1 /*
2 * YAFFS: Yet Another Flash File System. A NAND-flash specific file system.
3 *
4 * Copyright (C) 2002-2007 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 such ECC
20 * blocks are used on a 512-byte NAND page.
21 *
22 */
23
24 /* Table generated by gen-ecc.c
25 * Using a table means we do not have to calculate p1..p4 and p1'..p4'
26 * for each byte of data. These are instead provided in a table in bits7..2.
27 * Bit 0 of each entry indicates whether the entry has an odd or even parity, and therefore
28 * this bytes influence on the line parity.
29 */
30
31 const char *yaffs_ecc_c_version =
32 "$Id$";
33
34 #include "yportenv.h"
35
36 #include "yaffs_ecc.h"
37
38 static const unsigned char column_parity_table[] = {
39 0x00, 0x55, 0x59, 0x0c, 0x65, 0x30, 0x3c, 0x69,
40 0x69, 0x3c, 0x30, 0x65, 0x0c, 0x59, 0x55, 0x00,
41 0x95, 0xc0, 0xcc, 0x99, 0xf0, 0xa5, 0xa9, 0xfc,
42 0xfc, 0xa9, 0xa5, 0xf0, 0x99, 0xcc, 0xc0, 0x95,
43 0x99, 0xcc, 0xc0, 0x95, 0xfc, 0xa9, 0xa5, 0xf0,
44 0xf0, 0xa5, 0xa9, 0xfc, 0x95, 0xc0, 0xcc, 0x99,
45 0x0c, 0x59, 0x55, 0x00, 0x69, 0x3c, 0x30, 0x65,
46 0x65, 0x30, 0x3c, 0x69, 0x00, 0x55, 0x59, 0x0c,
47 0xa5, 0xf0, 0xfc, 0xa9, 0xc0, 0x95, 0x99, 0xcc,
48 0xcc, 0x99, 0x95, 0xc0, 0xa9, 0xfc, 0xf0, 0xa5,
49 0x30, 0x65, 0x69, 0x3c, 0x55, 0x00, 0x0c, 0x59,
50 0x59, 0x0c, 0x00, 0x55, 0x3c, 0x69, 0x65, 0x30,
51 0x3c, 0x69, 0x65, 0x30, 0x59, 0x0c, 0x00, 0x55,
52 0x55, 0x00, 0x0c, 0x59, 0x30, 0x65, 0x69, 0x3c,
53 0xa9, 0xfc, 0xf0, 0xa5, 0xcc, 0x99, 0x95, 0xc0,
54 0xc0, 0x95, 0x99, 0xcc, 0xa5, 0xf0, 0xfc, 0xa9,
55 0xa9, 0xfc, 0xf0, 0xa5, 0xcc, 0x99, 0x95, 0xc0,
56 0xc0, 0x95, 0x99, 0xcc, 0xa5, 0xf0, 0xfc, 0xa9,
57 0x3c, 0x69, 0x65, 0x30, 0x59, 0x0c, 0x00, 0x55,
58 0x55, 0x00, 0x0c, 0x59, 0x30, 0x65, 0x69, 0x3c,
59 0x30, 0x65, 0x69, 0x3c, 0x55, 0x00, 0x0c, 0x59,
60 0x59, 0x0c, 0x00, 0x55, 0x3c, 0x69, 0x65, 0x30,
61 0xa5, 0xf0, 0xfc, 0xa9, 0xc0, 0x95, 0x99, 0xcc,
62 0xcc, 0x99, 0x95, 0xc0, 0xa9, 0xfc, 0xf0, 0xa5,
63 0x0c, 0x59, 0x55, 0x00, 0x69, 0x3c, 0x30, 0x65,
64 0x65, 0x30, 0x3c, 0x69, 0x00, 0x55, 0x59, 0x0c,
65 0x99, 0xcc, 0xc0, 0x95, 0xfc, 0xa9, 0xa5, 0xf0,
66 0xf0, 0xa5, 0xa9, 0xfc, 0x95, 0xc0, 0xcc, 0x99,
67 0x95, 0xc0, 0xcc, 0x99, 0xf0, 0xa5, 0xa9, 0xfc,
68 0xfc, 0xa9, 0xa5, 0xf0, 0x99, 0xcc, 0xc0, 0x95,
69 0x00, 0x55, 0x59, 0x0c, 0x65, 0x30, 0x3c, 0x69,
70 0x69, 0x3c, 0x30, 0x65, 0x0c, 0x59, 0x55, 0x00,
71 };
72
73 /* Count the bits in an unsigned char or a U32 */
74
yaffs_CountBits(unsigned char x)75 static int yaffs_CountBits(unsigned char x)
76 {
77 int r = 0;
78 while (x) {
79 if (x & 1)
80 r++;
81 x >>= 1;
82 }
83 return r;
84 }
85
yaffs_CountBits32(unsigned x)86 static int yaffs_CountBits32(unsigned x)
87 {
88 int r = 0;
89 while (x) {
90 if (x & 1)
91 r++;
92 x >>= 1;
93 }
94 return r;
95 }
96
97 /* Calculate the ECC for a 256-byte block of data */
yaffs_ECCCalculate(const unsigned char * data,unsigned char * ecc)98 void yaffs_ECCCalculate(const unsigned char *data, unsigned char *ecc)
99 {
100 unsigned int i;
101
102 unsigned char col_parity = 0;
103 unsigned char line_parity = 0;
104 unsigned char line_parity_prime = 0;
105 unsigned char t;
106 unsigned char b;
107
108 for (i = 0; i < 256; i++) {
109 b = column_parity_table[*data++];
110 col_parity ^= b;
111
112 if (b & 0x01) { /* odd number of bits in the byte */
113 line_parity ^= i;
114 line_parity_prime ^= ~i;
115 }
116 }
117
118 ecc[2] = (~col_parity) | 0x03;
119
120 t = 0;
121 if (line_parity & 0x80)
122 t |= 0x80;
123 if (line_parity_prime & 0x80)
124 t |= 0x40;
125 if (line_parity & 0x40)
126 t |= 0x20;
127 if (line_parity_prime & 0x40)
128 t |= 0x10;
129 if (line_parity & 0x20)
130 t |= 0x08;
131 if (line_parity_prime & 0x20)
132 t |= 0x04;
133 if (line_parity & 0x10)
134 t |= 0x02;
135 if (line_parity_prime & 0x10)
136 t |= 0x01;
137 ecc[1] = ~t;
138
139 t = 0;
140 if (line_parity & 0x08)
141 t |= 0x80;
142 if (line_parity_prime & 0x08)
143 t |= 0x40;
144 if (line_parity & 0x04)
145 t |= 0x20;
146 if (line_parity_prime & 0x04)
147 t |= 0x10;
148 if (line_parity & 0x02)
149 t |= 0x08;
150 if (line_parity_prime & 0x02)
151 t |= 0x04;
152 if (line_parity & 0x01)
153 t |= 0x02;
154 if (line_parity_prime & 0x01)
155 t |= 0x01;
156 ecc[0] = ~t;
157
158 #ifdef CONFIG_YAFFS_ECC_WRONG_ORDER
159 /* Swap the bytes into the wrong order */
160 t = ecc[0];
161 ecc[0] = ecc[1];
162 ecc[1] = t;
163 #endif
164 }
165
166
167 /* Correct the ECC on a 256 byte block of data */
168
yaffs_ECCCorrect(unsigned char * data,unsigned char * read_ecc,const unsigned char * test_ecc)169 int yaffs_ECCCorrect(unsigned char *data, unsigned char *read_ecc,
170 const unsigned char *test_ecc)
171 {
172 unsigned char d0, d1, d2; /* deltas */
173
174 d0 = read_ecc[0] ^ test_ecc[0];
175 d1 = read_ecc[1] ^ test_ecc[1];
176 d2 = read_ecc[2] ^ test_ecc[2];
177
178 if ((d0 | d1 | d2) == 0)
179 return 0; /* no error */
180
181 if (((d0 ^ (d0 >> 1)) & 0x55) == 0x55 &&
182 ((d1 ^ (d1 >> 1)) & 0x55) == 0x55 &&
183 ((d2 ^ (d2 >> 1)) & 0x54) == 0x54) {
184 /* Single bit (recoverable) error in data */
185
186 unsigned byte;
187 unsigned bit;
188
189 #ifdef CONFIG_YAFFS_ECC_WRONG_ORDER
190 /* swap the bytes to correct for the wrong order */
191 unsigned char t;
192
193 t = d0;
194 d0 = d1;
195 d1 = t;
196 #endif
197
198 bit = byte = 0;
199
200 if (d1 & 0x80)
201 byte |= 0x80;
202 if (d1 & 0x20)
203 byte |= 0x40;
204 if (d1 & 0x08)
205 byte |= 0x20;
206 if (d1 & 0x02)
207 byte |= 0x10;
208 if (d0 & 0x80)
209 byte |= 0x08;
210 if (d0 & 0x20)
211 byte |= 0x04;
212 if (d0 & 0x08)
213 byte |= 0x02;
214 if (d0 & 0x02)
215 byte |= 0x01;
216
217 if (d2 & 0x80)
218 bit |= 0x04;
219 if (d2 & 0x20)
220 bit |= 0x02;
221 if (d2 & 0x08)
222 bit |= 0x01;
223
224 data[byte] ^= (1 << bit);
225
226 return 1; /* Corrected the error */
227 }
228
229 if ((yaffs_CountBits(d0) +
230 yaffs_CountBits(d1) +
231 yaffs_CountBits(d2)) == 1) {
232 /* Reccoverable error in ecc */
233
234 read_ecc[0] = test_ecc[0];
235 read_ecc[1] = test_ecc[1];
236 read_ecc[2] = test_ecc[2];
237
238 return 1; /* Corrected the error */
239 }
240
241 /* Unrecoverable error */
242
243 return -1;
244
245 }
246
247
248 /*
249 * ECCxxxOther does ECC calcs on arbitrary n bytes of data
250 */
yaffs_ECCCalculateOther(const unsigned char * data,unsigned nBytes,yaffs_ECCOther * eccOther)251 void yaffs_ECCCalculateOther(const unsigned char *data, unsigned nBytes,
252 yaffs_ECCOther *eccOther)
253 {
254 unsigned int i;
255
256 unsigned char col_parity = 0;
257 unsigned line_parity = 0;
258 unsigned line_parity_prime = 0;
259 unsigned char b;
260
261 for (i = 0; i < nBytes; i++) {
262 b = column_parity_table[*data++];
263 col_parity ^= b;
264
265 if (b & 0x01) {
266 /* odd number of bits in the byte */
267 line_parity ^= i;
268 line_parity_prime ^= ~i;
269 }
270
271 }
272
273 eccOther->colParity = (col_parity >> 2) & 0x3f;
274 eccOther->lineParity = line_parity;
275 eccOther->lineParityPrime = line_parity_prime;
276 }
277
yaffs_ECCCorrectOther(unsigned char * data,unsigned nBytes,yaffs_ECCOther * read_ecc,const yaffs_ECCOther * test_ecc)278 int yaffs_ECCCorrectOther(unsigned char *data, unsigned nBytes,
279 yaffs_ECCOther *read_ecc,
280 const yaffs_ECCOther *test_ecc)
281 {
282 unsigned char cDelta; /* column parity delta */
283 unsigned lDelta; /* line parity delta */
284 unsigned lDeltaPrime; /* line parity delta */
285 unsigned bit;
286
287 cDelta = read_ecc->colParity ^ test_ecc->colParity;
288 lDelta = read_ecc->lineParity ^ test_ecc->lineParity;
289 lDeltaPrime = read_ecc->lineParityPrime ^ test_ecc->lineParityPrime;
290
291 if ((cDelta | lDelta | lDeltaPrime) == 0)
292 return 0; /* no error */
293
294 if (lDelta == ~lDeltaPrime &&
295 (((cDelta ^ (cDelta >> 1)) & 0x15) == 0x15)) {
296 /* Single bit (recoverable) error in data */
297
298 bit = 0;
299
300 if (cDelta & 0x20)
301 bit |= 0x04;
302 if (cDelta & 0x08)
303 bit |= 0x02;
304 if (cDelta & 0x02)
305 bit |= 0x01;
306
307 if (lDelta >= nBytes)
308 return -1;
309
310 data[lDelta] ^= (1 << bit);
311
312 return 1; /* corrected */
313 }
314
315 if ((yaffs_CountBits32(lDelta) + yaffs_CountBits32(lDeltaPrime) +
316 yaffs_CountBits(cDelta)) == 1) {
317 /* Reccoverable error in ecc */
318
319 *read_ecc = *test_ecc;
320 return 1; /* corrected */
321 }
322
323 /* Unrecoverable error */
324
325 return -1;
326 }
327