+/*
+ * 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 <charles@aleph1.co.uk>
+ *
+ * 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;
+
+
+}
projects / yaffs / .git / commitdiff