
  • product - iProduct string, if present (static)
  • manufacturer - iManufacturer string, if present (static)
  • serial - iSerialNumber string, if present (static)
  • filelist - usbfs files that are open to this device
  • usb_classdev - USB class device that was created for usbfs device access from userspace
  • usbfs_dentry - usbfs dentry entry for the device
  • maxchild - number of ports if hub
  • children[USB_MAXCHILDREN] - child devices - USB devices that are attached to this hub
  • pm_usage_cnt - usage counter for autosuspend
  • quirks - quirks of the whole device
  • urbnum - number of URBs submitted for the whole device
  • active_duration - total time device is not suspended
  • autosuspend - for delayed autosuspends
  • autoresume - for autoresumes requested while in_interrupt
  • pm_mutex - protects PM operations
  • last_busy - time of last use
  • autosuspend_delay - in jiffies
  • connect_time - time device was first connected
  • auto_pm - autosuspend/resume in progress
  • do_remote_wakeup - remote wakeup should be enabled
  • reset_resume - needs reset instead of resume
  • autosuspend_disabled - autosuspend disabled by the user
  • autoresume_disabled - autoresume disabled by the user
  • skip_sys_resume - skip the next system resume
  • wusb_dev - if this is a Wireless USB device, link to the WUSB specific data for the device.
  • slot_id - Slot ID assigned by xHCI

Notes:

Usbcore drivers should not set usbdev→state directly. Instead use usb_set_device_state.

  • usb_interface_claimed - returns true iff an interface is claimed

Synopsis:

int usb_interface_claimed ( struct usb_interface * iface )

Arguments:

  • iface - the interface being checked

Description:

Returns true (nonzero) iff the interface is claimed, else false (zero). Callers must own the driver model's usb bus readlock. So driver probe entries don't need extra locking, but other call contexts may need to explicitly claim that lock.

  • usb_make_path - returns stable device path in the usb tree

Synopsis:

int usb_make_path ( struct usb_device * dev )

Arguments:

  • dev - the device whose path is being constructed
  • buf - where to put the string
  • size - how big is buf?

Description:

Returns length of the string (> 0) or negative if size was too small.

This identifier is intended to be stable, reflecting physical paths in hardware such as physical bus addresses for host controllers or ports on USB hubs. That makes it stay the same until systems are physically reconfigured, by re-cabling a tree of USB devices or by moving USB host controllers. Adding and removing devices, including virtual root hubs in host controller driver modules, does not change these path identifers; neither does rebooting or re-enumerating. These are more useful identifiers than changeable (unstable) ones like bus numbers or device addresses.

With a partial exception for devices connected to USB 2.0 root hubs, these identifiers are also predictable. So long as the device tree isn't changed, plugging any USB device into a given hub port always gives it the same path. Because of the use of companion controllers, devices connected to ports on USB 2.0 root hubs (EHCI host controllers) will get one path ID if they are high speed, and a different one if they are full or low speed.

  • USB_DEVICE - macro used to describe a specific usb device

Synopsis:

USB_DEVICE ( vend )

Arguments:

  • vend - the 16 bit USB Vendor ID
  • prod - the 16 bit USB Product ID

Description:

This macro is used to create a struct usb_device_id that matches a specific device.

  • USB_DEVICE_VER - describe a specific usb device with a version range

Synopsis:

USB_DEVICE_VER ( vend )

Arguments:

  • vend - the 16 bit USB Vendor ID
  • prod - the 16 bit USB Product ID
  • lo - the bcdDevice_lo value
  • hi - the bcdDevice_hi value

Description:

This macro is used to create a struct usb_device_id that matches a specific device, with a version range.

  • USB_DEVICE_INTERFACE_PROTOCOL - describe a usb device with a specific interface protocol

Synopsis:

USB_DEVICE_INTERFACE_PROTOCOL ( vend )

Arguments:

  • vend - the 16 bit USB Vendor ID
  • prod - the 16 bit USB Product ID
  • pr - bInterfaceProtocol value

Description:

This macro is used to create a struct usb_device_id that matches a specific interface protocol of devices.

  • USB_DEVICE_INFO - macro used to describe a class of usb devices

Synopsis:

USB_DEVICE_INFO ( cl )

Arguments:

  • cl - bDeviceClass value
  • sc - bDeviceSubClass value
  • pr - bDeviceProtocol value

Description:

This macro is used to create a struct usb_device_id that matches a specific class of devices.

  • USB_INTERFACE_INFO - macro used to describe a class of usb interfaces

Synopsis:

USB_INTERFACE_INFO ( cl )

Arguments:

  • cl - bInterfaceClass value
  • sc - bInterfaceSubClass value
  • pr - bInterfaceProtocol value

Description:

This macro is used to create a struct usb_device_id that matches a specific class of interfaces.

  • USB_DEVICE_AND_INTERFACE_INFO - describe a specific usb device with a class of usb interfaces

Synopsis:

USB_DEVICE_AND_INTERFACE_INFO ( vend )

Arguments:

  • vend - the 16 bit USB Vendor ID
  • prod - the 16 bit USB Product ID
  • cl - bInterfaceClass value
  • sc - bInterfaceSubClass value
  • pr - bInterfaceProtocol value

Description:

This macro is used to create a struct usb_device_id that matches a specific device with a specific class of interfaces.

This is especially useful when explicitly matching devices that have vendor specific bDeviceClass values, but standards-compliant interfaces.

  • struct usbdrv_wrap - wrapper for driver-model structure

Synopsis:

struct usbdrv_wrap {
  struct device_driver driver;
  int for_devices;
};  

Members:

  • driver - The driver-model core driver structure.
  • for_devices - Non-zero for device drivers, 0 for interface drivers.
  • struct usb_driver - identifies USB interface driver to usbcore

Synopsis:

struct usb_driver {
  const char * name;
  int (* probe) (struct usb_interface *intf,const struct usb_device_id *id);
  void (* disconnect) (struct usb_interface *intf);
  int (* ioctl) (struct usb_interface *intf, unsigned int code,void *buf);
  int (* suspend) (struct usb_interface *intf, pm_message_t message);
  int (* resume) (struct usb_interface *intf);
  int (* reset_resume) (struct usb_interface *intf);
  int (* pre_reset) (struct usb_interface *intf);
  int (* post_reset) (struct usb_interface *intf);
  const struct usb_device_id * id_table;
  struct usb_dynids dynids;
  struct usbdrv_wrap drvwrap;
  unsigned int no_dynamic_id:1;
  unsigned int supports_autosuspend:1;
  unsigned int soft_unbind:1;
};  

Members:

  • name - The driver name should be unique among USB drivers, and should normally be the same as the module name.
  • probe - Called to see if the driver is willing to manage a particular interface on a device. If it is, probe returns zero and uses usb_set_intfdata to associate driver-specific data with the interface. It may also use usb_set_interface to specify the appropriate altsetting. If unwilling to manage the interface, return -ENODEV, if genuine IO errors occured, an appropriate negative errno value.
  • disconnect - Called when the interface is no longer accessible, usually because its device has been (or is being) disconnected or the driver module is being unloaded.
  • ioctl - Used for drivers that want to talk to userspace through the usbfs filesystem. This lets devices provide ways to expose information to user space regardless of where they do (or don't) show up otherwise in the filesystem.
  • suspend - Called when the device is going to be suspended by the system.
  • resume - Called when the device is being resumed by the system.
  • reset_resume - Called when the suspended device has been reset instead of being resumed.
  • pre_reset - Called by usb_reset_device when the device is about to be reset.
  • post_reset - Called by usb_reset_device after the device has been reset
  • id_table - USB drivers use ID table to support hotplugging. Export this with MODULE_DEVICE_TABLE(usb,…). This must be set or your driver's probe function will never get called.
  • dynids - used internally to hold the list of dynamically added device ids for this driver.
  • drvwrap - Driver-model core structure wrapper.
  • no_dynamic_id - if set to 1, the USB core will not allow dynamic ids to be added to this driver by preventing the sysfs file from being created.
  • supports_autosuspend - if set to 0, the USB core will not allow autosuspend for interfaces bound to this driver.
  • soft_unbind - if set to 1, the USB core will not kill URBs and disable endpoints before calling the driver's disconnect method.

Description:

USB interface drivers must provide a name, probe and disconnect methods, and an id_table. Other driver fields are optional.

The id_table is used in hotplugging. It holds a set of descriptors, and specialized data may be associated with each entry. That table is used by both user and kernel mode hotplugging support.

The probe and disconnect methods are called in a context where they can sleep, but they should avoid abusing the privilege. Most work to connect to a device should be done when the device is opened, and undone at the last close. The disconnect code needs to address concurrency issues with respect to open and close methods, as well as forcing all pending I/O requests to complete (by unlinking them as necessary, and blocking until the unlinks complete).

  • struct usb_device_driver - identifies USB device driver to usbcore

Synopsis:

struct usb_device_driver {
  const char * name;
  int (* probe) (struct usb_device *udev);
  void (* disconnect) (struct usb_device *udev);
  int (* suspend) (struct usb_device *udev, pm_message_t message);
  int (* resume) (struct usb_device *udev, pm_message_t message);
  struct usbdrv_wrap drvwrap;
  unsigned int supports_autosuspend:1;
};  

Members:

  • name - The driver name should be unique among USB drivers, and should normally be the same as the module name.
  • probe - Called to see if the driver is willing to manage a particular device. If it is, probe returns zero and uses dev_set_drvdata to associate driver-specific data with the device. If unwilling to manage the device, return a negative errno value.
  • disconnect - Called when the device is no longer accessible, usually because it has been (or is being) disconnected or the driver's module is being unloaded.
  • suspend - Called when the device is going to be suspended by the system.
  • resume - Called when the device is being resumed by the system.
  • drvwrap - Driver-model core structure wrapper.
  • supports_autosuspend - if set to 0, the USB core will not allow autosuspend for devices bound to this driver.

Description:

USB drivers must provide all the fields listed above except drvwrap.

  • struct usb_class_driver - identifies a USB driver that wants to use the USB major number

Synopsis:

struct usb_class_driver {
  char * name;
  char *(* devnode) (struct device *dev, mode_t *mode);
  const struct file_operations * fops;
  int minor_base;
};  

Members:

  • name - the usb class device name for this driver. Will show up in sysfs.
  • devnode - Callback to provide a naming hint for a possible device node to create.
  • fops - pointer to the struct file_operations of this driver.
  • minor_base - the start of the minor range for this driver.

Description:

This structure is used for the usb_register_dev and usb_unregister_dev functions, to consolidate a number of the parameters used for them.

  • struct urb - USB Request Block

Synopsis:

struct urb {
  struct list_head urb_list;
  struct list_head anchor_list;
  struct usb_anchor * anchor;
  struct usb_device * dev;
  struct usb_host_endpoint * ep;
  unsigned int pipe;
  int status;
  unsigned int transfer_flags;
  void * transfer_buffer;
  dma_addr_t transfer_dma;
  struct usb_sg_request * sg;
  int num_sgs;
  u32 transfer_buffer_length;
  u32 actual_length;
  unsigned char * setup_packet;
  dma_addr_t setup_dma;
  int start_frame;
  int number_of_packets;
  int interval;
  int error_count;
  void * context;
  usb_complete_t complete;
  struct usb_iso_packet_descriptor iso_frame_desc[0];
};  

Members:

  • urb_list - For use by current owner of the URB.
  • anchor_list - membership in the list of an anchor
  • anchor - to anchor URBs to a common mooring
  • dev - Identifies the USB device to perform the request.
  • ep - Points to the endpoint's data structure. Will eventually replace pipe.
  • pipe - Holds endpoint number, direction, type, and more. Create these values with the eight macros available; usb_{snd,rcv}TYPEpipe(dev,endpoint), where the TYPE is ctrl (control), bulk, int (interrupt), or iso (isochronous). For example usb_sndbulkpipe or usb_rcvintpipe. Endpoint numbers range from zero to fifteen. Note that in endpoint two is a different endpoint (and pipe) from out endpoint two. The current configuration controls the existence, type, and maximum packet size of any given endpoint.
  • status - This is read in non-iso completion functions to get the status of the particular request. ISO requests only use it to tell whether the URB was unlinked; detailed status for each frame is in the fields of the iso_frame-desc.
  • transfer_flags - A variety of flags may be used to affect how URB submission, unlinking, or operation are handled. Different kinds of URB can use different flags.
  • transfer_buffer - This identifies the buffer to (or from) which the I/O request will be performed unless URB_NO_TRANSFER_DMA_MAP is set (however, do not leave garbage in transfer_buffer even then). This buffer must be suitable for DMA; allocate it with kmalloc or equivalent. For transfers to in endpoints, contents of this buffer will be modified. This buffer is used for the data stage of control transfers.
  • transfer_dma - When transfer_flags includes URB_NO_TRANSFER_DMA_MAP, the device driver is saying that it provided this DMA address, which the host controller driver should use in preference to the transfer_buffer.
  • sg - scatter gather buffer list
  • num_sgs - number of entries in the sg list
  • transfer_buffer_length - How big is transfer_buffer. The transfer may be broken up into chunks according to the current maximum packet size for the endpoint, which is a function of the configuration and is encoded in the pipe. When the length is zero, neither transfer_buffer nor transfer_dma is used.
  • actual_length - This is read in non-iso completion functions, and it tells how many bytes (out of transfer_buffer_length) were transferred. It will normally be the same as requested, unless either an error was reported or a short read was performed. The URB_SHORT_NOT_OK transfer flag may be used to make such short reads be reported as errors.
  • setup_packet - Only used for control transfers, this points to eight bytes of setup data. Control transfers always start by sending this data to the device. Then transfer_buffer is read or written, if needed.
  • setup_dma - For control transfers with URB_NO_SETUP_DMA_MAP set, the device driver has provided this DMA address for the setup packet. The host controller driver should use this in preference to setup_packet, but the HCD may chose to ignore the address if it must copy the setup packet into internal structures. Therefore, setup_packet must always point to a valid buffer.
  • start_frame - Returns the initial frame for isochronous transfers.
  • number_of_packets - Lists the number of ISO transfer buffers.
  • interval - Specifies the polling interval for interrupt or isochronous transfers. The units are frames (milliseconds) for full and low speed devices, and microframes (1/8 millisecond) for highspeed ones.
  • error_count - Returns the number of ISO transfers that reported errors.
  • context - For use in completion functions. This normally points to request-specific driver context.
  • complete - Completion handler. This URB is passed as the parameter to the completion function. The completion function may then do what it likes with the URB, including resubmitting or freeing it.
  • iso_frame_desc[0] - Used to provide arrays of ISO transfer buffers and to collect the transfer status for each buffer.

Description:

This structure identifies USB transfer requests. URBs must be allocated by calling usb_alloc_urb and freed with a call to usb_free_urb. Initialization may be done using various usb_fill_*_urb functions. URBs are submitted using usb_submit_urb, and pending requests may be canceled using usb_unlink_urb or usb_kill_urb.

Data Transfer Buffers:

Normally drivers provide I/O buffers allocated with kmalloc or otherwise taken from the general page pool. That is provided by transfer_buffer (control requests also use setup_packet), and host controller drivers perform a dma mapping (and unmapping) for each buffer transferred. Those mapping operations can be expensive on some platforms (perhaps using a dma bounce buffer or talking to an IOMMU), although they're cheap on commodity x86 and ppc hardware.

Alternatively, drivers may pass the URB_NO_xxx_DMA_MAP transfer flags, which tell the host controller driver that no such mapping is needed since the device driver is DMA-aware. For example, a device driver might allocate a DMA buffer with usb_buffer_alloc or call usb_buffer_map. When these transfer flags are provided, host controller drivers will attempt to use the dma addresses found in the transfer_dma and/or setup_dma fields rather than determining a dma address themselves.

Note that transfer_buffer must still be set if the controller does not support DMA (as indicated by bus.uses_dma) and when talking to root hub. If you have to trasfer between highmem zone and the device on such controller, create a bounce buffer or bail out with an error. If transfer_buffer cannot be set (is in highmem) and the controller is DMA capable, assign NULL to it, so that usbmon knows not to use the value. The setup_packet must always be set, so it cannot be located in highmem.

Initialization:

All URBs submitted must initialize the dev, pipe, transfer_flags (may be zero), and complete fields. All URBs must also initialize transfer_buffer and transfer_buffer_length. They may provide the URB_SHORT_NOT_OK transfer flag, indicating that short reads are to be treated as errors; that flag is invalid for write requests.

Bulk URBs may use the URB_ZERO_PACKET transfer flag, indicating that bulk OUT transfers should always terminate with a short packet, even if it means adding an extra zero length packet.

Control URBs must provide a setup_packet. The setup_packet and transfer_buffer may each be mapped for DMA or not, independently of the other. The transfer_flags bits URB_NO_TRANSFER_DMA_MAP and URB_NO_SETUP_DMA_MAP indicate which buffers have already been mapped. URB_NO_SETUP_DMA_MAP is ignored for non-control URBs.

Interrupt URBs must provide an interval, saying how often (in milliseconds or, for highspeed devices, 125 microsecond units) to poll for transfers. After the URB has been submitted, the interval field reflects how the transfer was actually scheduled. The polling interval may be more frequent than requested. For example, some controllers have a maximum interval of 32 milliseconds, while others support intervals of up to 1024 milliseconds. Isochronous URBs also have transfer intervals. (Note that for isochronous endpoints, as well as high speed interrupt endpoints, the encoding of the transfer interval in the endpoint descriptor is logarithmic. Device drivers must convert that value to linear units themselves.)

Isochronous URBs normally use the URB_ISO_ASAP transfer flag, telling the host controller to schedule the transfer as soon as bandwidth utilization allows, and then set start_frame to reflect the actual frame selected during submission. Otherwise drivers must specify the start_frame and handle the case where the transfer can't begin then. However, drivers won't know how bandwidth is currently allocated, and while they can find the current frame using usb_get_current_frame_number () they can't know the range for that frame number. (Ranges for frame counter values are HC-specific, and can go from 256 to 65536 frames from now.)

Isochronous URBs have a different data transfer model, in part because the quality of service is only best effort. Callers provide specially allocated URBs, with number_of_packets worth of iso_frame_desc structures at the end. Each such packet is an individual ISO transfer. Isochronous URBs are normally queued, submitted by drivers to arrange that transfers are at least double buffered, and then explicitly resubmitted in completion handlers, so that data (such as audio or video) streams at as constant a rate as the host controller scheduler can support.

Completion Callbacks:

The completion callback is made in_interrupt, and one of the first things that a completion handler should do is check the status field. The status field is provided for all URBs. It is used to report unlinked URBs, and status for all non-ISO transfers. It should not be examined before the URB is returned to the completion handler.

The context field is normally used to link URBs back to the relevant driver or request state.

When the completion callback is invoked for non-isochronous URBs, the actual_length field tells how many bytes were transferred. This field is updated even when the URB terminated with an error or was unlinked.

ISO transfer status is reported in the status and actual_length fields of the iso_frame_desc array, and the number of errors is reported in error_count. Completion callbacks for ISO transfers will normally (re)submit URBs to ensure a constant transfer rate.

Note that even fields marked public should not be touched by the driver when the urb is owned by the hcd, that is, since the call to usb_submit_urb till the entry into the completion routine.

  • usb_fill_control_urb - initializes a control urb

Synopsis:

void usb_fill_control_urb ( struct urb * urb )

Arguments:

  • urb - pointer to the urb to initialize.
  • dev - pointer to the struct usb_device for this urb.
  • pipe - the endpoint pipe
  • setup_packet - pointer to the setup_packet buffer
  • transfer_buffer - pointer to the transfer buffer
  • buffer_length - length of the transfer buffer
  • complete_fn - pointer to the usb_complete_t function
  • context - what to set the urb context to.

Description:

Initializes a control urb with the proper information needed to submit it to a device.

  • usb_fill_bulk_urb - macro to help initialize a bulk urb

Synopsis:

void usb_fill_bulk_urb ( struct urb * urb )

Arguments:

  • urb - pointer to the urb to initialize.
  • dev - pointer to the struct usb_device for this urb.
  • pipe - the endpoint pipe
  • transfer_buffer - pointer to the transfer buffer
  • buffer_length - length of the transfer buffer
  • complete_fn - pointer to the usb_complete_t function
  • context - what to set the urb context to.

Description:

Initializes a bulk urb with the proper information needed to submit it to a device.

  • usb_fill_int_urb - macro to help initialize a interrupt urb

Synopsis:

void usb_fill_int_urb ( struct urb * urb )

Arguments:

  • urb - pointer to the urb to initialize.
  • dev - pointer to the struct usb_device for this urb.
  • pipe - the endpoint pipe
  • transfer_buffer - pointer to the transfer buffer
  • buffer_length - length of the transfer buffer
  • complete_fn - pointer to the usb_complete_t function
  • context - what to set the urb context to.
  • interval - what to set the urb interval to, encoded like the endpoint descriptor's bInterval value.

Description:

Initializes a interrupt urb with the proper information needed to submit it to a device. Note that high speed interrupt endpoints use a logarithmic encoding of the endpoint interval, and express polling intervals in microframes (eight per millisecond) rather than in frames (one per millisecond).

  • usb_urb_dir_in - check if an URB describes an IN transfer

Synopsis:

int usb_urb_dir_in ( struct urb * urb )

Arguments:

  • urb - URB to be checked

Description:

Returns 1 if urb describes an IN transfer (device-to-host), otherwise 0.

  • usb_urb_dir_out - check if an URB describes an OUT transfer

Synopsis:

int usb_urb_dir_out ( struct urb * urb )

Arguments:

  • urb - URB to be checked

Description:

Returns 1 if urb describes an OUT transfer (host-to-device), otherwise 0.

  • struct usb_sg_request - support for scatter/gather I/O

Synopsis:

struct usb_sg_request {
  int status;
  size_t bytes;
};  

Members:

  • status - zero indicates success, else negative errno
  • bytes - counts bytes transferred.

Description:

These requests are initialized using usb_sg_init, and then are used as request handles passed to usb_sg_wait or usb_sg_cancel. Most members of the request object aren't for driver access.

The status and bytecount values are valid only after usb_sg_wait returns. If the status is zero, then the bytecount matches the total from the request.

After an error completion, drivers may need to clear a halt condition on the endpoint.

USB Core APIs

There are two basic I/O models in the USB API. The most elemental one is asynchronous: drivers submit requests in the form of an URB, and the URB's completion callback handle the next step. All USB transfer types support that model, although there are special cases for control URBs (which always have setup and status stages, but may not have a data stage) and isochronous URBs (which allow large packets and include per-packet fault reports). Built on top of that is synchronous API support, where a driver calls a routine that allocates one or more URBs, submits them, and waits until they complete. There are synchronous wrappers for single-buffer control and bulk transfers (which are awkward to use in some driver disconnect scenarios), and for scatterlist based streaming i/o (bulk or interrupt).

USB drivers need to provide buffers that can be used for DMA, although they don't necessarily need to provide the DMA mapping themselves. There are APIs to use used when allocating DMA buffers, which can prevent use of bounce buffers on some systems. In some cases, drivers may be able to rely on 64bit DMA to eliminate another kind of bounce buffer.

  • usb_init_urb - initializes a urb so that it can be used by a USB driver

Synopsis:

void usb_init_urb ( struct urb * urb )

Arguments:

  • urb - pointer to the urb to initialize

Description:

Initializes a urb so that the USB subsystem can use it properly.

If a urb is created with a call to usb_alloc_urb it is not necessary to call this function. Only use this if you allocate the space for a struct urb on your own. If you call this function, be careful when freeing the memory for your urb that it is no longer in use by the USB core.

Only use this function if you _really_ understand what you are doing.

  • usb_alloc_urb - creates a new urb for a USB driver to use

Synopsis:

struct urb * usb_alloc_urb ( int iso_packets )

Arguments:

  • iso_packets - number of iso packets for this urb
  • mem_flags - the type of memory to allocate, see kmalloc for a list of valid options for this.

Description:

Creates an urb for the USB driver to use, initializes a few internal structures, incrementes the usage counter, and returns a pointer to it.

If no memory is available, NULL is returned.

If the driver want to use this urb for interrupt, control, or bulk endpoints, pass '0' as the number of iso packets.

The driver must call usb_free_urb when it is finished with the urb.

  • usb_free_urb - frees the memory used by a urb when all users of it are finished

Synopsis:

void usb_free_urb ( struct urb * urb )

Arguments:

  • urb - pointer to the urb to free, may be NULL

Description:

Must be called when a user of a urb is finished with it. When the last user of the urb calls this function, the memory of the urb is freed.

Note:

The transfer buffer associated with the urb is not freed unless the URB_FREE_BUFFER transfer flag is set.

  • usb_get_urb - increments the reference count of the urb

Synopsis:

struct urb * usb_get_urb ( struct urb * urb )

Arguments:

  • urb - pointer to the urb to modify, may be NULL

Description:

This must be called whenever a urb is transferred from a device driver to a host controller driver. This allows proper reference counting to happen for urbs.

A pointer to the urb with the incremented reference counter is returned.

  • usb_anchor_urb - anchors an URB while it is processed

Synopsis:

void usb_anchor_urb ( struct urb * urb )

Arguments:

  • urb - pointer to the urb to anchor
  • anchor - pointer to the anchor

Description:

This can be called to have access to URBs which are to be executed without bothering to track them

  • usb_unanchor_urb - unanchors an URB

Synopsis:

void usb_unanchor_urb ( struct urb * urb )

Arguments:

  • urb - pointer to the urb to anchor

Description:

Call this to stop the system keeping track of this URB

  • usb_submit_urb - issue an asynchronous transfer request for an endpoint

Synopsis:

int usb_submit_urb ( struct urb * urb )

Arguments:

  • urb - pointer to the urb describing the request
  • mem_flags - the type of memory to allocate, see kmalloc for a list of valid options for this.

Description:

This submits a transfer request, and transfers control of the URB describing that request to the USB subsystem. Request completion will be indicated later, asynchronously, by calling the completion handler. The three types of completion are success, error, and unlink (a software-induced fault, also called request cancellation).

URBs may be submitted in interrupt context.

The caller must have correctly initialized the URB before submitting it. Functions such as usb_fill_bulk_urb and usb_fill_control_urb are available to ensure that most fields are correctly initialized, for the particular kind of transfer, although they will not initialize any transfer flags.

Successful submissions return 0; otherwise this routine returns a negative error number. If the submission is successful, the complete callback from the URB will be called exactly once, when the USB core and Host Controller Driver (HCD) are finished with the URB. When the completion function is called, control of the URB is returned to the device driver which issued the request. The completion handler may then immediately free or reuse that URB.

With few exceptions, USB device drivers should never access URB fields provided by usbcore or the HCD until its complete is called. The exceptions relate to periodic transfer scheduling. For both interrupt and isochronous urbs, as part of successful URB submission urb→interval is modified to reflect the actual transfer period used (normally some power of two units). And for isochronous urbs, urb→start_frame is modified to reflect when the URB's transfers were scheduled to start. Not all isochronous transfer scheduling policies will work, but most host controller drivers should easily handle ISO queues going from now until 10-200 msec into the future.

For control endpoints, the synchronous usb_control_msg call is often used (in non-interrupt context) instead of this call. That is often used through convenience wrappers, for the requests that are standardized in the USB 2.0 specification. For bulk endpoints, a synchronous usb_bulk_msg call is available.

Request Queuing:

URBs may be submitted to endpoints before previous ones complete, to minimize the impact of interrupt latencies and system overhead on data throughput. With that queuing policy, an endpoint's queue would never be empty. This is required for continuous isochronous data streams, and may also be required for some kinds of interrupt transfers. Such queuing also maximizes bandwidth utilization by letting USB controllers start work on later requests before driver software has finished the completion processing for earlier (successful) requests.

As of Linux 2.6, all USB endpoint transfer queues support depths greater than one. This was previously a HCD-specific behavior, except for ISO transfers. Non-isochronous endpoint queues are inactive during cleanup after faults (transfer errors or cancellation).

Reserved Bandwidth Transfers:

Periodic transfers (interrupt or isochronous) are performed repeatedly, using the interval specified in the urb. Submitting the first urb to the endpoint reserves the bandwidth necessary to make those transfers. If the USB subsystem can't allocate sufficient bandwidth to perform the periodic request, submitting such a periodic request should fail.

For devices under xHCI, the bandwidth is reserved at configuration time, or when the alt setting is selected. If there is not enough bus bandwidth, the configuration/alt setting request will fail. Therefore, submissions to periodic endpoints on devices under xHCI should never fail due to bandwidth constraints.

Device drivers must explicitly request that repetition, by ensuring that some URB is always on the endpoint's queue (except possibly for short periods during completion callacks). When there is no longer an urb queued, the endpoint's bandwidth reservation is canceled. This means drivers can use their completion handlers to ensure they keep bandwidth they need, by reinitializing and resubmitting the just-completed urb until the driver longer needs that periodic bandwidth.

Memory Flags:

The general rules for how to decide which mem_flags to use are the same as for kmalloc. There are four different possible values; GFP_KERNEL, GFP_NOFS, GFP_NOIO and GFP_ATOMIC.

GFP_NOFS is not ever used, as it has not been implemented yet.

GFP_ATOMIC is used when (a) you are inside a completion handler, an interrupt, bottom half, tasklet or timer, or (b) you are holding a spinlock or rwlock (does not apply to semaphores), or © current→state != TASK_RUNNING, this is the case only after you've changed it.

GFP_NOIO is used in the block io path and error handling of storage devices.

All other situations use GFP_KERNEL.

Some more specific rules for mem_flags can be inferred, such as (1) start_xmit, timeout, and receive methods of network drivers must use GFP_ATOMIC (they are called with a spinlock held); (2) queuecommand methods of scsi drivers must use GFP_ATOMIC (also called with a spinlock held); (3) If you use a kernel thread with a network driver you must use GFP_NOIO, unless (b) or © apply; (4) after you have done a down you can use GFP_KERNEL, unless (b) or © apply or your are in a storage driver's block io path; (5) USB probe and disconnect can use GFP_KERNEL unless (b) or © apply; and (6) changing firmware on a running storage or net device uses GFP_NOIO, unless b) or c) apply

  • usb_unlink_urb - abort/cancel a transfer request for an endpoint

Synopsis:

int usb_unlink_urb ( struct urb * urb )

Arguments:

  • urb - pointer to urb describing a previously submitted request, may be NULL

Description:

This routine cancels an in-progress request. URBs complete only once per submission, and may be canceled only once per submission. Successful cancellation means termination of urb will be expedited and the completion handler will be called with a status code indicating that the request has been canceled (rather than any other code).

Drivers should not call this routine or related routines, such as usb_kill_urb or usb_unlink_anchored_urbs, after their disconnect method has returned. The disconnect function should synchronize with a driver's I/O routines to insure that all URB-related activity has completed before it returns.

This request is always asynchronous. Success is indicated by returning -EINPROGRESS, at which time the URB will probably not yet have been given back to the device driver. When it is eventually called, the completion function will see urb→status == -ECONNRESET. Failure is indicated by usb_unlink_urb returning any other value. Unlinking will fail when urb is not currently linked (i.e., it was never submitted, or it was unlinked before, or the hardware is already finished with it), even if the completion handler has not yet run.

Unlinking and Endpoint Queues:

[The behaviors and guarantees described below do not apply to virtual root hubs but only to endpoint queues for physical USB devices.]

Host Controller Drivers (HCDs) place all the URBs for a particular endpoint in a queue. Normally the queue advances as the controller hardware processes each request. But when an URB terminates with an error its queue generally stops (see below), at least until that URB's completion routine returns. It is guaranteed that a stopped queue will not restart until all its unlinked URBs have been fully retired, with their completion routines run, even if that's not until some time after the original completion handler returns. The same behavior and guarantee apply when an URB terminates because it was unlinked.

Bulk and interrupt endpoint queues are guaranteed to stop whenever an URB terminates with any sort of error, including -ECONNRESET, -ENOENT, and -EREMOTEIO. Control endpoint queues behave the same way except that they are not guaranteed to stop for -EREMOTEIO errors. Queues for isochronous endpoints are treated differently, because they must advance at fixed rates. Such queues do not stop when an URB encounters an error or is unlinked. An unlinked isochronous URB may leave a gap in the stream of packets; it is undefined whether such gaps can be filled in.

Note that early termination of an URB because a short packet was received will generate a -EREMOTEIO error if and only if the URB_SHORT_NOT_OK flag is set. By setting this flag, USB device drivers can build deep queues for large or complex bulk transfers and clean them up reliably after any sort of aborted transfer by unlinking all pending URBs at the first fault.

When a control URB terminates with an error other than -EREMOTEIO, it is quite likely that the status stage of the transfer will not take place.

  • usb_kill_urb - cancel a transfer request and wait for it to finish

Synopsis:

void usb_kill_urb ( struct urb * urb )

Arguments:

  • urb - pointer to URB describing a previously submitted request, may be NULL

Description:

This routine cancels an in-progress request. It is guaranteed that upon return all completion handlers will have finished and the URB will be totally idle and available for reuse. These features make this an ideal way to stop I/O in a disconnect callback or close function. If the request has not already finished or been unlinked the completion handler will see urb→status == -ENOENT.

While the routine is running, attempts to resubmit the URB will fail with error -EPERM. Thus even if the URB's completion handler always tries to resubmit, it will not succeed and the URB will become idle.

This routine may not be used in an interrupt context (such as a bottom half or a completion handler), or when holding a spinlock, or in other situations where the caller can't schedule.

This routine should not be called by a driver after its disconnect method has returned.

  • usb_poison_urb - reliably kill a transfer and prevent further use of an URB

Synopsis:

void usb_poison_urb ( struct urb * urb )

Arguments:

  • urb - pointer to URB describing a previously submitted request, may be NULL

Description:

This routine cancels an in-progress request. It is guaranteed that upon return all completion handlers will have finished and the URB will be totally idle and cannot be reused. These features make this an ideal way to stop I/O in a disconnect callback. If the request has not already finished or been unlinked the completion handler will see urb→status == -ENOENT.

After and while the routine runs, attempts to resubmit the URB will fail with error -EPERM. Thus even if the URB's completion handler always tries to resubmit, it will not succeed and the URB will become idle.

This routine may not be used in an interrupt context (such as a bottom half or a completion handler), or when holding a spinlock, or in other situations where the caller can't schedule.

This routine should not be called by a driver after its disconnect method has returned.

  • usb_kill_anchored_urbs - cancel transfer requests en masse

Synopsis:

void usb_kill_anchored_urbs ( struct usb_anchor * anchor )

Arguments:

  • anchor - anchor the requests are bound to

Description:

this allows all outstanding URBs to be killed starting from the back of the queue

This routine should not be called by a driver after its disconnect method has returned.

  • usb_poison_anchored_urbs - cease all traffic from an anchor

Synopsis:

void usb_poison_anchored_urbs ( struct usb_anchor * anchor )

Arguments:

  • anchor - anchor the requests are bound to

Description:

this allows all outstanding URBs to be poisoned starting from the back of the queue. Newly added URBs will also be poisoned

This routine should not be called by a driver after its disconnect method has returned.

  • usb_unpoison_anchored_urbs - let an anchor be used successfully again

Synopsis:

void usb_unpoison_anchored_urbs ( struct usb_anchor * anchor )

Arguments:

  • anchor - anchor the requests are bound to

Description:

Reverses the effect of usb_poison_anchored_urbs the anchor can be used normally after it returns

  • usb_unlink_anchored_urbs - asynchronously cancel transfer requests en masse

Synopsis:

void usb_unlink_anchored_urbs ( struct usb_anchor * anchor )

Arguments:

  • anchor - anchor the requests are bound to

Description:

this allows all outstanding URBs to be unlinked starting from the back of the queue. This function is asynchronous. The unlinking is just tiggered. It may happen after this function has returned.

This routine should not be called by a driver after its disconnect method has returned.

  • usb_wait_anchor_empty_timeout - wait for an anchor to be unused

Synopsis:

int usb_wait_anchor_empty_timeout ( struct usb_anchor * anchor )

Arguments:

  • anchor - the anchor you want to become unused
  • timeout - how long you are willing to wait in milliseconds

Description:

Call this is you want to be sure all an anchor's URBs have finished

  • usb_get_from_anchor - get an anchor's oldest urb

Synopsis:

struct urb * usb_get_from_anchor ( struct usb_anchor * anchor )

Arguments:

  • anchor - the anchor whose urb you want

Description:

this will take the oldest urb from an anchor, unanchor and return it

  • usb_scuttle_anchored_urbs - unanchor all an anchor's urbs

Synopsis:

void usb_scuttle_anchored_urbs ( struct usb_anchor * anchor )

Arguments:

  • anchor - the anchor whose urbs you want to unanchor

Description:

use this to get rid of all an anchor's urbs

  • usb_anchor_empty - is an anchor empty

Synopsis:

int usb_anchor_empty ( struct usb_anchor * anchor )

Arguments:

  • anchor - the anchor you want to query

Description:

returns 1 if the anchor has no urbs associated with it

  • usb_control_msg - Builds a control urb, sends it off and waits for completion

Synopsis:

int usb_control_msg ( struct usb_device * dev )

Arguments:

  • dev - pointer to the usb device to send the message to
  • pipe - endpoint pipe to send the message to
  • request - USB message request value
  • requesttype - USB message request type value
  • value - USB message value
  • index - USB message index value
  • data - pointer to the data to send
  • size - length in bytes of the data to send
  • timeout - time in msecs to wait for the message to complete before timing out (if 0 the wait is forever)

Context:

!in_interrupt ()

Description:

This function sends a simple control message to a specified endpoint and waits for the message to complete, or timeout.

If successful, it returns the number of bytes transferred, otherwise a negative error number.

Don't use this function from within an interrupt context, like a bottom half handler. If you need an asynchronous message, or need to send a message from within interrupt context, use usb_submit_urb. If a thread in your driver uses this call, make sure your disconnect method can wait for it to complete. Since you don't have a handle on the URB used, you can't cancel the request.

  • usb_interrupt_msg - Builds an interrupt urb, sends it off and waits for completion

Synopsis:

int usb_interrupt_msg ( struct usb_device * usb_dev )

Arguments:

  • usb_dev - pointer to the usb device to send the message to
  • pipe - endpoint pipe to send the message to
  • data - pointer to the data to send
  • len - length in bytes of the data to send
  • actual_length - pointer to a location to put the actual length transferred in bytes
  • timeout - time in msecs to wait for the message to complete before timing out (if 0 the wait is forever)

Context:

!in_interrupt ()

Description:

This function sends a simple interrupt message to a specified endpoint and waits for the message to complete, or timeout.

If successful, it returns 0, otherwise a negative error number. The number of actual bytes transferred will be stored in the actual_length paramater.

Don't use this function from within an interrupt context, like a bottom half handler. If you need an asynchronous message, or need to send a message from within interrupt context, use usb_submit_urb If a thread in your driver uses this call, make sure your disconnect method can wait for it to complete. Since you don't have a handle on the URB used, you can't cancel the request.

  • usb_bulk_msg - Builds a bulk urb, sends it off and waits for completion

Synopsis:

int usb_bulk_msg ( struct usb_device * usb_dev )

Arguments:

  • usb_dev - pointer to the usb device to send the message to
  • pipe - endpoint pipe to send the message to
  • data - pointer to the data to send
  • len - length in bytes of the data to send
  • actual_length - pointer to a location to put the actual length transferred in bytes
  • timeout - time in msecs to wait for the message to complete before timing out (if 0 the wait is forever)

Context:

!in_interrupt ()

Description:

This function sends a simple bulk message to a specified endpoint and waits for the message to complete, or timeout.

If successful, it returns 0, otherwise a negative error number. The number of actual bytes transferred will be stored in the actual_length paramater.

Don't use this function from within an interrupt context, like a bottom half handler. If you need an asynchronous message, or need to send a message from within interrupt context, use usb_submit_urb If a thread in your driver uses this call, make sure your disconnect method can wait for it to complete. Since you don't have a handle on the URB used, you can't cancel the request.

Because there is no usb_interrupt_msg and no USBDEVFS_INTERRUPT ioctl, users are forced to abuse this routine by using it to submit URBs for interrupt endpoints. We will take the liberty of creating an interrupt URB (with the default interval) if the target is an interrupt endpoint.

  • usb_sg_init - initializes scatterlist-based bulk/interrupt I/O request

Synopsis:

int usb_sg_init ( struct usb_sg_request * io )

Arguments:

  • io - request block being initialized. until usb_sg_wait returns, treat this as a pointer to an opaque block of memory,
  • dev - the usb device that will send or receive the data
  • pipe - endpoint pipe used to transfer the data
  • period - polling rate for interrupt endpoints, in frames or (for high speed endpoints) microframes; ignored for bulk
  • sg - scatterlist entries
  • nents - how many entries in the scatterlist
  • length - how many bytes to send from the scatterlist, or zero to send every byte identified in the list.
  • mem_flags - SLAB_* flags affecting memory allocations in this call

Description:

Returns zero for success, else a negative errno value. This initializes a scatter/gather request, allocating resources such as I/O mappings and urb memory (except maybe memory used by USB controller drivers).

The request must be issued using usb_sg_wait, which waits for the I/O to complete (or to be canceled) and then cleans up all resources allocated by usb_sg_init.

The request may be canceled with usb_sg_cancel, either before or after usb_sg_wait is called.

  • usb_sg_wait - synchronously execute scatter/gather request

Synopsis:

void usb_sg_wait ( struct usb_sg_request * io )

Arguments:

  • io - request block handle, as initialized with usb_sg_init. some fields become accessible when this call returns.

Context:

!in_interrupt ()

Description:

This function blocks until the specified I/O operation completes. It leverages the grouping of the related I/O requests to get good transfer rates, by queueing the requests. At higher speeds, such queuing can significantly improve USB throughput.

There are three kinds of completion for this function. (1) success, where io→status is zero. The number of io→bytes transferred is as requested. (2) error, where io→status is a negative errno value. The number of io→bytes transferred before the error is usually less than requested, and can be nonzero. (3) cancellation, a type of error with status -ECONNRESET that is initiated by usb_sg_cancel.

When this function returns, all memory allocated through usb_sg_init or this call will have been freed. The request block parameter may still be passed to usb_sg_cancel, or it may be freed. It could also be reinitialized and then reused.

Data Transfer Rates:

Bulk transfers are valid for full or high speed endpoints. The best full speed data rate is 19 packets of 64 bytes each per frame, or 1216 bytes per millisecond. The best high speed data rate is 13 packets of 512 bytes each per microframe, or 52 KBytes per millisecond.

The reason to use interrupt transfers through this API would most likely be to reserve high speed bandwidth, where up to 24 KBytes per millisecond could be transferred. That capability is less useful for low or full speed interrupt endpoints, which allow at most one packet per millisecond, of at most 8 or 64 bytes (respectively).

It is not necessary to call this function to reserve bandwidth for devices under an xHCI host controller, as the bandwidth is reserved when the configuration or interface alt setting is selected.

  • usb_sg_cancel - stop scatter/gather i/o issued by usb_sg_wait

Synopsis:

void usb_sg_cancel ( struct usb_sg_request * io )

Arguments:

  • io - request block, initialized with usb_sg_init

Description:

This stops a request after it has been started by usb_sg_wait. It can also prevents one initialized by usb_sg_init from starting, so that call just frees resources allocated to the request.

  • usb_get_descriptor - issues a generic GET_DESCRIPTOR request

Synopsis:

int usb_get_descriptor ( struct usb_device * dev )

Arguments:

  • dev - the device whose descriptor is being retrieved
  • type - the descriptor type (USB_DT_*)
  • index - the number of the descriptor
  • buf - where to put the descriptor
  • size - how big is buf?

Context:

!in_interrupt ()

Description:

Gets a USB descriptor. Convenience functions exist to simplify getting some types of descriptors. Use usb_get_string or usb_string for USB_DT_STRING. Device (USB_DT_DEVICE) and configuration descriptors (USB_DT_CONFIG) are part of the device structure. In addition to a number of USB-standard descriptors, some devices also use class-specific or vendor-specific descriptors.

This call is synchronous, and may not be used in an interrupt context.

Returns the number of bytes received on success, or else the status code returned by the underlying usb_control_msg call.

  • usb_string - returns UTF-8 version of a string descriptor

Synopsis:

int usb_string ( struct usb_device * dev )

Arguments:

  • dev - the device whose string descriptor is being retrieved
  • index - the number of the descriptor
  • buf - where to put the string
  • size - how big is buf?

Context:

!in_interrupt ()

Description:

This converts the UTF-16LE encoded strings returned by devices, from usb_get_string_descriptor, to null-terminated UTF-8 encoded ones that are more usable in most kernel contexts. Note that this function chooses strings in the first language supported by the device.

This call is synchronous, and may not be used in an interrupt context.

Returns length of the string (>= 0) or usb_control_msg status (< 0).

  • usb_get_status - issues a GET_STATUS call

Synopsis:

int usb_get_status ( struct usb_device * dev )

Arguments:

  • dev - the device whose status is being checked
  • type - USB_RECIP_*; for device, interface, or endpoint
  • target - zero (for device), else interface or endpoint number
  • data - pointer to two bytes of bitmap data

Context:

!in_interrupt ()

Description:

Returns device, interface, or endpoint status. Normally only of interest to see if the device is self powered, or has enabled the remote wakeup facility; or whether a bulk or interrupt endpoint is halted (stalled).

Bits in these status bitmaps are set using the SET_FEATURE request, and cleared using the CLEAR_FEATURE request. The usb_clear_halt function should be used to clear halt (stall) status.

This call is synchronous, and may not be used in an interrupt context.

Returns the number of bytes received on success, or else the status code returned by the underlying usb_control_msg call.

  • usb_clear_halt - tells device to clear endpoint halt/stall condition

Synopsis:

int usb_clear_halt ( struct usb_device * dev )

Arguments:

  • dev - device whose endpoint is halted
  • pipe - endpoint pipe being cleared

Context:

!in_interrupt ()

Description:

This is used to clear halt conditions for bulk and interrupt endpoints, as reported by URB completion status. Endpoints that are halted are sometimes referred to as being stalled. Such endpoints are unable to transmit or receive data until the halt status is cleared. Any URBs queued for such an endpoint should normally be unlinked by the driver before clearing the halt condition, as described in sections 5.7.5 and 5.8.5 of the USB 2.0 spec.

Note that control and isochronous endpoints don't halt, although control endpoints report protocol stall (for unsupported requests) using the same status code used to report a true stall.

This call is synchronous, and may not be used in an interrupt context.

Returns zero on success, or else the status code returned by the underlying usb_control_msg call.

  • usb_reset_endpoint - Reset an endpoint's state.

Synopsis:

void usb_reset_endpoint ( struct usb_device * dev )

Arguments:

  • dev - the device whose endpoint is to be reset
  • epaddr - the endpoint's address. Endpoint number for output, endpoint number + USB_DIR_IN for input

Description:

Resets any host-side endpoint state such as the toggle bit, sequence number or current window.

  • usb_set_interface - Makes a particular alternate setting be current

Synopsis:

int usb_set_interface ( struct usb_device * dev )

Arguments:

  • dev - the device whose interface is being updated
  • interface - the interface being updated
  • alternate - the setting being chosen.

Context:

!in_interrupt ()

Description:

This is used to enable data transfers on interfaces that may not be enabled by default. Not all devices support such configurability. Only the driver bound to an interface may change its setting.

Within any given configuration, each interface may have several alternative settings. These are often used to control levels of bandwidth consumption. For example, the default setting for a high speed interrupt endpoint may not send more than 64 bytes per microframe, while interrupt transfers of up to 3KBytes per microframe are legal. Also, isochronous endpoints may never be part of an interface's default setting. To access such bandwidth, alternate interface settings must be made current.

Note that in the Linux USB subsystem, bandwidth associated with an endpoint in a given alternate setting is not reserved until an URB is submitted that needs that bandwidth. Some other operating systems allocate bandwidth early, when a configuration is chosen.

This call is synchronous, and may not be used in an interrupt context. Also, drivers must not change altsettings while urbs are scheduled for endpoints in that interface; all such urbs must first be completed (perhaps forced by unlinking).

Returns zero on success, or else the status code returned by the underlying usb_control_msg call.

  • usb_reset_configuration - lightweight device reset

Synopsis:

int usb_reset_configuration ( struct usb_device * dev )

Arguments:

  • dev - the device whose configuration is being reset

Description:

This issues a standard SET_CONFIGURATION request to the device using the current configuration. The effect is to reset most USB-related state in the device, including interface altsettings (reset to zero), endpoint halts (cleared), and endpoint state (only for bulk and interrupt endpoints). Other usbcore state is unchanged, including bindings of usb device drivers to interfaces.

Because this affects multiple interfaces, avoid using this with composite (multi-interface) devices. Instead, the driver for each interface may use usb_set_interface on the interfaces it claims. Be careful though; some devices don't support the SET_INTERFACE request, and others won't reset all the interface state (notably endpoint state). Resetting the whole configuration would affect other drivers' interfaces.

The caller must own the device lock.

Returns zero on success, else a negative error code.

  • usb_driver_set_configuration - Provide a way for drivers to change device configurations

Synopsis:

int usb_driver_set_configuration ( struct usb_device * udev )

Arguments:

  • udev - the device whose configuration is being updated
  • config - the configuration being chosen.

Context:

In process context, must be able to sleep

Description:

Device interface drivers are not allowed to change device configurations. This is because changing configurations will destroy the interface the driver is bound to and create new ones; it would be like a floppy-disk driver telling the computer to replace the floppy-disk drive with a tape drive!

Still, in certain specialized circumstances the need may arise. This routine gets around the normal restrictions by using a work thread to submit the change-config request.

Returns 0 if the request was successfully queued, error code otherwise. The caller has no way to know whether the queued request will eventually succeed.

  • usb_register_dev - register a USB device, and ask for a minor number

Synopsis:

int usb_register_dev ( struct usb_interface * intf )

Arguments:

  • intf - pointer to the usb_interface that is being registered
  • class_driver - pointer to the usb_class_driver for this device

Description:

This should be called by all USB drivers that use the USB major number. If CONFIG_USB_DYNAMIC_MINORS is enabled, the minor number will be dynamically allocated out of the list of available ones. If it is not enabled, the minor number will be based on the next available free minor, starting at the class_driver→minor_base.

This function also creates a usb class device in the sysfs tree.

usb_deregister_dev must be called when the driver is done with the minor numbers given out by this function.

Returns -EINVAL if something bad happens with trying to register a device, and 0 on success.

  • usb_deregister_dev - deregister a USB device's dynamic minor.

Synopsis:

void usb_deregister_dev ( struct usb_interface * intf )

Arguments:

  • intf - pointer to the usb_interface that is being deregistered
  • class_driver - pointer to the usb_class_driver for this device

Description:

Used in conjunction with usb_register_dev. This function is called when the USB driver is finished with the minor numbers gotten from a call to usb_register_dev (usually when the device is disconnected from the system.)

This function also removes the usb class device from the sysfs tree.

This should be called by all drivers that use the USB major number.

  • usb_driver_claim_interface - bind a driver to an interface

Synopsis:

int usb_driver_claim_interface ( struct usb_driver * driver )

Arguments:

  • driver - the driver to be bound
  • iface - the interface to which it will be bound; must be in the usb device's active configuration
  • priv - driver data associated with that interface

Description:

This is used by usb device drivers that need to claim more than one interface on a device when probing (audio and acm are current examples). No device driver should directly modify internal usb_interface or usb_device structure members.

Few drivers should need to use this routine, since the most natural way to bind to an interface is to return the private data from the driver's probe method.

Callers must own the device lock, so driver probe entries don't need extra locking, but other call contexts may need to explicitly claim that lock.

  • usb_driver_release_interface - unbind a driver from an interface

Synopsis:

void usb_driver_release_interface ( struct usb_driver * driver )

Arguments:

  • driver - the driver to be unbound
  • iface - the interface from which it will be unbound

Description:

This can be used by drivers to release an interface without waiting for their disconnect methods to be called. In typical cases this also causes the driver disconnect method to be called.

This call is synchronous, and may not be used in an interrupt context. Callers must own the device lock, so driver disconnect entries don't need extra locking, but other call contexts may need to explicitly claim that lock.

  • usb_match_id - find first usb_device_id matching device or interface

Synopsis:

const struct usb_device_id * usb_match_id ( struct usb_interface * interface )

Arguments:

  • interface - the interface of interest
  • id - array of usb_device_id structures, terminated by zero entry

Description:

usb_match_id searches an array of usb_device_id's and returns the first one matching the device or interface, or null. This is used when binding (or rebinding) a driver to an interface. Most USB device drivers will use this indirectly, through the usb core, but some layered driver frameworks use it directly. These device tables are exported with MODULE_DEVICE_TABLE, through modutils, to support the driver loading functionality of USB hotplugging.

What Matches:

The match_flags element in a usb_device_id controls which members are used. If the corresponding bit is set, the value in the device_id must match its corresponding member in the device or interface descriptor, or else the device_id does not match.

driver_info is normally used only by device drivers, but you can create a wildcard matches anything usb_device_id as a driver's modules.usbmap entry if you provide an id with only a nonzero driver_info field. If you do this, the USB device driver's probe routine should use additional intelligence to decide whether to bind to the specified interface.

What Makes Good usb_device_id Tables:

The match algorithm is very simple, so that intelligence in driver selection must come from smart driver id records. Unless you have good reasons to use another selection policy, provide match elements only in related groups, and order match specifiers from specific to general. Use the macros provided for that purpose if you can.

The most specific match specifiers use device descriptor data. These are commonly used with product-specific matches; the USB_DEVICE macro lets you provide vendor and product IDs, and you can also match against ranges of product revisions. These are widely used for devices with application or vendor specific bDeviceClass values.

Matches based on device class/subclass/protocol specifications are slightly more general; use the USB_DEVICE_INFO macro, or its siblings. These are used with single-function devices where bDeviceClass doesn't specify that each interface has its own class.

Matches based on interface class/subclass/protocol are the most general; they let drivers bind to any interface on a multiple-function device. Use the USB_INTERFACE_INFO macro, or its siblings, to match class-per-interface style devices (as recorded in bInterfaceClass).

Note that an entry created by USB_INTERFACE_INFO won't match any interface if the device class is set to Vendor-Specific. This is deliberate; according to the USB spec the meanings of the interface class/subclass/protocol for these devices are also vendor-specific, and hence matching against a standard product class wouldn't work anyway. If you really want to use an interface-based match for such a device, create a match record that also specifies the vendor ID. (Unforunately there isn't a standard macro for creating records like this.)

Within those groups, remember that not all combinations are meaningful. For example, don't give a product version range without vendor and product IDs; or specify a protocol without its associated class and subclass.

  • usb_register_device_driver - register a USB device (not interface) driver

Synopsis:

int usb_register_device_driver ( struct usb_device_driver * new_udriver )

Arguments:

  • new_udriver - USB operations for the device driver
  • owner - module owner of this driver.

Description:

Registers a USB device driver with the USB core. The list of unattached devices will be rescanned whenever a new driver is added, allowing the new driver to attach to any recognized devices. Returns a negative error code on failure and 0 on success.

  • usb_deregister_device_driver - unregister a USB device (not interface) driver

Synopsis:

void usb_deregister_device_driver ( struct usb_device_driver * udriver )

Arguments:

  • udriver - USB operations of the device driver to unregister

Context:

must be able to sleep

Description:

Unlinks the specified driver from the internal USB driver list.

  • usb_register_driver - register a USB interface driver

Synopsis:

int usb_register_driver ( struct usb_driver * new_driver )

Arguments:

  • new_driver - USB operations for the interface driver
  • owner - module owner of this driver.
  • mod_name - module name string

Description:

Registers a USB interface driver with the USB core. The list of unattached interfaces will be rescanned whenever a new driver is added, allowing the new driver to attach to any recognized interfaces. Returns a negative error code on failure and 0 on success.

NOTE:

if you want your driver to use the USB major number, you must call usb_register_dev to enable that functionality. This function no longer takes care of that.

  • usb_deregister - unregister a USB interface driver

Synopsis:

void usb_deregister ( struct usb_driver * driver )

Arguments:

  • driver - USB operations of the interface driver to unregister

Context:

must be able to sleep

Description:

Unlinks the specified driver from the internal USB driver list.

NOTE:

If you called usb_register_dev, you still need to call usb_deregister_dev to clean up your driver's allocated minor numbers, this * call will no longer do it for you.

  • usb_autopm_put_interface - decrement a USB interface's PM-usage counter

Synopsis:

void usb_autopm_put_interface ( struct usb_interface * intf )

Arguments:

  • intf - the usb_interface whose counter should be decremented

Description:

This routine should be called by an interface driver when it is finished using intf and wants to allow it to autosuspend. A typical example would be a character-device driver when its device file is closed.

The routine decrements intf's usage counter. When the counter reaches 0, a delayed autosuspend request for intf's device is queued. When the delay expires, if intf→pm_usage_cnt is still ⇐ 0 along with all the other usage counters for the sibling interfaces and intf's usb_device, the device and all its interfaces will be autosuspended.

Note that intf→pm_usage_cnt is owned by the interface driver. The core will not change its value other than the increment and decrement in usb_autopm_get_interface and usb_autopm_put_interface. The driver may use this simple counter-oriented discipline or may set the value any way it likes.

If the driver has set intf→needs_remote_wakeup then autosuspend will take place only if the device's remote-wakeup facility is enabled.

Suspend method calls queued by this routine can arrive at any time while intf is resumed and its usage counter is equal to 0. They are not protected by the usb_device's lock but only by its pm_mutex. Drivers must provide their own synchronization.

This routine can run only in process context.

  • usb_autopm_put_interface_async - decrement a USB interface's PM-usage counter

Synopsis:

void usb_autopm_put_interface_async ( struct usb_interface * intf )

Arguments:

  • intf - the usb_interface whose counter should be decremented

Description:

This routine does essentially the same thing as usb_autopm_put_interface: it decrements intf's usage counter and queues a delayed autosuspend request if the counter is ⇐ 0. The difference is that it does not acquire the device's pm_mutex; callers must handle all synchronization issues themselves.

Typically a driver would call this routine during an URB's completion handler, if no more URBs were pending.

This routine can run in atomic context.

  • usb_autopm_get_interface - increment a USB interface's PM-usage counter

Synopsis:

int usb_autopm_get_interface ( struct usb_interface * intf )

Arguments:

  • intf - the usb_interface whose counter should be incremented

Description:

This routine should be called by an interface driver when it wants to use intf and needs to guarantee that it is not suspended. In addition, the routine prevents intf from being autosuspended subsequently. (Note that this will not prevent suspend events originating in the PM core.) This prevention will persist until usb_autopm_put_interface is called or intf is unbound. A typical example would be a character-device driver when its device file is opened.

The routine increments intf's usage counter. (However if the autoresume fails then the counter is re-decremented.) So long as the counter is greater than 0, autosuspend will not be allowed for intf or its usb_device. When the driver is finished using intf it should call usb_autopm_put_interface to decrement the usage counter and queue a delayed autosuspend request (if the counter is ⇐ 0).

Note that intf→pm_usage_cnt is owned by the interface driver. The core will not change its value other than the increment and decrement in usb_autopm_get_interface and usb_autopm_put_interface. The driver may use this simple counter-oriented discipline or may set the value any way it likes.

Resume method calls generated by this routine can arrive at any time while intf is suspended. They are not protected by the usb_device's lock but only by its pm_mutex. Drivers must provide their own synchronization.

This routine can run only in process context.

  • usb_autopm_get_interface_async - increment a USB interface's PM-usage counter

Synopsis:

int usb_autopm_get_interface_async ( struct usb_interface * intf )

Arguments:

  • intf - the usb_interface whose counter should be incremented

Description:

This routine does much the same thing as usb_autopm_get_interface: it increments intf's usage counter and queues an autoresume request if the result is > 0. The differences are that it does not acquire the device's pm_mutex (callers must handle all synchronization issues themselves), and it does not autoresume the device directly (it only queues a request). After a successful call, the device will generally not yet be resumed.

This routine can run in atomic context.

  • usb_autopm_set_interface - set a USB interface's autosuspend state

Synopsis:

int usb_autopm_set_interface ( struct usb_interface * intf )

Arguments:

  • intf - the usb_interface whose state should be set

Description:

This routine sets the autosuspend state of intf's device according to intf's usage counter, which the caller must have set previously. If the counter is ⇐ 0, the device is autosuspended (if it isn't already suspended and if nothing else prevents the autosuspend). If the counter is > 0, the device is autoresumed (if it isn't already awake).

  • usb_ifnum_to_if - get the interface object with a given interface number

Synopsis:

struct usb_interface * usb_ifnum_to_if ( const struct usb_device * dev )

Arguments:

  • dev - the device whose current configuration is considered
  • ifnum - the desired interface

Description:

This walks the device descriptor for the currently active configuration and returns a pointer to the interface with that particular interface number, or null.

Note that configuration descriptors are not required to assign interface numbers sequentially, so that it would be incorrect to assume that the first interface in that descriptor corresponds to interface zero. This routine helps device drivers avoid such mistakes. However, you should make sure that you do the right thing with any alternate settings available for this interfaces.

Don't call this function unless you are bound to one of the interfaces on this device or you have locked the device!

  • usb_altnum_to_altsetting - get the altsetting structure with a given alternate setting number.

Synopsis:

struct usb_host_interface * usb_altnum_to_altsetting ( const struct usb_interface * intf )

Arguments:

  • intf - the interface containing the altsetting in question
  • altnum - the desired alternate setting number

Description:

This searches the altsetting array of the specified interface for an entry with the correct bAlternateSetting value and returns a pointer to that entry, or null.

Note that altsettings need not be stored sequentially by number, so it would be incorrect to assume that the first altsetting entry in the array corresponds to altsetting zero. This routine helps device drivers avoid such mistakes.

Don't call this function unless you are bound to the intf interface or you have locked the device!

  • usb_find_interface - find usb_interface pointer for driver and device

Synopsis:

struct usb_interface * usb_find_interface ( struct usb_driver * drv )

Arguments:

  • drv - the driver whose current configuration is considered
  • minor - the minor number of the desired device

Description:

This walks the bus device list and returns a pointer to the interface with the matching minor and driver. Note, this only works for devices that share the USB major number.

  • usb_get_dev - increments the reference count of the usb device structure

Synopsis:

struct usb_device * usb_get_dev ( struct usb_device * dev )

Arguments:

  • dev - the device being referenced

Description:

Each live reference to a device should be refcounted.

Drivers for USB interfaces should normally record such references in their probe methods, when they bind to an interface, and release them by calling usb_put_dev, in their disconnect methods.

A pointer to the device with the incremented reference counter is returned.

  • usb_put_dev - release a use of the usb device structure

Synopsis:

void usb_put_dev ( struct usb_device * dev )

Arguments:

  • dev - device that's been disconnected

Description:

Must be called when a user of a device is finished with it. When the last user of the device calls this function, the memory of the device is freed.

  • usb_get_intf - increments the reference count of the usb interface structure

Synopsis:

struct usb_interface * usb_get_intf ( struct usb_interface * intf )

Arguments:

  • intf - the interface being referenced

Description:

Each live reference to a interface must be refcounted.

Drivers for USB interfaces should normally record such references in their probe methods, when they bind to an interface, and release them by calling usb_put_intf, in their disconnect methods.

A pointer to the interface with the incremented reference counter is returned.

  • usb_put_intf - release a use of the usb interface structure

Synopsis:

void usb_put_intf ( struct usb_interface * intf )

Arguments:

  • intf - interface that's been decremented

Description:

Must be called when a user of an interface is finished with it. When the last user of the interface calls this function, the memory of the interface is freed.

  • usb_lock_device_for_reset - cautiously acquire the lock for a usb device structure

Synopsis:

int usb_lock_device_for_reset ( struct usb_device * udev )

Arguments:

  • udev - device that's being locked
  • iface - interface bound to the driver making the request (optional)

Description:

Attempts to acquire the device lock, but fails if the device is NOTATTACHED or SUSPENDED, or if iface is specified and the interface is neither BINDING nor BOUND. Rather than sleeping to wait for the lock, the routine polls repeatedly. This is to prevent deadlock with disconnect; in some drivers (such as usb-storage) the disconnect or suspend method will block waiting for a device reset to complete.

Returns a negative error code for failure, otherwise 0.

  • usb_get_current_frame_number - return current bus frame number

Synopsis:

int usb_get_current_frame_number ( struct usb_device * dev )

Arguments:

  • dev - the device whose bus is being queried

Description:

Returns the current frame number for the USB host controller used with the given USB device. This can be used when scheduling isochronous requests.

Note that different kinds of host controller have different scheduling horizons. While one type might support scheduling only 32 frames into the future, others could support scheduling up to 1024 frames into the future.

  • usb_buffer_alloc - allocate dma-consistent buffer for URB_NO_xxx_DMA_MAP

Synopsis:

void * usb_buffer_alloc ( struct usb_device * dev )

Arguments:

  • dev - device the buffer will be used with
  • size - requested buffer size
  • mem_flags - affect whether allocation may block
  • dma - used to return DMA address of buffer

Description:

Return value is either null (indicating no buffer could be allocated), or the cpu-space pointer to a buffer that may be used to perform DMA to the specified device. Such cpu-space buffers are returned along with the DMA address (through the pointer provided).

These buffers are used with URB_NO_xxx_DMA_MAP set in urb→transfer_flags to avoid behaviors like using DMA bounce buffers, or thrashing IOMMU hardware during URB completion/resubmit. The implementation varies between platforms, depending on details of how DMA will work to this device. Using these buffers also eliminates cacheline sharing problems on architectures where CPU caches are not DMA-coherent. On systems without bus-snooping caches, these buffers are uncached.

When the buffer is no longer used, free it with usb_buffer_free.

  • usb_buffer_free - free memory allocated with usb_buffer_alloc

Synopsis:

void usb_buffer_free ( struct usb_device * dev )

Arguments:

  • dev - device the buffer was used with
  • size - requested buffer size
  • addr - CPU address of buffer
  • dma - DMA address of buffer

Description:

This reclaims an I/O buffer, letting it be reused. The memory must have been allocated using usb_buffer_alloc, and the parameters must match those provided in that allocation request.

  • usb_buffer_map - create DMA mapping(s) for an urb

Synopsis:

struct urb * usb_buffer_map ( struct urb * urb )

Arguments:

  • urb - urb whose transfer_buffer/setup_packet will be mapped

Description:

Return value is either null (indicating no buffer could be mapped), or the parameter. URB_NO_TRANSFER_DMA_MAP and URB_NO_SETUP_DMA_MAP are added to urb→transfer_flags if the operation succeeds. If the device is connected to this system through a non-DMA controller, this operation always succeeds.

This call would normally be used for an urb which is reused, perhaps as the target of a large periodic transfer, with usb_buffer_dmasync calls to synchronize memory and dma state.

Reverse the effect of this call with usb_buffer_unmap.

  • usb_buffer_dmasync - synchronize DMA and CPU view of buffer(s)

Synopsis:

void usb_buffer_dmasync ( struct urb * urb )

Arguments:

  • urb - urb whose transfer_buffer/setup_packet will be synchronized
  • usb_buffer_unmap - free DMA mapping(s) for an urb

Synopsis:

void usb_buffer_unmap ( struct urb * urb )

Arguments:

  • urb - urb whose transfer_buffer will be unmapped

Description:

Reverses the effect of usb_buffer_map.

  • usb_buffer_map_sg - create scatterlist DMA mapping(s) for an endpoint

Synopsis:

int usb_buffer_map_sg ( const struct usb_device * dev )

Arguments:

  • dev - device to which the scatterlist will be mapped
  • is_in - mapping transfer direction
  • sg - the scatterlist to map
  • nents - the number of entries in the scatterlist

Description:

Return value is either < 0 (indicating no buffers could be mapped), or the number of DMA mapping array entries in the scatterlist.

The caller is responsible for placing the resulting DMA addresses from the scatterlist into URB transfer buffer pointers, and for setting the URB_NO_TRANSFER_DMA_MAP transfer flag in each of those URBs.

Top I/O rates come from queuing URBs, instead of waiting for each one to complete before starting the next I/O. This is particularly easy to do with scatterlists. Just allocate and submit one URB for each DMA mapping entry returned, stopping on the first error or when all succeed. Better yet, use the usb_sg_*() calls, which do that (and more) for you.

This call would normally be used when translating scatterlist requests, rather than usb_buffer_map, since on some hardware (with IOMMUs) it may be able to coalesce mappings for improved I/O efficiency.

Reverse the effect of this call with usb_buffer_unmap_sg.

  • usb_buffer_dmasync_sg - synchronize DMA and CPU view of scatterlist buffer(s)

Synopsis:

void usb_buffer_dmasync_sg ( const struct usb_device * dev )

Arguments:

  • dev - device to which the scatterlist will be mapped
  • is_in - mapping transfer direction
  • sg - the scatterlist to synchronize
  • n_hw_ents - the positive return value from usb_buffer_map_sg

Description:

Use this when you are re-using a scatterlist's data buffers for another USB request.

  • usb_buffer_unmap_sg - free DMA mapping(s) for a scatterlist

Synopsis:

void usb_buffer_unmap_sg ( const struct usb_device * dev )

Arguments:

  • dev - device to which the scatterlist will be mapped
  • is_in - mapping transfer direction
  • sg - the scatterlist to unmap
  • n_hw_ents - the positive return value from usb_buffer_map_sg

Description:

Reverses the effect of usb_buffer_map_sg.

  • usb_hub_clear_tt_buffer - clear control/bulk TT state in high speed hub

Synopsis:

int usb_hub_clear_tt_buffer ( struct urb * urb )

Arguments:

  • urb - an URB associated with the failed or incomplete split transaction

Description:

High speed HCDs use this to tell the hub driver that some split control or bulk transaction failed in a way that requires clearing internal state of a transaction translator. This is normally detected (and reported) from interrupt context.

It may not be possible for that hub to handle additional full (or low) speed transactions until that state is fully cleared out.

  • usb_set_device_state - change a device's current state (usbcore, hcds)

Synopsis:

void usb_set_device_state ( struct usb_device * udev )

Arguments:

  • udev - pointer to device whose state should be changed
  • new_state - new state value to be stored

Description:

udev→state is _not_ fully protected by the device lock. Although most transitions are made only while holding the lock, the state can can change to USB_STATE_NOTATTACHED at almost any time. This is so that devices can be marked as disconnected as soon as possible, without having to wait for any semaphores to be released. As a result, all changes to any device's state must be protected by the device_state_lock spinlock.

Once a device has been added to the device tree, all changes to its state should be made using this routine. The state should _not_ be set directly.

If udev→state is already USB_STATE_NOTATTACHED then no change is made. Otherwise udev→state is set to new_state, and if new_state is USB_STATE_NOTATTACHED then all of udev's descendants' states are also set to USB_STATE_NOTATTACHED.

  • usb_root_hub_lost_power - called by HCD if the root hub lost Vbus power

Synopsis:

void usb_root_hub_lost_power ( struct usb_device * rhdev )

Arguments:

  • rhdev - struct usb_device for the root hub

Description:

The USB host controller driver calls this function when its root hub is resumed and Vbus power has been interrupted or the controller has been reset. The routine marks rhdev as having lost power. When the hub driver is resumed it will take notice and carry out power-session recovery for all the USB-PERSIST-enabled child devices; the others will be disconnected.

  • usb_reset_device - warn interface drivers and perform a USB port reset

Synopsis:

int usb_reset_device ( struct usb_device * udev )

Arguments:

  • udev - device to reset (not in SUSPENDED or NOTATTACHED state)

Description:

Warns all drivers bound to registered interfaces (using their pre_reset method), performs the port reset, and then lets the drivers know that the reset is over (using their post_reset method).

Return value is the same as for usb_reset_and_verify_device.

The caller must own the device lock. For example, it's safe to use this from a driver probe routine after downloading new firmware. For calls that might not occur during probe, drivers should lock the device using usb_lock_device_for_reset.

If an interface is currently being probed or disconnected, we assume its driver knows how to handle resets. For all other interfaces, if the driver doesn't have pre_reset and post_reset methods then we attempt to unbind it and rebind afterward.

  • usb_queue_reset_device - Reset a USB device from an atomic context

Synopsis:

void usb_queue_reset_device ( struct usb_interface * iface )

Arguments:

  • iface - USB interface belonging to the device to reset

Description:

This function can be used to reset a USB device from an atomic context, where usb_reset_device won't work (as it blocks).

Doing a reset via this method is functionally equivalent to calling usb_reset_device, except for the fact that it is delayed to a workqueue. This means that any drivers bound to other interfaces might be unbound, as well as users from usbfs in user space.

Corner cases:

- Scheduling two resets at the same time from two different drivers attached to two different interfaces of the same device is possible; depending on how the driver attached to each interface handles →pre_reset, the second reset might happen or not.

- If a driver is unbound and it had a pending reset, the reset will be cancelled.

- This function can be called during .probe or .disconnect times. On return from .disconnect, any pending resets will be cancelled.

There is no no need to lock/unlock the reset_ws as schedule_work does its own.

NOTE:

We don't do any reference count tracking because it is not needed. The lifecycle of the work_struct is tied to the usb_interface. Before destroying the interface we cancel the work_struct, so the fact that work_struct is queued and or running means the interface (and thus, the device) exist and are referenced.

Host Controller APIs

These APIs are only for use by host controller drivers, most of which implement standard register interfaces such as EHCI, OHCI, or UHCI. UHCI was one of the first interfaces, designed by Intel and also used by VIA; it doesn't do much in hardware. OHCI was designed later, to have the hardware do more work (bigger transfers, tracking protocol state, and so on). EHCI was designed with USB 2.0; its design has features that resemble OHCI (hardware does much more work) as well as UHCI (some parts of ISO support, TD list processing).

There are host controllers other than the “big three”, although most PCI based controllers (and a few non-PCI based ones) use one of those interfaces. Not all host controllers use DMA; some use PIO, and there is also a simulator.

The same basic APIs are available to drivers for all those controllers. For historical reasons they are in two layers: struct usb_bus is a rather thin layer that became available in the 2.2 kernels, while struct usb_hcd is a more featureful layer (available in later 2.4 kernels and in 2.5) that lets HCDs share common code, to shrink driver size and significantly reduce hcd-specific behaviors.

  • usb_calc_bus_time - approximate periodic transaction time in nanoseconds

Synopsis:

long usb_calc_bus_time ( int speed )

Arguments:

  • speed - from dev→speed; USB_SPEED_{LOW,FULL,HIGH}
  • is_input - true iff the transaction sends data to the host
  • isoc - true for isochronous transactions, false for interrupt ones
  • bytecount - how many bytes in the transaction.

Description:

Returns approximate bus time in nanoseconds for a periodic transaction. See USB 2.0 spec section 5.11.3; only periodic transfers need to be scheduled in software, this function is only used for such scheduling.

  • usb_hcd_link_urb_to_ep - add an URB to its endpoint queue

Synopsis:

int usb_hcd_link_urb_to_ep ( struct usb_hcd * hcd )

Arguments:

  • hcd - host controller to which urb was submitted
  • urb - URB being submitted

Description:

Host controller drivers should call this routine in their enqueue method. The HCD's private spinlock must be held and interrupts must be disabled. The actions carried out here are required for URB submission, as well as for endpoint shutdown and for usb_kill_urb.

Returns 0 for no error, otherwise a negative error code (in which case the enqueue method must fail). If no error occurs but enqueue fails anyway, it must call usb_hcd_unlink_urb_from_ep before releasing the private spinlock and returning.

  • usb_hcd_check_unlink_urb - check whether an URB may be unlinked

Synopsis:

int usb_hcd_check_unlink_urb ( struct usb_hcd * hcd )

Arguments:

  • hcd - host controller to which urb was submitted
  • urb - URB being checked for unlinkability
  • status - error code to store in urb if the unlink succeeds

Description:

Host controller drivers should call this routine in their dequeue method. The HCD's private spinlock must be held and interrupts must be disabled. The actions carried out here are required for making sure than an unlink is valid.

Returns 0 for no error, otherwise a negative error code (in which case the dequeue method must fail). The possible error codes are:

-EIDRM: urb was not submitted or has already completed. The completion function may not have been called yet.

-EBUSY: urb has already been unlinked.

  • usb_hcd_unlink_urb_from_ep - remove an URB from its endpoint queue

Synopsis:

void usb_hcd_unlink_urb_from_ep ( struct usb_hcd * hcd )

Arguments:

  • hcd - host controller to which urb was submitted
  • urb - URB being unlinked

Description:

Host controller drivers should call this routine before calling usb_hcd_giveback_urb. The HCD's private spinlock must be held and interrupts must be disabled. The actions carried out here are required for URB completion.

  • usb_hcd_giveback_urb - return URB from HCD to device driver

Synopsis:

void usb_hcd_giveback_urb ( struct usb_hcd * hcd )

Arguments:

  • hcd - host controller returning the URB
  • urb - urb being returned to the USB device driver.
  • status - completion status code for the URB.

Context:

in_interrupt

Description:

This hands the URB from HCD to its USB device driver, using its completion function. The HCD has freed all per-urb resources (and is done using urb→hcpriv). It also released all HCD locks; the device driver won't cause problems if it frees, modifies, or resubmits this URB.

If urb was unlinked, the value of status will be overridden by urb→unlinked. Erroneous short transfers are detected in case the HCD hasn't checked for them.

  • usb_hcd_resume_root_hub - called by HCD to resume its root hub

Synopsis:

void usb_hcd_resume_root_hub ( struct usb_hcd * hcd )

Arguments:

  • hcd - host controller for this root hub

Description:

The USB host controller calls this function when its root hub is suspended (with the remote wakeup feature enabled) and a remote wakeup request is received. The routine submits a workqueue request to resume the root hub (that is, manage its downstream ports again).

  • usb_bus_start_enum - start immediate enumeration (for OTG)

Synopsis:

int usb_bus_start_enum ( struct usb_bus * bus )

Arguments:

  • bus - the bus (must use hcd framework)
  • port_num - 1-based number of port; usually bus→otg_port

Context:

in_interrupt

Description:

Starts enumeration, with an immediate reset followed later by khubd identifying and possibly configuring the device. This is needed by OTG controller drivers, where it helps meet HNP protocol timing requirements for starting a port reset.

  • usb_hc_died - report abnormal shutdown of a host controller (bus glue)

Synopsis:

void usb_hc_died ( struct usb_hcd * hcd )

Arguments:

  • hcd - pointer to the HCD representing the controller

Description:

This is called by bus glue to report a USB host controller that died while operations may still have been pending. It's called automatically by the PCI glue, so only glue for non-PCI busses should need to call it.

  • usb_create_hcd - create and initialize an HCD structure

Synopsis:

struct usb_hcd * usb_create_hcd ( const struct hc_driver * driver )

Arguments:

  • driver - HC driver that will use this hcd
  • dev - device for this HC, stored in hcd→self.controller
  • bus_name - value to store in hcd→self.bus_name

Context:

!in_interrupt

Description:

Allocate a struct usb_hcd, with extra space at the end for the HC driver's private data. Initialize the generic members of the hcd structure.

If memory is unavailable, returns NULL.

  • usb_add_hcd - finish generic HCD structure initialization and register

Synopsis:

int usb_add_hcd ( struct usb_hcd * hcd )

Arguments:

  • hcd - the usb_hcd structure to initialize
  • irqnum - Interrupt line to allocate
  • irqflags - Interrupt type flags

Finish the remaining parts of generic HCD initialization:

allocate the buffers of consistent memory, register the bus, request the IRQ line, and call the driver's reset and start routines.

  • usb_remove_hcd - shutdown processing for generic HCDs

Synopsis:

void usb_remove_hcd ( struct usb_hcd * hcd )

Arguments:

  • hcd - the usb_hcd structure to remove

Context:

!in_interrupt

Description:

Disconnects the root hub, then reverses the effects of usb_add_hcd, invoking the HCD's stop method.

  • usb_hcd_pci_probe - initialize PCI-based HCDs

Synopsis:

int usb_hcd_pci_probe ( struct pci_dev * dev )

Arguments:

  • dev - USB Host Controller being probed
  • id - pci hotplug id connecting controller to HCD framework

Context:

!in_interrupt

Description:

Allocates basic PCI resources for this USB host controller, and then invokes the start method for the HCD associated with it through the hotplug entry's driver_data.

Store this function in the HCD's struct pci_driver as probe.

  • usb_hcd_pci_remove - shutdown processing for PCI-based HCDs

Synopsis:

void usb_hcd_pci_remove ( struct pci_dev * dev )

Arguments:

  • dev - USB Host Controller being removed

Context:

!in_interrupt

Description:

Reverses the effect of usb_hcd_pci_probe, first invoking the HCD's stop method. It is always called from a thread context, normally rmmod, apmd, or something similar.

Store this function in the HCD's struct pci_driver as remove.

  • usb_hcd_pci_shutdown - shutdown host controller

Synopsis:

void usb_hcd_pci_shutdown ( struct pci_dev * dev )

Arguments:

  • dev - USB Host Controller being shutdown
  • hcd_buffer_create - initialize buffer pools

Synopsis:

int hcd_buffer_create ( struct usb_hcd * hcd )

Arguments:

  • hcd - the bus whose buffer pools are to be initialized

Context:

!in_interrupt

Description:

Call this as part of initializing a host controller that uses the dma memory allocators. It initializes some pools of dma-coherent memory that will be shared by all drivers using that controller, or returns a negative errno value on error.

Call hcd_buffer_destroy to clean up after using those pools.

  • hcd_buffer_destroy - deallocate buffer pools

Synopsis:

void hcd_buffer_destroy ( struct usb_hcd * hcd )

Arguments:

  • hcd - the bus whose buffer pools are to be destroyed

Context:

!in_interrupt

Description:

This frees the buffer pools created by hcd_buffer_create.

The USB Filesystem (usbfs)

libusbjUSBThis chapter presents the Linux usbfs. You may prefer to avoid writing new kernel code for your USB driver; that's the problem that usbfs set out to solve. User mode device drivers are usually packaged as applications or libraries, and may use usbfs through some programming library that wraps it. Such libraries include libusb for C/C++, and jUSB for Java.

Configure usbfs into Linux kernels by enabling the USB filesystem option (CONFIG_USB_DEVICEFS), and you get basic support for user mode USB device drivers. Until relatively recently it was often (confusingly) called usbdevfs although it wasn't solving what devfs was. Every USB device will appear in usbfs, regardless of whether or not it has a kernel driver.

What files are in "usbfs"?

Conventionally mounted at /proc/bus/usb, usbfs features include: /proc/bus/usb/devices … a text file showing each of the USB devices on known to the kernel, and their configuration descriptors. You can also poll() this to learn about new devices. /proc/bus/usb/BBB/DDD … magic files exposing the each device's configuration descriptors, and supporting a series of ioctls for making device requests, including I/O to devices. (Purely for access by programs.)

Each bus is given a number (BBB) based on when it was enumerated; within each bus, each device is given a similar number (DDD). Those BBB/DDD paths are not “stable” identifiers; expect them to change even if you always leave the devices plugged in to the same hub port. Don't even think of saving these in application configuration files. Stable identifiers are available, for user mode applications that want to use them. HID and networking devices expose these stable IDs, so that for example you can be sure that you told the right UPS to power down its second server. “usbfs” doesn't (yet) expose those IDs.

Mounting and Access Control

There are a number of mount options for usbfs, which will be of most interest to you if you need to override the default access control policy. That policy is that only root may read or write device files (/proc/bus/BBB/DDD) although anyone may read the devices or drivers files. I/O requests to the device also need the CAP_SYS_RAWIO capability,

The significance of that is that by default, all user mode device drivers need super-user privileges. You can change modes or ownership in a driver setup when the device hotplugs, or maye just start the driver right then, as a privileged server (or some activity within one). That's the most secure approach for multi-user systems, but for single user systems (“trusted” by that user) it's more convenient just to grant everyone all access (using the devmode=0666 option) so the driver can start whenever it's needed.

The mount options for usbfs, usable in /etc/fstab or in command line invocations of mount, are: busgid=NNNNNControls the GID used for the /proc/bus/usb/BBB directories. (Default: 0)busmode=MMMControls the file mode used for the /proc/bus/usb/BBB directories. (Default: 0555) busuid=NNNNNControls the UID used for the /proc/bus/usb/BBB directories. (Default: 0)devgid=NNNNNControls the GID used for the /proc/bus/usb/BBB/DDD files. (Default: 0)devmode=MMMControls the file mode used for the /proc/bus/usb/BBB/DDD files. (Default: 0644)devuid=NNNNNControls the UID used for the /proc/bus/usb/BBB/DDD files. (Default: 0)listgid=NNNNNControls the GID used for the /proc/bus/usb/devices and drivers files. (Default: 0)listmode=MMMControls the file mode used for the /proc/bus/usb/devices and drivers files. (Default: 0444)listuid=NNNNNControls the UID used for the /proc/bus/usb/devices and drivers files. (Default: 0)

Note that many Linux distributions hard-wire the mount options for usbfs in their init scripts, such as /etc/rc.d/rc.sysinit, rather than making it easy to set this per-system policy in /etc/fstab.

/proc/bus/usb/devices

This file is handy for status viewing tools in user mode, which can scan the text format and ignore most of it. More detailed device status (including class and vendor status) is available from device-specific files. For information about the current format of this file, see the Documentation/usb/proc_usb_info.txt file in your Linux kernel sources.

This file, in combination with the poll() system call, can also be used to detect when devices are added or removed: int fd; struct pollfd pfd; fd = open(”/proc/bus/usb/devices”, O_RDONLY); pfd = { fd, POLLIN, 0 }; for (;;) { /* The first time through, this call will return immediately. */ poll(&pfd, 1, -1); /* To see what's changed, compare the file's previous and current contents or scan the filesystem. (Scanning is more precise.) */ } Note that this behavior is intended to be used for informational and debug purposes. It would be more appropriate to use programs such as udev or HAL to initialize a device or start a user-mode helper program, for instance.

/proc/bus/usb/BBB/DDD

Use these files in one of these basic ways:

They can be read, producing first the device descriptor (18 bytes) and then the descriptors for the current configuration. See the USB 2.0 spec for details about those binary data formats. You'll need to convert most multibyte values from little endian format to your native host byte order, although a few of the fields in the device descriptor (both of the BCD-encoded fields, and the vendor and product IDs) will be byteswapped for you. Note that configuration descriptors include descriptors for interfaces, altsettings, endpoints, and maybe additional class descriptors.

Perform USB operations using ioctl() requests to make endpoint I/O requests (synchronously or asynchronously) or manage the device. These requests need the CAP_SYS_RAWIO capability, as well as filesystem access permissions. Only one ioctl request can be made on one of these device files at a time. This means that if you are synchronously reading an endpoint from one thread, you won't be able to write to a different endpoint from another thread until the read completes. This works for half duplex protocols, but otherwise you'd use asynchronous i/o requests.

Life Cycle of User Mode Drivers

Such a driver first needs to find a device file for a device it knows how to handle. Maybe it was told about it because a /sbin/hotplug event handling agent chose that driver to handle the new device. Or maybe it's an application that scans all the /proc/bus/usb device files, and ignores most devices. In either case, it should read() all the descriptors from the device file, and check them against what it knows how to handle. It might just reject everything except a particular vendor and product ID, or need a more complex policy.

Never assume there will only be one such device on the system at a time! If your code can't handle more than one device at a time, at least detect when there's more than one, and have your users choose which device to use.

Once your user mode driver knows what device to use, it interacts with it in either of two styles. The simple style is to make only control requests; some devices don't need more complex interactions than those. (An example might be software using vendor-specific control requests for some initialization or configuration tasks, with a kernel driver for the rest.)

More likely, you need a more complex style driver: one using non-control endpoints, reading or writing data and claiming exclusive use of an interface. Bulk transfers are easiest to use, but only their sibling interrupt transfers work with low speed devices. Both interrupt and isochronous transfers offer service guarantees because their bandwidth is reserved. Such “periodic” transfers are awkward to use through usbfs, unless you're using the asynchronous calls. However, interrupt transfers can also be used in a synchronous “one shot” style.

Your user-mode driver should never need to worry about cleaning up request state when the device is disconnected, although it should close its open file descriptors as soon as it starts seeing the ENODEV errors.

The ioctl() Requests

To use these ioctls, you need to include the following headers in your userspace program: #include <linux/usb.h> #include <linux/usbdevice_fs.h> #include <asm/byteorder.h> The standard USB device model requests, from “Chapter 9” of the USB 2.0 specification, are automatically included from the <linux/usb/ch9.h> header.

Unless noted otherwise, the ioctl requests described here will update the modification time on the usbfs file to which they are applied (unless they fail). A return of zero indicates success; otherwise, a standard USB error code is returned. (These are documented in Documentation/usb/error-codes.txt in your kernel sources.)

Each of these files multiplexes access to several I/O streams, one per endpoint. Each device has one control endpoint (endpoint zero) which supports a limited RPC style RPC access. Devices are configured by khubd (in the kernel) setting a device-wide configuration that affects things like power consumption and basic functionality. The endpoints are part of USB interfaces, which may have altsettings affecting things like which endpoints are available. Many devices only have a single configuration and interface, so drivers for them will ignore configurations and altsettings.

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