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Linux generic IRQ handling

Introduction

The generic interrupt handling layer is designed to provide a complete abstraction of interrupt handling for device drivers. It is able to handle all the different types of interrupt controller hardware. Device drivers use generic API functions to request, enable, disable and free interrupts. The drivers do not have to know anything about interrupt hardware details, so they can be used on different platforms without code changes.

This documentation is provided to developers who want to implement an interrupt subsystem based for their architecture, with the help of the generic IRQ handling layer.

Rationale

The original implementation of interrupt handling in Linux is using the __do_IRQ() super-handler, which is able to deal with every type of interrupt logic.

Originally, Russell King identified different types of handlers to build a quite universal set for the ARM interrupt handler implementation in Linux 2.5/2.6. He distinguished between: Level typeEdge typeSimple type In the SMP world of the __do_IRQ() super-handler another type was identified: Per CPU type

This split implementation of highlevel IRQ handlers allows us to optimize the flow of the interrupt handling for each specific interrupt type. This reduces complexity in that particular codepath and allows the optimized handling of a given type.

The original general IRQ implementation used hw_interrupt_type structures and their →ack(), →end() [etc.] callbacks to differentiate the flow control in the super-handler. This leads to a mix of flow logic and lowlevel hardware logic, and it also leads to unnecessary code duplication: for example in i386, there is a ioapic_level_irq and a ioapic_edge_irq irq-type which share many of the lowlevel details but have different flow handling.

A more natural abstraction is the clean separation of the 'irq flow' and the 'chip details'.

Analysing a couple of architecture's IRQ subsystem implementations reveals that most of them can use a generic set of 'irq flow' methods and only need to add the chip level specific code. The separation is also valuable for (sub)architectures which need specific quirks in the irq flow itself but not in the chip-details - and thus provides a more transparent IRQ subsystem design.

Each interrupt descriptor is assigned its own highlevel flow handler, which is normally one of the generic implementations. (This highlevel flow handler implementation also makes it simple to provide demultiplexing handlers which can be found in embedded platforms on various architectures.)

The separation makes the generic interrupt handling layer more flexible and extensible. For example, an (sub)architecture can use a generic irq-flow implementation for 'level type' interrupts and add a (sub)architecture specific 'edge type' implementation.

To make the transition to the new model easier and prevent the breakage of existing implementations, the __do_IRQ() super-handler is still available. This leads to a kind of duality for the time being. Over time the new model should be used in more and more architectures, as it enables smaller and cleaner IRQ subsystems.

Known Bugs And Assumptions

None (knock on wood).

Abstraction layers

There are three main levels of abstraction in the interrupt code: Highlevel driver APIHighlevel IRQ flow handlersChiplevel hardware encapsulation

Interrupt control flow

Each interrupt is described by an interrupt descriptor structure irq_desc. The interrupt is referenced by an 'unsigned int' numeric value which selects the corresponding interrupt decription structure in the descriptor structures array. The descriptor structure contains status information and pointers to the interrupt flow method and the interrupt chip structure which are assigned to this interrupt.

Whenever an interrupt triggers, the lowlevel arch code calls into the generic interrupt code by calling desc→handle_irq(). This highlevel IRQ handling function only uses desc→chip primitives referenced by the assigned chip descriptor structure.

Highlevel Driver API

The highlevel Driver API consists of following functions: request_irq()free_irq()disable_irq()enable_irq()disable_irq_nosync() (SMP only)synchronize_irq() (SMP only)set_irq_type()set_irq_wake()set_irq_data()set_irq_chip()set_irq_chip_data() See the autogenerated function documentation for details.

Highlevel IRQ flow handlers

The generic layer provides a set of pre-defined irq-flow methods: handle_level_irqhandle_edge_irqhandle_simple_irqhandle_percpu_irq The interrupt flow handlers (either predefined or architecture specific) are assigned to specific interrupts by the architecture either during bootup or during device initialization.

Chiplevel hardware encapsulation

The chip level hardware descriptor structure irq_chip contains all the direct chip relevant functions, which can be utilized by the irq flow implementations. ack()mask_ack() - Optional, recommended for performancemask()unmask()retrigger() - Optionalset_type() - Optionalset_wake() - Optional These primitives are strictly intended to mean what they say: ack means ACK, masking means masking of an IRQ line, etc. It is up to the flow handler(s) to use these basic units of lowlevel functionality.

__do_IRQ entry point

The original implementation __do_IRQ() is an alternative entry point for all types of interrupts.

This handler turned out to be not suitable for all interrupt hardware and was therefore reimplemented with split functionality for egde/level/simple/percpu interrupts. This is not only a functional optimization. It also shortens code paths for interrupts.

To make use of the split implementation, replace the call to __do_IRQ by a call to desc→chip→handle_irq() and associate the appropriate handler function to desc→chip→handle_irq(). In most cases the generic handler implementations should be sufficient.

Locking on SMP

The locking of chip registers is up to the architecture that defines the chip primitives. There is a chip→lock field that can be used for serialization, but the generic layer does not touch it. The per-irq structure is protected via desc→lock, by the generic layer.

Structures

This chapter contains the autogenerated documentation of the structures which are used in the generic IRQ layer.

  • struct irq_chip - hardware interrupt chip descriptor

Synopsis:

struct irq_chip {
  const char * name;
  unsigned int	(* startup) (unsigned int irq);
  void (* shutdown) (unsigned int irq);
  void (* enable) (unsigned int irq);
  void (* disable) (unsigned int irq);
  void (* ack) (unsigned int irq);
  void (* mask) (unsigned int irq);
  void (* mask_ack) (unsigned int irq);
  void (* unmask) (unsigned int irq);
  void (* eoi) (unsigned int irq);
  void (* end) (unsigned int irq);
  int (* set_affinity) (unsigned int irq,const struct cpumask *dest);
  int (* retrigger) (unsigned int irq);
  int (* set_type) (unsigned int irq, unsigned int flow_type);
  int (* set_wake) (unsigned int irq, unsigned int on);
  void (* bus_lock) (unsigned int irq);
  void (* bus_sync_unlock) (unsigned int irq);
#ifdef CONFIG_IRQ_RELEASE_METHOD
  void (* release) (unsigned int irq, void *dev_id);
#endif
  const char * typename;
};  

Members:

  • name - name for /proc/interrupts
  • startup - start up the interrupt (defaults to →enable if NULL)
  • shutdown - shut down the interrupt (defaults to →disable if NULL)
  • enable - enable the interrupt (defaults to chip→unmask if NULL)
  • disable - disable the interrupt (defaults to chip→mask if NULL)
  • ack - start of a new interrupt
  • mask - mask an interrupt source
  • mask_ack - ack and mask an interrupt source
  • unmask - unmask an interrupt source
  • eoi - end of interrupt - chip level
  • end - end of interrupt - flow level
  • set_affinity - set the CPU affinity on SMP machines
  • retrigger - resend an IRQ to the CPU
  • set_type - set the flow type (IRQ_TYPE_LEVEL/etc.) of an IRQ
  • set_wake - enable/disable power-management wake-on of an IRQ
  • bus_lock - function to lock access to slow bus (i2c) chips
  • bus_sync_unlock - function to sync and unlock slow bus (i2c) chips
  • release - release function solely used by UML
  • typename - obsoleted by name, kept as migration helper
  • struct irq_desc - interrupt descriptor

Synopsis:

struct irq_desc {
  unsigned int irq;
  struct timer_rand_state * timer_rand_state;
  unsigned int * kstat_irqs;
#ifdef CONFIG_INTR_REMAP
  struct irq_2_iommu * irq_2_iommu;
#endif
  irq_flow_handler_t handle_irq;
  struct irq_chip * chip;
  struct msi_desc * msi_desc;
  void * handler_data;
  void * chip_data;
  struct irqaction * action;
  unsigned int status;
  unsigned int depth;
  unsigned int wake_depth;
  unsigned int irq_count;
  unsigned long last_unhandled;
  unsigned int irqs_unhandled;
  spinlock_t lock;
#ifdef CONFIG_SMP
  cpumask_var_t affinity;
  unsigned int node;
#ifdef CONFIG_GENERIC_PENDING_IRQ
  cpumask_var_t pending_mask;
#endif
#endif
  atomic_t threads_active;
  wait_queue_head_t wait_for_threads;
#ifdef CONFIG_PROC_FS
  struct proc_dir_entry * dir;
#endif
  const char * name;
};  

Members:

  • irq - interrupt number for this descriptor
  • timer_rand_state - pointer to timer rand state struct
  • kstat_irqs - irq stats per cpu
  • irq_2_iommu - iommu with this irq
  • handle_irq - highlevel irq-events handler [if NULL, __do_IRQ]
  • chip - low level interrupt hardware access
  • msi_desc - MSI descriptor
  • handler_data - per-IRQ data for the irq_chip methods
  • chip_data - platform-specific per-chip private data for the chip methods, to allow shared chip implementations
  • action - the irq action chain
  • status - status information
  • depth - disable-depth, for nested irq_disable calls
  • wake_depth - enable depth, for multiple set_irq_wake callers
  • irq_count - stats field to detect stalled irqs
  • last_unhandled - aging timer for unhandled count
  • irqs_unhandled - stats field for spurious unhandled interrupts
  • lock - locking for SMP
  • affinity - IRQ affinity on SMP
  • node - node index useful for balancing
  • pending_mask - pending rebalanced interrupts
  • threads_active - number of irqaction threads currently running
  • wait_for_threads - wait queue for sync_irq to wait for threaded handlers
  • dir - /proc/irq/ procfs entry
  • name - flow handler name for /proc/interrupts output
  • alloc_desc_masks - allocate cpumasks for irq_desc

Synopsis:

bool alloc_desc_masks ( struct irq_desc * desc )

Arguments:

  • desc - pointer to irq_desc struct
  • node - node which will be handling the cpumasks
  • boot - true if need bootmem

Description:

Allocates affinity and pending_mask cpumask if required. Returns true if successful (or not required).

  • init_copy_desc_masks - copy cpumasks for irq_desc

Synopsis:

void init_copy_desc_masks ( struct irq_desc * old_desc )

Arguments:

  • old_desc - pointer to old irq_desc struct
  • new_desc - pointer to new irq_desc struct

Description:

Insures affinity and pending_masks are copied to new irq_desc. If !CONFIG_CPUMASKS_OFFSTACK the cpumasks are embedded in the irq_desc struct so the copy is redundant.

  • struct irqaction - per interrupt action descriptor

Synopsis:

struct irqaction {
  irq_handler_t handler;
  unsigned long flags;
  const char * name;
  void * dev_id;
  struct irqaction * next;
  int irq;
  struct proc_dir_entry * dir;
  irq_handler_t thread_fn;
  struct task_struct * thread;
  unsigned long thread_flags;
};  

Members:

  • handler - interrupt handler function
  • flags - flags (see IRQF_* above)
  • name - name of the device
  • dev_id - cookie to identify the device
  • next - pointer to the next irqaction for shared interrupts
  • irq - interrupt number
  • dir - pointer to the proc/irq/NN/name entry
  • thread_fn - interupt handler function for threaded interrupts
  • thread - thread pointer for threaded interrupts
  • thread_flags - flags related to thread

Public Functions Provided

This chapter contains the autogenerated documentation of the kernel API functions which are exported.

  • synchronize_irq - wait for pending IRQ handlers (on other CPUs)

Synopsis:

void synchronize_irq ( unsigned int irq )

Arguments:

  • irq - interrupt number to wait for

Description:

This function waits for any pending IRQ handlers for this interrupt to complete before returning. If you use this function while holding a resource the IRQ handler may need you will deadlock.

This function may be called - with care - from IRQ context.

  • disable_irq_nosync - disable an irq without waiting

Synopsis:

void disable_irq_nosync ( unsigned int irq )

Arguments:

  • irq - Interrupt to disable

Description:

Disable the selected interrupt line. Disables and Enables are nested. Unlike disable_irq, this function does not ensure existing instances of the IRQ handler have completed before returning.

This function may be called from IRQ context.

  • disable_irq - disable an irq and wait for completion

Synopsis:

void disable_irq ( unsigned int irq )

Arguments:

  • irq - Interrupt to disable

Description:

Disable the selected interrupt line. Enables and Disables are nested. This function waits for any pending IRQ handlers for this interrupt to complete before returning. If you use this function while holding a resource the IRQ handler may need you will deadlock.

This function may be called - with care - from IRQ context.

  • enable_irq - enable handling of an irq

Synopsis:

void enable_irq ( unsigned int irq )

Arguments:

  • irq - Interrupt to enable

Description:

Undoes the effect of one call to disable_irq. If this matches the last disable, processing of interrupts on this IRQ line is re-enabled.

This function may be called from IRQ context only when desc→chip→bus_lock and desc→chip→bus_sync_unlock are NULL !

  • set_irq_wake - control irq power management wakeup

Synopsis:

int set_irq_wake ( unsigned int irq )

Arguments:

  • irq - interrupt to control
  • on - enable/disable power management wakeup

Description:

Enable/disable power management wakeup mode, which is disabled by default. Enables and disables must match, just as they match for non-wakeup mode support.

Wakeup mode lets this IRQ wake the system from sleep states like suspend to RAM.

  • setup_irq - setup an interrupt

Synopsis:

int setup_irq ( unsigned int irq )

Arguments:

  • irq - Interrupt line to setup
  • act - irqaction for the interrupt

Description:

Used to statically setup interrupts in the early boot process.

  • remove_irq - free an interrupt

Synopsis:

void remove_irq ( unsigned int irq )

Arguments:

  • irq - Interrupt line to free
  • act - irqaction for the interrupt

Description:

Used to remove interrupts statically setup by the early boot process.

  • free_irq - free an interrupt allocated with request_irq

Synopsis:

void free_irq ( unsigned int irq )

Arguments:

  • irq - Interrupt line to free
  • dev_id - Device identity to free

Description:

Remove an interrupt handler. The handler is removed and if the interrupt line is no longer in use by any driver it is disabled. On a shared IRQ the caller must ensure the interrupt is disabled on the card it drives before calling this function. The function does not return until any executing interrupts for this IRQ have completed.

This function must not be called from interrupt context.

  • request_threaded_irq - allocate an interrupt line

Synopsis:

int request_threaded_irq ( unsigned int irq )

Arguments:

  • irq - Interrupt line to allocate
  • handler - Function to be called when the IRQ occurs. Primary handler for threaded interrupts If NULL and thread_fn != NULL the default primary handler is installed
  • thread_fn - Function called from the irq handler thread If NULL, no irq thread is created
  • irqflags - Interrupt type flags
  • devname - An ascii name for the claiming device
  • dev_id - A cookie passed back to the handler function

Description:

This call allocates interrupt resources and enables the interrupt line and IRQ handling. From the point this call is made your handler function may be invoked. Since your handler function must clear any interrupt the board raises, you must take care both to initialise your hardware and to set up the interrupt handler in the right order.

If you want to set up a threaded irq handler for your device then you need to supply handler and thread_fn. handler ist still called in hard interrupt context and has to check whether the interrupt originates from the device. If yes it needs to disable the interrupt on the device and return IRQ_WAKE_THREAD which will wake up the handler thread and run thread_fn. This split handler design is necessary to support shared interrupts.

Dev_id must be globally unique. Normally the address of the device data structure is used as the cookie. Since the handler receives this value it makes sense to use it.

If your interrupt is shared you must pass a non NULL dev_id as this is required when freeing the interrupt.

Flags:

IRQF_SHARED Interrupt is shared IRQF_DISABLED Disable local interrupts while processing IRQF_SAMPLE_RANDOM The interrupt can be used for entropy IRQF_TRIGGER_* Specify active edge(s) or level

  • set_irq_chip - set the irq chip for an irq

Synopsis:

int set_irq_chip ( unsigned int irq )

Arguments:

  • irq - irq number
  • chip - pointer to irq chip description structure
  • set_irq_type - set the irq trigger type for an irq

Synopsis:

int set_irq_type ( unsigned int irq )

Arguments:

  • irq - irq number
  • type - IRQ_TYPE_{LEVEL,EDGE}_* value - see include/linux/irq.h
  • set_irq_data - set irq type data for an irq

Synopsis:

int set_irq_data ( unsigned int irq )

Arguments:

  • irq - Interrupt number
  • data - Pointer to interrupt specific data

Description:

Set the hardware irq controller data for an irq

  • set_irq_chip_data - set irq chip data for an irq

Synopsis:

int set_irq_chip_data ( unsigned int irq )

Arguments:

  • irq - Interrupt number
  • data - Pointer to chip specific data

Description:

Set the hardware irq chip data for an irq

  • set_irq_nested_thread - Set/Reset the IRQ_NESTED_THREAD flag of an irq

Synopsis:

void set_irq_nested_thread ( unsigned int irq )

Arguments:

  • irq - Interrupt number
  • nest - 0 to clear / 1 to set the IRQ_NESTED_THREAD flag

Description:

The IRQ_NESTED_THREAD flag indicates that on request_threaded_irq no separate interrupt thread should be created for the irq as the handler are called nested in the context of a demultiplexing interrupt handler thread.

  • handle_level_irq - Level type irq handler

Synopsis:

void handle_level_irq ( unsigned int irq )

Arguments:

  • irq - the interrupt number
  • desc - the interrupt description structure for this irq

Description:

Level type interrupts are active as long as the hardware line has the active level. This may require to mask the interrupt and unmask it after the associated handler has acknowledged the device, so the interrupt line is back to inactive.

Internal Functions Provided

This chapter contains the autogenerated documentation of the internal functions.

  • handle_bad_irq - handle spurious and unhandled irqs

Synopsis:

void handle_bad_irq ( unsigned int irq )

Arguments:

  • irq - the interrupt number
  • desc - description of the interrupt

Description:

Handles spurious and unhandled IRQ's. It also prints a debugmessage.

  • handle_IRQ_event - irq action chain handler

Synopsis:

irqreturn_t handle_IRQ_event ( unsigned int irq )

Arguments:

  • irq - the interrupt number
  • action - the interrupt action chain for this irq

Description:

Handles the action chain of an irq event

  • __do_IRQ - original all in one highlevel IRQ handler

Synopsis:

unsigned int __do_IRQ ( unsigned int irq )

Arguments:

  • irq - the interrupt number

Description:

__do_IRQ handles all normal device IRQ's (the special SMP cross-CPU interrupts have their own specific handlers).

This is the original x86 implementation which is used for every interrupt type.

  • dynamic_irq_init - initialize a dynamically allocated irq

Synopsis:

void dynamic_irq_init ( unsigned int irq )

Arguments:

  • irq - irq number to initialize
  • dynamic_irq_cleanup - cleanup a dynamically allocated irq

Synopsis:

void dynamic_irq_cleanup ( unsigned int irq )

Arguments:

  • irq - irq number to initialize
  • set_irq_msi - set irq type data for an irq

Synopsis:

int set_irq_msi ( unsigned int irq )

Arguments:

  • irq - Interrupt number
  • entry - Pointer to MSI descriptor data

Description:

Set the hardware irq controller data for an irq

  • handle_simple_irq - Simple and software-decoded IRQs.

Synopsis:

void handle_simple_irq ( unsigned int irq )

Arguments:

  • irq - the interrupt number
  • desc - the interrupt description structure for this irq

Description:

Simple interrupts are either sent from a demultiplexing interrupt handler or come from hardware, where no interrupt hardware control is necessary.

Note:

The caller is expected to handle the ack, clear, mask and unmask issues if necessary.

  • handle_fasteoi_irq - irq handler for transparent controllers

Synopsis:

void handle_fasteoi_irq ( unsigned int irq )

Arguments:

  • irq - the interrupt number
  • desc - the interrupt description structure for this irq

Only a single callback will be issued to the chip:

an →eoi call when the interrupt has been serviced. This enables support for modern forms of interrupt handlers, which handle the flow details in hardware, transparently.

  • handle_edge_irq - edge type IRQ handler

Synopsis:

void handle_edge_irq ( unsigned int irq )

Arguments:

  • irq - the interrupt number
  • desc - the interrupt description structure for this irq

Description:

Interrupt occures on the falling and/or rising edge of a hardware signal. The occurence is latched into the irq controller hardware and must be acked in order to be reenabled. After the ack another interrupt can happen on the same source even before the first one is handled by the assosiacted event handler. If this happens it might be necessary to disable (mask) the interrupt depending on the controller hardware. This requires to reenable the interrupt inside of the loop which handles the interrupts which have arrived while the handler was running. If all pending interrupts are handled, the loop is left.

  • handle_percpu_irq - Per CPU local irq handler

Synopsis:

void handle_percpu_irq ( unsigned int irq )

Arguments:

  • irq - the interrupt number
  • desc - the interrupt description structure for this irq

Description:

Per CPU interrupts on SMP machines without locking requirements

Credits

The following people have contributed to this document: Thomas Gleixnertglx@linutronix.deIngo Molnarmingo@elte.hu

About This Book

Authors

  • Thomas Gleixner
  • Ingo Molnar

Copyright 2005-2006

  • Thomas Gleixner

Copyright 2005-2006

  • Ingo Molnar

Legal Notice

This documentation is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License version 2 as published by the Free Software Foundation.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this program; if not, write to the Free Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA

For more details see the file COPYING in the source distribution of Linux.