yaffs_ecc.c

YAFFS (Yet Another Flash File System) is now in its second generation and provides a fast robust fil

/*
 * YAFFS: Yet another FFS. A NAND-flash specific file system. 
 *
 * yaffs_ecc.c: ECC generation/correction algorithms.
 *
 * Copyright (C) 2002 Aleph One Ltd.
 *
 * Created by Charles Manning <[e-mail]>
 *
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public License
 * version 2.1 as published by the Free Software Foundation.
 */
 
 /*
 * This code implements the ECC algorithm used in SmartMedia.
 *
 * The ECC comprises 22 bits of parity information and is stuffed into 3 bytes. 
 * The two unused bit are set to 1.
 * The ECC can correct single bit errors in a 256-byte page of data. Thus, two such ECC 
 * blocks are used on a 512-byte NAND page.
 *
 */

// Table generated by gen-ecc.c
// Using a table means we do not have to calculate p1..p4 and p1'..p4'
// for each byte of data. These are instead provided in a table in bits7..2.
// Bit 0 of each entry indicates whether the entry has an odd or even parity, and therefore
// this bytes influence on the line parity.

const char *yaffs_ecc_c_version = "$Id: yaffs_ecc.c,v 1.2 2003/05/20 23:20:53 charles Exp $";


#include "yaffs_ecc.h"

static const unsigned char column_parity_table[] = {
0x00, 0x55, 0x59, 0x0c, 0x65, 0x30, 0x3c, 0x69, 0x69, 0x3c, 0x30, 0x65, 0x0c, 0x59, 0x55, 0x00, 
0x95, 0xc0, 0xcc, 0x99, 0xf0, 0xa5, 0xa9, 0xfc, 0xfc, 0xa9, 0xa5, 0xf0, 0x99, 0xcc, 0xc0, 0x95, 
0x99, 0xcc, 0xc0, 0x95, 0xfc, 0xa9, 0xa5, 0xf0, 0xf0, 0xa5, 0xa9, 0xfc, 0x95, 0xc0, 0xcc, 0x99, 
0x0c, 0x59, 0x55, 0x00, 0x69, 0x3c, 0x30, 0x65, 0x65, 0x30, 0x3c, 0x69, 0x00, 0x55, 0x59, 0x0c, 
0xa5, 0xf0, 0xfc, 0xa9, 0xc0, 0x95, 0x99, 0xcc, 0xcc, 0x99, 0x95, 0xc0, 0xa9, 0xfc, 0xf0, 0xa5, 
0x30, 0x65, 0x69, 0x3c, 0x55, 0x00, 0x0c, 0x59, 0x59, 0x0c, 0x00, 0x55, 0x3c, 0x69, 0x65, 0x30, 
0x3c, 0x69, 0x65, 0x30, 0x59, 0x0c, 0x00, 0x55, 0x55, 0x00, 0x0c, 0x59, 0x30, 0x65, 0x69, 0x3c, 
0xa9, 0xfc, 0xf0, 0xa5, 0xcc, 0x99, 0x95, 0xc0, 0xc0, 0x95, 0x99, 0xcc, 0xa5, 0xf0, 0xfc, 0xa9, 
0xa9, 0xfc, 0xf0, 0xa5, 0xcc, 0x99, 0x95, 0xc0, 0xc0, 0x95, 0x99, 0xcc, 0xa5, 0xf0, 0xfc, 0xa9, 
0x3c, 0x69, 0x65, 0x30, 0x59, 0x0c, 0x00, 0x55, 0x55, 0x00, 0x0c, 0x59, 0x30, 0x65, 0x69, 0x3c, 
0x30, 0x65, 0x69, 0x3c, 0x55, 0x00, 0x0c, 0x59, 0x59, 0x0c, 0x00, 0x55, 0x3c, 0x69, 0x65, 0x30, 
0xa5, 0xf0, 0xfc, 0xa9, 0xc0, 0x95, 0x99, 0xcc, 0xcc, 0x99, 0x95, 0xc0, 0xa9, 0xfc, 0xf0, 0xa5, 
0x0c, 0x59, 0x55, 0x00, 0x69, 0x3c, 0x30, 0x65, 0x65, 0x30, 0x3c, 0x69, 0x00, 0x55, 0x59, 0x0c, 
0x99, 0xcc, 0xc0, 0x95, 0xfc, 0xa9, 0xa5, 0xf0, 0xf0, 0xa5, 0xa9, 0xfc, 0x95, 0xc0, 0xcc, 0x99, 
0x95, 0xc0, 0xcc, 0x99, 0xf0, 0xa5, 0xa9, 0xfc, 0xfc, 0xa9, 0xa5, 0xf0, 0x99, 0xcc, 0xc0, 0x95, 
0x00, 0x55, 0x59, 0x0c, 0x65, 0x30, 0x3c, 0x69, 0x69, 0x3c, 0x30, 0x65, 0x0c, 0x59, 0x55, 0x00, 
};


static int yaffs_CountBits(unsigned char x)
{
	int r = 0;
	while(x)
	{
		if(x & 1) r++;
		x >>= 1;
	}
	return r;
}

void yaffs_ECCCalculate(const unsigned char *data,unsigned char *ecc)
{
	unsigned int i;
	
	unsigned char col_parity = 0;
	unsigned char line_parity = 0;
	unsigned char line_parity_prime = 0;
	unsigned char t;
	unsigned char b;
	
	for(i = 0; i < 256; i++)
	{
		b = column_parity_table[*data++];
		col_parity ^= b;

		if(b & 0x01) // odd number of bits in the byte
		{
			line_parity ^= i;
			line_parity_prime ^= ~i;
		}
		
	}
	
	ecc[2] = (~col_parity) | 0x03;
	
	t = 0;
	if(line_parity       & 0x80) t |= 0x80;
	if(line_parity_prime & 0x80) t |= 0x40;
	if(line_parity       & 0x40) t |= 0x20;
	if(line_parity_prime & 0x40) t |= 0x10;
	if(line_parity       & 0x20) t |= 0x08;
	if(line_parity_prime & 0x20) t |= 0x04;
	if(line_parity       & 0x10) t |= 0x02;
	if(line_parity_prime & 0x10) t |= 0x01;
	ecc[1] = ~t;
	
	t = 0;
	if(line_parity       & 0x08) t |= 0x80;
	if(line_parity_prime & 0x08) t |= 0x40;
	if(line_parity       & 0x04) t |= 0x20;
	if(line_parity_prime & 0x04) t |= 0x10;
	if(line_parity       & 0x02) t |= 0x08;
	if(line_parity_prime & 0x02) t |= 0x04;
	if(line_parity       & 0x01) t |= 0x02;
	if(line_parity_prime & 0x01) t |= 0x01;
	ecc[0] = ~t;

#if CONFIG_YAFFS_ECC_WRONG_ORDER
	// Swap the bytes into the wrong order
	t = ecc[0];
	ecc[0] = ecc[1];
	ecc[1] = t;
#endif 
}

int yaffs_ECCCorrect(unsigned char *data, unsigned char *read_ecc, const unsigned char *test_ecc)
{
	unsigned char d0, d1, d2; // deltas 

	d0 = read_ecc[0] ^ test_ecc[0];
	d1 = read_ecc[1] ^ test_ecc[1];
	d2 = read_ecc[2] ^ test_ecc[2];
	
	
	
	if((d0 | d1 | d2) == 0)
	{
		// no error
		return 0;
	}
	
	if( ((d0 ^ (d0 >> 1)) & 0x55) == 0x55 &&
	    ((d1 ^ (d1 >> 1)) & 0x55) == 0x55 &&
	    ((d2 ^ (d2 >> 1)) & 0x54) == 0x54)
	{
		// Single bit (recoverable) error in data

		unsigned byte;
		unsigned bit;

#if CONFIG_YAFFS_ECC_WRONG_ORDER
		// swap the bytes to correct for the wrong order
		unsigned char t;
		
		t = d0;
		d0 = d1;
		d1 = t;
#endif
		
		bit = byte = 0;
		
		
		if(d1 & 0x80) byte |= 0x80;
		if(d1 & 0x20) byte |= 0x40;
		if(d1 & 0x08) byte |= 0x20;
		if(d1 & 0x02) byte |= 0x10;
		if(d0 & 0x80) byte |= 0x08;
		if(d0 & 0x20) byte |= 0x04;
		if(d0 & 0x08) byte |= 0x02;
		if(d0 & 0x02) byte |= 0x01;

		if(d2 & 0x80) bit |= 0x04;
		if(d2 & 0x20) bit |= 0x02;
		if(d2 & 0x08) bit |= 0x01;

		data[byte] ^= (1 << bit);
		
		return 1;
	}
	
	if((yaffs_CountBits(d0)+yaffs_CountBits(d1)+yaffs_CountBits(d2)) == 1)
	{
		// Reccoverable error in ecc
		
		read_ecc[0] = test_ecc[0];
		read_ecc[1] = test_ecc[1];
		read_ecc[2] = test_ecc[2];
		
		return 1;
	}
	
	// Unrecoverable error
	
	return -1;
	    

}