reed_solomon.c

/* 
 * lib/reed_solomon/rslib.c
 *
 * Overview:
 *   Generic Reed Solomon encoder / decoder library
 *   
 * Copyright (C) 2004 Thomas Gleixner (tglx@linutronix.de)
 *
 * Reed Solomon code lifted from reed solomon library written by Phil Karn
 * Copyright 2002 Phil Karn, KA9Q
 *
 * $Id: rslib.c,v 1.5 2004/10/22 15:41:47 gleixner Exp $
 *
 * This program 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.
 *
 * Description:
 *	
 * The generic Reed Solomon library provides runtime configurable
 * encoding / decoding of RS codes.
 * Each user must call init_rs to get a pointer to a rs_control
 * structure for the given rs parameters. This structure is either
 * generated or a already available matching control structure is used.
 * If a structure is generated then the polynomial arrays for
 * fast encoding / decoding are built. This can take some time so
 * make sure not to call this function from a time critical path.
 * Usually a module / driver should initialize the necessary 
 * rs_control structure on module / driver init and release it
 * on exit.
 * The encoding puts the calculated syndrome into a given syndrome 
 * buffer. 
 * The decoding is a two step process. The first step calculates
 * the syndrome over the received (data + syndrome) and calls the
 * second stage, which does the decoding / error correction itself.
 * Many hw encoders provide a syndrome calculation over the received
 * data + syndrome and can call the second stage directly.
 *
 */

#include <linux/errno.h>
#include <linux/kernel.h>
#include <linux/init.h>
#include <linux/module.h>
#include <linux/rslib.h>
#include <linux/slab.h>
#include <asm/semaphore.h>

/* This list holds all currently allocated rs control structures */
static LIST_HEAD (rslist);
/* Protection for the list */
static DECLARE_MUTEX(rslistlock);

/** 
 * rs_init - Initialize a Reed-Solomon codec
 *
 * @symsize:	symbol size, bits (1-8)
 * @gfpoly:	Field generator polynomial coefficients
 * @fcr:	first root of RS code generator polynomial, index form
 * @prim:	primitive element to generate polynomial roots
 * @nroots:	RS code generator polynomial degree (number of roots)
 *
 * Allocate a control structure and the polynom arrays for faster
 * en/decoding. Fill the arrays according to the given parameters
 */
static struct rs_control *rs_init(int symsize, int gfpoly, int fcr, 
				   int prim, int nroots)
{
	struct rs_control *rs;
	int i, j, sr, root, iprim;

	/* Allocate the control structure */
	rs = kmalloc(sizeof (struct rs_control), GFP_KERNEL);
	if (rs == NULL)
		return NULL;

	INIT_LIST_HEAD(&rs->list);

	rs->mm = symsize;
	rs->nn = (1 << symsize) - 1;
	rs->fcr = fcr;
	rs->prim = prim;
	rs->nroots = nroots;
	rs->gfpoly = gfpoly;

	/* Allocate the arrays */
	rs->alpha_to = kmalloc(sizeof(uint16_t) * (rs->nn + 1), GFP_KERNEL);
	if (rs->alpha_to == NULL)
		goto errrs;

	rs->index_of = kmalloc(sizeof(uint16_t) * (rs->nn + 1), GFP_KERNEL);
	if (rs->index_of == NULL)
		goto erralp;

	rs->genpoly = kmalloc(sizeof(uint16_t) * (rs->nroots + 1), GFP_KERNEL);
	if(rs->genpoly == NULL)
		goto erridx;

	/* Generate Galois field lookup tables */
	rs->index_of[0] = rs->nn;	/* log(zero) = -inf */
	rs->alpha_to[rs->nn] = 0;	/* alpha**-inf = 0 */
	sr = 1;
	for (i = 0; i < rs->nn; i++) {
		rs->index_of[sr] = i;
		rs->alpha_to[i] = sr;
		sr <<= 1;
		if (sr & (1 << symsize))
			sr ^= gfpoly;
		sr &= rs->nn;
	}
	/* If it's not primitive, exit */
	if(sr != 1)
		goto errpol;

	/* Find prim-th root of 1, used in decoding */
	for(iprim = 1; (iprim % prim) != 0; iprim += rs->nn);
	/* prim-th root of 1, index form */
	rs->iprim = iprim / prim;

	/* Form RS code generator polynomial from its roots */
	rs->genpoly[0] = 1;
	for (i = 0, root = fcr * prim; i < nroots; i++, root += prim) {
		rs->genpoly[i + 1] = 1;
		/* Multiply rs->genpoly[] by  @**(root + x) */
		for (j = i; j > 0; j--) {
			if (rs->genpoly[j] != 0) {
				rs->genpoly[j] = rs->genpoly[j -1] ^ 
					rs->alpha_to[rs_modnn(rs, 
					rs->index_of[rs->genpoly[j]] + root)];
			} else
				rs->genpoly[j] = rs->genpoly[j - 1];
		}
		/* rs->genpoly[0] can never be zero */
		rs->genpoly[0] = 
			rs->alpha_to[rs_modnn(rs, 
				rs->index_of[rs->genpoly[0]] + root)];
	}
	/* convert rs->genpoly[] to index form for quicker encoding */
	for (i = 0; i <= nroots; i++)
		rs->genpoly[i] = rs->index_of[rs->genpoly[i]];
	return rs;

	/* Error exit */
errpol:
	kfree(rs->genpoly);
erridx:
	kfree(rs->index_of);
erralp:
	kfree(rs->alpha_to);
errrs:
	kfree(rs);
	return NULL;
}


/** 
 *  free_rs - Free the rs control structure, if its not longer used
 *
 *  @rs:	the control structure which is not longer used by the
 *		caller
 */
void free_rs(struct rs_control *rs)
{
	down(&rslistlock);
	rs->users--;
	if(!rs->users) {
		list_del(&rs->list);
		kfree(rs->alpha_to);
		kfree(rs->index_of);
		kfree(rs->genpoly);
		kfree(rs);
	}
	up(&rslistlock);
}

/** 
 * init_rs - Find a matching or allocate a new rs control structure
 *
 *  @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)
 */
struct rs_control *init_rs(int symsize, int gfpoly, int fcr, int prim, 
			   int nroots)
{
	struct list_head	*tmp;
	struct rs_control	*rs;

	/* Sanity checks */
	if (symsize < 1)
		return NULL;
	if (fcr < 0 || fcr >= (1<<symsize))
    		return NULL;
	if (prim <= 0 || prim >= (1<<symsize))
    		return NULL;
	if (nroots < 0 || nroots >= (1<<symsize) || nroots > 8)
		return NULL;
	
	down(&rslistlock);

	/* Walk through the list and look for a matching entry */
	list_for_each(tmp, &rslist) {
		rs = list_entry(tmp, struct rs_control, list);
		if (symsize != rs->mm)
			continue;
		if (gfpoly != rs->gfpoly)
			continue;
		if (fcr != rs->fcr)
			continue;	
		if (prim != rs->prim)
			continue;	
		if (nroots != rs->nroots)
			continue;
		/* We have a matching one already */
		rs->users++;
		goto out;
	}

	/* Create a new one */
	rs = rs_init(symsize, gfpoly, fcr, prim, nroots);
	if (rs) {
		rs->users = 1;
		list_add(&rs->list, &rslist);
	}
out:	
	up(&rslistlock);
	return rs;
}

#ifdef CONFIG_REED_SOLOMON_ENC8
/** 
 *  encode_rs8 - Calculate the parity for data values (8bit data width)
 *
 *  @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)
 *
 *  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.
 */
int encode_rs8(struct rs_control *rs, uint8_t *data, int len, uint16_t *par, 
	       uint16_t invmsk)
{
#include "encode_rs.c"
}
EXPORT_SYMBOL_GPL(encode_rs8);
#endif

#ifdef CONFIG_REED_SOLOMON_DEC8
/** 
 *  decode_rs8 - Decode codeword (8bit data width)
 *
 *  @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
 *
 *  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.
 */
int decode_rs8(struct rs_control *rs, uint8_t *data, uint16_t *par, int len,
	       uint16_t *s, int no_eras, int *eras_pos, uint16_t invmsk, 
	       uint16_t *corr)
{
#include "decode_rs.c"
}
EXPORT_SYMBOL_GPL(decode_rs8);
#endif

#ifdef CONFIG_REED_SOLOMON_ENC16
/**
 *  encode_rs16 - Calculate the parity for data values (16bit data width)
 *
 *  @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!)
 *
 *  Each field in the data array contains up to symbol size bits of valid data.
 */
int encode_rs16(struct rs_control *rs, uint16_t *data, int len, uint16_t *par, 
	uint16_t invmsk)
{
#include "encode_rs.c"
}
EXPORT_SYMBOL_GPL(encode_rs16);
#endif

#ifdef CONFIG_REED_SOLOMON_DEC16
/** 
 *  decode_rs16 - Decode codeword (16bit data width)
 *
 *  @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
 *
 *  Each field in the data array contains up to symbol size bits of valid data.
 */
int decode_rs16(struct rs_control *rs, uint16_t *data, uint16_t *par, int len,
		uint16_t *s, int no_eras, int *eras_pos, uint16_t invmsk, 
		uint16_t *corr)
{
#include "decode_rs.c"
}
EXPORT_SYMBOL_GPL(decode_rs16);
#endif

EXPORT_SYMBOL_GPL(init_rs);
EXPORT_SYMBOL_GPL(free_rs);

MODULE_LICENSE("GPL");
MODULE_DESCRIPTION("Reed Solomon encoder/decoder");
MODULE_AUTHOR("Phil Karn, Thomas Gleixner");