1 /*
2 * YAFFS: Yet Another Flash File System. A NAND-flash specific file system.
3 *
4 * Copyright (C) 2002-2011 Aleph One Ltd.
5 * for Toby Churchill Ltd and Brightstar Engineering
6 *
7 * Created by Charles Manning <charles@aleph1.co.uk>
8 *
9 * This program is free software; you can redistribute it and/or modify
10 * it under the terms of the GNU General Public License version 2 as
11 * published by the Free Software Foundation.
12 */
13
14 /*
15 * This code implements the ECC algorithm used in SmartMedia.
16 *
17 * The ECC comprises 22 bits of parity information and is stuffed into 3 bytes.
18 * The two unused bit are set to 1.
19 * The ECC can correct single bit errors in a 256-byte page of data. Thus, two
20 * such ECC blocks are used on a 512-byte NAND page.
21 *
22 */
23
24 #include "yportenv.h"
25
26 #include "yaffs_ecc.h"
27
28 /* Table generated by gen-ecc.c
29 * Using a table means we do not have to calculate p1..p4 and p1'..p4'
30 * for each byte of data. These are instead provided in a table in bits7..2.
31 * Bit 0 of each entry indicates whether the entry has an odd or even parity,
32 * and therefore this bytes influence on the line parity.
33 */
34
35 static const unsigned char column_parity_table[] = {
36 0x00, 0x55, 0x59, 0x0c, 0x65, 0x30, 0x3c, 0x69,
37 0x69, 0x3c, 0x30, 0x65, 0x0c, 0x59, 0x55, 0x00,
38 0x95, 0xc0, 0xcc, 0x99, 0xf0, 0xa5, 0xa9, 0xfc,
39 0xfc, 0xa9, 0xa5, 0xf0, 0x99, 0xcc, 0xc0, 0x95,
40 0x99, 0xcc, 0xc0, 0x95, 0xfc, 0xa9, 0xa5, 0xf0,
41 0xf0, 0xa5, 0xa9, 0xfc, 0x95, 0xc0, 0xcc, 0x99,
42 0x0c, 0x59, 0x55, 0x00, 0x69, 0x3c, 0x30, 0x65,
43 0x65, 0x30, 0x3c, 0x69, 0x00, 0x55, 0x59, 0x0c,
44 0xa5, 0xf0, 0xfc, 0xa9, 0xc0, 0x95, 0x99, 0xcc,
45 0xcc, 0x99, 0x95, 0xc0, 0xa9, 0xfc, 0xf0, 0xa5,
46 0x30, 0x65, 0x69, 0x3c, 0x55, 0x00, 0x0c, 0x59,
47 0x59, 0x0c, 0x00, 0x55, 0x3c, 0x69, 0x65, 0x30,
48 0x3c, 0x69, 0x65, 0x30, 0x59, 0x0c, 0x00, 0x55,
49 0x55, 0x00, 0x0c, 0x59, 0x30, 0x65, 0x69, 0x3c,
50 0xa9, 0xfc, 0xf0, 0xa5, 0xcc, 0x99, 0x95, 0xc0,
51 0xc0, 0x95, 0x99, 0xcc, 0xa5, 0xf0, 0xfc, 0xa9,
52 0xa9, 0xfc, 0xf0, 0xa5, 0xcc, 0x99, 0x95, 0xc0,
53 0xc0, 0x95, 0x99, 0xcc, 0xa5, 0xf0, 0xfc, 0xa9,
54 0x3c, 0x69, 0x65, 0x30, 0x59, 0x0c, 0x00, 0x55,
55 0x55, 0x00, 0x0c, 0x59, 0x30, 0x65, 0x69, 0x3c,
56 0x30, 0x65, 0x69, 0x3c, 0x55, 0x00, 0x0c, 0x59,
57 0x59, 0x0c, 0x00, 0x55, 0x3c, 0x69, 0x65, 0x30,
58 0xa5, 0xf0, 0xfc, 0xa9, 0xc0, 0x95, 0x99, 0xcc,
59 0xcc, 0x99, 0x95, 0xc0, 0xa9, 0xfc, 0xf0, 0xa5,
60 0x0c, 0x59, 0x55, 0x00, 0x69, 0x3c, 0x30, 0x65,
61 0x65, 0x30, 0x3c, 0x69, 0x00, 0x55, 0x59, 0x0c,
62 0x99, 0xcc, 0xc0, 0x95, 0xfc, 0xa9, 0xa5, 0xf0,
63 0xf0, 0xa5, 0xa9, 0xfc, 0x95, 0xc0, 0xcc, 0x99,
64 0x95, 0xc0, 0xcc, 0x99, 0xf0, 0xa5, 0xa9, 0xfc,
65 0xfc, 0xa9, 0xa5, 0xf0, 0x99, 0xcc, 0xc0, 0x95,
66 0x00, 0x55, 0x59, 0x0c, 0x65, 0x30, 0x3c, 0x69,
67 0x69, 0x3c, 0x30, 0x65, 0x0c, 0x59, 0x55, 0x00,
68 };
69
70
71 /* Calculate the ECC for a 256-byte block of data */
yaffs_ecc_calc(const unsigned char * data,unsigned char * ecc)72 void yaffs_ecc_calc(const unsigned char *data, unsigned char *ecc)
73 {
74 unsigned int i;
75 unsigned char col_parity = 0;
76 unsigned char line_parity = 0;
77 unsigned char line_parity_prime = 0;
78 unsigned char t;
79 unsigned char b;
80
81 for (i = 0; i < 256; i++) {
82 b = column_parity_table[*data++];
83 col_parity ^= b;
84
85 if (b & 0x01) { /* odd number of bits in the byte */
86 line_parity ^= i;
87 line_parity_prime ^= ~i;
88 }
89 }
90
91 ecc[2] = (~col_parity) | 0x03;
92
93 t = 0;
94 if (line_parity & 0x80)
95 t |= 0x80;
96 if (line_parity_prime & 0x80)
97 t |= 0x40;
98 if (line_parity & 0x40)
99 t |= 0x20;
100 if (line_parity_prime & 0x40)
101 t |= 0x10;
102 if (line_parity & 0x20)
103 t |= 0x08;
104 if (line_parity_prime & 0x20)
105 t |= 0x04;
106 if (line_parity & 0x10)
107 t |= 0x02;
108 if (line_parity_prime & 0x10)
109 t |= 0x01;
110 ecc[1] = ~t;
111
112 t = 0;
113 if (line_parity & 0x08)
114 t |= 0x80;
115 if (line_parity_prime & 0x08)
116 t |= 0x40;
117 if (line_parity & 0x04)
118 t |= 0x20;
119 if (line_parity_prime & 0x04)
120 t |= 0x10;
121 if (line_parity & 0x02)
122 t |= 0x08;
123 if (line_parity_prime & 0x02)
124 t |= 0x04;
125 if (line_parity & 0x01)
126 t |= 0x02;
127 if (line_parity_prime & 0x01)
128 t |= 0x01;
129 ecc[0] = ~t;
130
131 }
132
133 /* Correct the ECC on a 256 byte block of data */
134
yaffs_ecc_correct(unsigned char * data,unsigned char * read_ecc,const unsigned char * test_ecc)135 int yaffs_ecc_correct(unsigned char *data, unsigned char *read_ecc,
136 const unsigned char *test_ecc)
137 {
138 unsigned char d0, d1, d2; /* deltas */
139
140 d0 = read_ecc[0] ^ test_ecc[0];
141 d1 = read_ecc[1] ^ test_ecc[1];
142 d2 = read_ecc[2] ^ test_ecc[2];
143
144 if ((d0 | d1 | d2) == 0)
145 return 0; /* no error */
146
147 if (((d0 ^ (d0 >> 1)) & 0x55) == 0x55 &&
148 ((d1 ^ (d1 >> 1)) & 0x55) == 0x55 &&
149 ((d2 ^ (d2 >> 1)) & 0x54) == 0x54) {
150 /* Single bit (recoverable) error in data */
151
152 unsigned byte;
153 unsigned bit;
154
155 bit = byte = 0;
156
157 if (d1 & 0x80)
158 byte |= 0x80;
159 if (d1 & 0x20)
160 byte |= 0x40;
161 if (d1 & 0x08)
162 byte |= 0x20;
163 if (d1 & 0x02)
164 byte |= 0x10;
165 if (d0 & 0x80)
166 byte |= 0x08;
167 if (d0 & 0x20)
168 byte |= 0x04;
169 if (d0 & 0x08)
170 byte |= 0x02;
171 if (d0 & 0x02)
172 byte |= 0x01;
173
174 if (d2 & 0x80)
175 bit |= 0x04;
176 if (d2 & 0x20)
177 bit |= 0x02;
178 if (d2 & 0x08)
179 bit |= 0x01;
180
181 data[byte] ^= (1 << bit);
182
183 return 1; /* Corrected the error */
184 }
185
186 if ((hweight8(d0) + hweight8(d1) + hweight8(d2)) == 1) {
187 /* Reccoverable error in ecc */
188
189 read_ecc[0] = test_ecc[0];
190 read_ecc[1] = test_ecc[1];
191 read_ecc[2] = test_ecc[2];
192
193 return 1; /* Corrected the error */
194 }
195
196 /* Unrecoverable error */
197
198 return -1;
199
200 }
201
202 /*
203 * ECCxxxOther does ECC calcs on arbitrary n bytes of data
204 */
yaffs_ecc_calc_other(const unsigned char * data,unsigned n_bytes,struct yaffs_ecc_other * ecc_other)205 void yaffs_ecc_calc_other(const unsigned char *data, unsigned n_bytes,
206 struct yaffs_ecc_other *ecc_other)
207 {
208 unsigned int i;
209 unsigned char col_parity = 0;
210 unsigned line_parity = 0;
211 unsigned line_parity_prime = 0;
212 unsigned char b;
213
214 for (i = 0; i < n_bytes; i++) {
215 b = column_parity_table[*data++];
216 col_parity ^= b;
217
218 if (b & 0x01) {
219 /* odd number of bits in the byte */
220 line_parity ^= i;
221 line_parity_prime ^= ~i;
222 }
223
224 }
225
226 ecc_other->col_parity = (col_parity >> 2) & 0x3f;
227 ecc_other->line_parity = line_parity;
228 ecc_other->line_parity_prime = line_parity_prime;
229 }
230
yaffs_ecc_correct_other(unsigned char * data,unsigned n_bytes,struct yaffs_ecc_other * read_ecc,const struct yaffs_ecc_other * test_ecc)231 int yaffs_ecc_correct_other(unsigned char *data, unsigned n_bytes,
232 struct yaffs_ecc_other *read_ecc,
233 const struct yaffs_ecc_other *test_ecc)
234 {
235 unsigned char delta_col; /* column parity delta */
236 unsigned delta_line; /* line parity delta */
237 unsigned delta_line_prime; /* line parity delta */
238 unsigned bit;
239
240 delta_col = read_ecc->col_parity ^ test_ecc->col_parity;
241 delta_line = read_ecc->line_parity ^ test_ecc->line_parity;
242 delta_line_prime =
243 read_ecc->line_parity_prime ^ test_ecc->line_parity_prime;
244
245 if ((delta_col | delta_line | delta_line_prime) == 0)
246 return 0; /* no error */
247
248 if (delta_line == ~delta_line_prime &&
249 (((delta_col ^ (delta_col >> 1)) & 0x15) == 0x15)) {
250 /* Single bit (recoverable) error in data */
251
252 bit = 0;
253
254 if (delta_col & 0x20)
255 bit |= 0x04;
256 if (delta_col & 0x08)
257 bit |= 0x02;
258 if (delta_col & 0x02)
259 bit |= 0x01;
260
261 if (delta_line >= n_bytes)
262 return -1;
263
264 data[delta_line] ^= (1 << bit);
265
266 return 1; /* corrected */
267 }
268
269 if ((hweight32(delta_line) +
270 hweight32(delta_line_prime) +
271 hweight8(delta_col)) == 1) {
272 /* Reccoverable error in ecc */
273
274 *read_ecc = *test_ecc;
275 return 1; /* corrected */
276 }
277
278 /* Unrecoverable error */
279
280 return -1;
281 }
282