yaffs_ecc.h和yaffs_ecc.c源码分析

/*
 * 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.
 *
 */
 //ECC包含22位基偶校验信息，它共占用3个字节，另外两个未用的bit被置为1.
//ECC可以修复页大小为256字节flash的位错误，因此，在
//页大小为512字节的flash中需要使用两个ECC块。
#ifndef __YAFFS_ECC_H__
#define __YAFFS_ECC_H__


typedef struct {
unsigned char colParity;//列基偶校验
unsigned lineParity;//行基偶校验
unsigned lineParityPrime;
} yaffs_ECCOther;

void yaffs_ECCCalculate(const unsigned char *data, unsigned char *ecc);
int yaffs_ECCCorrect(unsigned char *data, unsigned char *read_ecc,
const unsigned char *test_ecc);

void yaffs_ECCCalculateOther(const unsigned char *data, unsigned nBytes,
yaffs_ECCOther *ecc);
int yaffs_ECCCorrectOther(unsigned char *data, unsigned nBytes,
yaffs_ECCOther *read_ecc,
const yaffs_ECCOther *test_ecc);
#endif

=============================================================================================
const char *yaffs_ecc_c_version =
"$Id: yaffs_ecc.c,v 1.11 2009-03-06 17:20:50 wookey Exp $";

#include "yportenv.h"

#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,
};

/* Count the bits in an unsigned char or a U32 */

static int yaffs_CountBits(unsigned char x)//计算uchar型数据中位为1的bit位个数。
{
int r = 0;
while (x) {
if (x & 1)
r++;
x >>= 1;
}
return r;
}

static int yaffs_CountBits32(unsigned x)//计算u32型数据中位为1的bit位个数。
{
int r = 0;
while (x) {
if (x & 1)
r++;
x >>= 1;
}
return r;
}

/* Calculate the ECC for a 256-byte block of data */
//计算256字节数据块的ECC。
void yaffs_ECCCalculate(const unsigned char *data, unsigned char *ecc)
{//入参为256字节的数据块，和用于保存最终计算ecc结果的数组。
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++) {//遍历该数据块中的每一个字节。
b = column_parity_table[*data++];//得到该Byte数据行校验和列校验值。
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;

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

/* Correct the ECC on a 256 byte block of data */
//若该256字节数据块中只有一个bit错误，可纠正；否则无法纠正错误。
//方法:将原ECC校验和新ECC校验和按位异或，若结果为0，则表示不存在错；
//若异或结果中存在11个比特位为1，表示存在一个比特错误，且可纠正


//若该256字节数据块中有2bit以上错误，不保证能检测。
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)
return 0; /* no error */

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;

#ifdef 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; /* Corrected the error */
}

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; /* Corrected the error */
}

/* Unrecoverable error */

return -1;

}

/*
 * ECCxxxOther does ECC calcs on arbitrary n bytes of data
 */
 //该函数用于对任意n字节数据进行ecc校验，而不是固定的256字节数据块。
void yaffs_ECCCalculateOther(const unsigned char *data, unsigned nBytes,
yaffs_ECCOther *eccOther)
{
unsigned int i;

unsigned char col_parity = 0;
unsigned line_parity = 0;
unsigned line_parity_prime = 0;
unsigned char b;

for (i = 0; i < nBytes; 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;
}

}

eccOther->colParity = (col_parity >> 2) & 0x3f;
eccOther->lineParity = line_parity;
eccOther->lineParityPrime = line_parity_prime;
}

int yaffs_ECCCorrectOther(unsigned char *data, unsigned nBytes,
yaffs_ECCOther *read_ecc,
const yaffs_ECCOther *test_ecc)
{
unsigned char cDelta; /* column parity delta */
unsigned lDelta; /* line parity delta */
unsigned lDeltaPrime; /* line parity delta */
unsigned bit;

cDelta = read_ecc->colParity ^ test_ecc->colParity;
lDelta = read_ecc->lineParity ^ test_ecc->lineParity;
lDeltaPrime = read_ecc->lineParityPrime ^ test_ecc->lineParityPrime;

if ((cDelta | lDelta | lDeltaPrime) == 0)
return 0; /* no error */

if (lDelta == ~lDeltaPrime &&
   (((cDelta ^ (cDelta >> 1)) & 0x15) == 0x15)) {
/* Single bit (recoverable) error in data */

bit = 0;

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

if (lDelta >= nBytes)
return -1;

data[lDelta] ^= (1 << bit);

return 1; /* corrected */
}

if ((yaffs_CountBits32(lDelta) + yaffs_CountBits32(lDeltaPrime) +
yaffs_CountBits(cDelta)) == 1) {
/* Reccoverable error in ecc */

*read_ecc = *test_ecc;
return 1; /* corrected */
}

/* Unrecoverable error */

return -1;
}