X-Git-Url: http://www.aleph1.co.uk/gitweb/?p=yaffs%2F.git;a=blobdiff_plain;f=yaffs_ecc.c;fp=yaffs_ecc.c;h=1420da2491445efb795f4b4a79ecb9ebbb1845f2;hp=0000000000000000000000000000000000000000;hb=64a12f248950e23dd994ff29fd090480e0921930;hpb=b0029bde1249255cf0e4771146ba8652da7ca883 diff --git a/yaffs_ecc.c b/yaffs_ecc.c new file mode 100644 index 0000000..1420da2 --- /dev/null +++ b/yaffs_ecc.c @@ -0,0 +1,194 @@ +/* + * 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 + * + * 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. + * + */ + + /* + * 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 1.1 2003-05-20 22:32:24 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; + + +}