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Reed-Solomon Library Programming Interface

Introduction

The generic Reed-Solomon Library provides encoding, decoding and error correction functions.

Reed-Solomon codes are used in communication and storage applications to ensure data integrity.

This documentation is provided for developers who want to utilize the functions provided by the library.

Known Bugs And Assumptions

None.

Usage

This chapter provides examples of how to use the library.

Initializing

The init function init_rs returns a pointer to an rs decoder structure, which holds the necessary information for encoding, decoding and error correction with the given polynomial. It either uses an existing matching decoder or creates a new one. On creation all the lookup tables for fast en/decoding are created. The function may take a while, so make sure not to call it in critical code paths.

/* the Reed Solomon control structure */
static struct rs_control *rs_decoder;

/* Symbolsize is 10 (bits)
 * Primitive polynomial is x^10+x^3+1
 * first consecutive root is 0
 * primitive element to generate roots = 1
 * generator polynomial degree (number of roots) = 6
 */
rs_decoder = init_rs (10, 0x409, 0, 1, 6);
		

Encoding

The encoder calculates the Reed-Solomon code over the given data length and stores the result in the parity buffer. Note that the parity buffer must be initialized before calling the encoder.

The expanded data can be inverted on the fly by providing a non-zero inversion mask. The expanded data is XOR'ed with the mask. This is used e.g. for FLASH ECC, where the all 0xFF is inverted to an all 0x00. The Reed-Solomon code for all 0x00 is all 0x00. The code is inverted before storing to FLASH so it is 0xFF too. This prevents that reading from an erased FLASH results in ECC errors.

The databytes are expanded to the given symbol size on the fly. There is no support for encoding continuous bitstreams with a symbol size != 8 at the moment. If it is necessary it should be not a big deal to implement such functionality.

/* Parity buffer. Size = number of roots */
uint16_t par[6];
/* Initialize the parity buffer */
memset(par, 0, sizeof(par));
/* Encode 512 byte in data8. Store parity in buffer par */
encode_rs8 (rs_decoder, data8, 512, par, 0);
		

Decoding

The decoder calculates the syndrome over the given data length and the received parity symbols and corrects errors in the data.

If a syndrome is available from a hardware decoder then the syndrome calculation is skipped.

The correction of the data buffer can be suppressed by providing a correction pattern buffer and an error location buffer to the decoder. The decoder stores the calculated error location and the correction bitmask in the given buffers. This is useful for hardware decoders which use a weird bit ordering scheme.

The databytes are expanded to the given symbol size on the fly. There is no support for decoding continuous bitstreams with a symbolsize != 8 at the moment. If it is necessary it should be not a big deal to implement such functionality.

Cleanup

The function free_rs frees the allocated resources, if the caller is the last user of the decoder.

/* Release resources */
free_rs(rs_decoder);
		

Structures

This chapter contains the autogenerated documentation of the structures which are used in the Reed-Solomon Library and are relevant for a developer.

  • struct rs_control - rs control structure

Synopsis:

struct rs_control {
  int mm;
  int nn;
  uint16_t * alpha_to;
  uint16_t * index_of;
  uint16_t * genpoly;
  int nroots;
  int fcr;
  int prim;
  int iprim;
  int gfpoly;
  int (* gffunc) (int);
  int users;
  struct list_head list;
};  

Members:

  • mm - Bits per symbol
  • nn - Symbols per block (= (1«mm)-1)
  • alpha_to - log lookup table
  • index_of - Antilog lookup table
  • genpoly - Generator polynomial
  • nroots - Number of generator roots = number of parity symbols
  • fcr - First consecutive root, index form
  • prim - Primitive element, index form
  • iprim - prim-th root of 1, index form
  • gfpoly - The primitive generator polynominal
  • gffunc - Function to generate the field, if non-canonical representation
  • users - Users of this structure
  • list - List entry for the rs control list

Public Functions Provided

This chapter contains the autogenerated documentation of the Reed-Solomon functions which are exported.

  • free_rs - Free the rs control structure, if it is no longer used

Synopsis:

void free_rs ( struct rs_control * rs )

Arguments:

  • rs - the control structure which is not longer used by the caller
  • init_rs - Find a matching or allocate a new rs control structure

Synopsis:

struct rs_control * init_rs ( int symsize )

Arguments:

  • symsize - the symbol size (number of bits)
  • gfpoly - the extended Galois field generator polynomial coefficients, with the 0th coefficient in the low order bit. The polynomial must be primitive;
  • fcr - the first consecutive root of the rs code generator polynomial in index form
  • prim - primitive element to generate polynomial roots
  • nroots - RS code generator polynomial degree (number of roots)
  • init_rs_non_canonical - Find a matching or allocate a new rs control structure, for fields with non-canonical representation

Synopsis:

struct rs_control * init_rs_non_canonical ( int symsize )

Arguments:

  • symsize - the symbol size (number of bits)
  • gffunc - pointer to function to generate the next field element, or the multiplicative identity element if given 0. Used instead of gfpoly if gfpoly is 0
  • fcr - the first consecutive root of the rs code generator polynomial in index form
  • prim - primitive element to generate polynomial roots
  • nroots - RS code generator polynomial degree (number of roots)
  • encode_rs8 - Calculate the parity for data values (8bit data width)

Synopsis:

int encode_rs8 ( struct rs_control * rs )

Arguments:

  • rs - the rs control structure
  • data - data field of a given type
  • len - data length
  • par - parity data, must be initialized by caller (usually all 0)
  • invmsk - invert data mask (will be xored on data)

Description:

The parity uses a uint16_t data type to enable symbol size > 8. The calling code must take care of encoding of the syndrome result for storage itself.

  • decode_rs8 - Decode codeword (8bit data width)

Synopsis:

int decode_rs8 ( struct rs_control * rs )

Arguments:

  • rs - the rs control structure
  • data - data field of a given type
  • par - received parity data field
  • len - data length
  • s - syndrome data field (if NULL, syndrome is calculated)
  • no_eras - number of erasures
  • eras_pos - position of erasures, can be NULL
  • invmsk - invert data mask (will be xored on data, not on parity!)
  • corr - buffer to store correction bitmask on eras_pos

Description:

The syndrome and parity uses a uint16_t data type to enable symbol size > 8. The calling code must take care of decoding of the syndrome result and the received parity before calling this code. Returns the number of corrected bits or -EBADMSG for uncorrectable errors.

  • encode_rs16 - Calculate the parity for data values (16bit data width)

Synopsis:

int encode_rs16 ( struct rs_control * rs )

Arguments:

  • rs - the rs control structure
  • data - data field of a given type
  • len - data length
  • par - parity data, must be initialized by caller (usually all 0)
  • invmsk - invert data mask (will be xored on data, not on parity!)

Description:

Each field in the data array contains up to symbol size bits of valid data.

  • decode_rs16 - Decode codeword (16bit data width)

Synopsis:

int decode_rs16 ( struct rs_control * rs )

Arguments:

  • rs - the rs control structure
  • data - data field of a given type
  • par - received parity data field
  • len - data length
  • s - syndrome data field (if NULL, syndrome is calculated)
  • no_eras - number of erasures
  • eras_pos - position of erasures, can be NULL
  • invmsk - invert data mask (will be xored on data, not on parity!)
  • corr - buffer to store correction bitmask on eras_pos

Description:

Each field in the data array contains up to symbol size bits of valid data. Returns the number of corrected bits or -EBADMSG for uncorrectable errors.

Credits

The library code for encoding and decoding was written by Phil Karn.

		Copyright 2002, Phil Karn, KA9Q
 		May be used under the terms of the GNU General Public License (GPL)
	

The wrapper functions and interfaces are written by Thomas Gleixner.

Many users have provided bugfixes, improvements and helping hands for testing. Thanks a lot.

The following people have contributed to this document:

Thomas Gleixnertglx@linutronix.de

About This Book

Authors

  • Thomas Gleixner

Copyright 2004

  • Thomas Gleixner

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