1 /*
2 * YAFFS: Yet another FFS. A NAND-flash specific file system.
3 *
4 * yaffs_ecc.c: ECC generation/correction algorithms.
5 *
6 * Copyright (C) 2002 Aleph One Ltd.
7 *
8 * Created by Charles Manning <charles@aleph1.co.uk>
9 *
10 *
11 * This program is free software; you can redistribute it and/or
12 * modify it under the terms of the GNU Lesser General Public License
13 * version 2.1 as published by the Free Software Foundation.
14 */
15
16 /*
17 * This code implements the ECC algorithm used in SmartMedia.
18 *
19 * The ECC comprises 22 bits of parity information and is stuffed into 3 bytes.
20 * The two unused bit are set to 1.
21 * The ECC can correct single bit errors in a 256-byte page of data. Thus, two such ECC
22 * blocks are used on a 512-byte NAND page.
23 *
24 */
25
26 /* Table generated by gen-ecc.c
27 * Using a table means we do not have to calculate p1..p4 and p1'..p4'
28 * for each byte of data. These are instead provided in a table in bits7..2.
29 * Bit 0 of each entry indicates whether the entry has an odd or even parity, and therefore
30 * this bytes influence on the line parity.
31 */
32
33 const char *yaffs_ecc_c_version =
34 "$Id: yaffs_ecc.c,v 1.7 2006/09/14 22:02:46 charles Exp $";
35
36 #include "yportenv.h"
37
38 #include "yaffs_ecc.h"
39
40 static const unsigned char column_parity_table[] = {
41 0x00, 0x55, 0x59, 0x0c, 0x65, 0x30, 0x3c, 0x69,
42 0x69, 0x3c, 0x30, 0x65, 0x0c, 0x59, 0x55, 0x00,
43 0x95, 0xc0, 0xcc, 0x99, 0xf0, 0xa5, 0xa9, 0xfc,
44 0xfc, 0xa9, 0xa5, 0xf0, 0x99, 0xcc, 0xc0, 0x95,
45 0x99, 0xcc, 0xc0, 0x95, 0xfc, 0xa9, 0xa5, 0xf0,
46 0xf0, 0xa5, 0xa9, 0xfc, 0x95, 0xc0, 0xcc, 0x99,
47 0x0c, 0x59, 0x55, 0x00, 0x69, 0x3c, 0x30, 0x65,
48 0x65, 0x30, 0x3c, 0x69, 0x00, 0x55, 0x59, 0x0c,
49 0xa5, 0xf0, 0xfc, 0xa9, 0xc0, 0x95, 0x99, 0xcc,
50 0xcc, 0x99, 0x95, 0xc0, 0xa9, 0xfc, 0xf0, 0xa5,
51 0x30, 0x65, 0x69, 0x3c, 0x55, 0x00, 0x0c, 0x59,
52 0x59, 0x0c, 0x00, 0x55, 0x3c, 0x69, 0x65, 0x30,
53 0x3c, 0x69, 0x65, 0x30, 0x59, 0x0c, 0x00, 0x55,
54 0x55, 0x00, 0x0c, 0x59, 0x30, 0x65, 0x69, 0x3c,
55 0xa9, 0xfc, 0xf0, 0xa5, 0xcc, 0x99, 0x95, 0xc0,
56 0xc0, 0x95, 0x99, 0xcc, 0xa5, 0xf0, 0xfc, 0xa9,
57 0xa9, 0xfc, 0xf0, 0xa5, 0xcc, 0x99, 0x95, 0xc0,
58 0xc0, 0x95, 0x99, 0xcc, 0xa5, 0xf0, 0xfc, 0xa9,
59 0x3c, 0x69, 0x65, 0x30, 0x59, 0x0c, 0x00, 0x55,
60 0x55, 0x00, 0x0c, 0x59, 0x30, 0x65, 0x69, 0x3c,
61 0x30, 0x65, 0x69, 0x3c, 0x55, 0x00, 0x0c, 0x59,
62 0x59, 0x0c, 0x00, 0x55, 0x3c, 0x69, 0x65, 0x30,
63 0xa5, 0xf0, 0xfc, 0xa9, 0xc0, 0x95, 0x99, 0xcc,
64 0xcc, 0x99, 0x95, 0xc0, 0xa9, 0xfc, 0xf0, 0xa5,
65 0x0c, 0x59, 0x55, 0x00, 0x69, 0x3c, 0x30, 0x65,
66 0x65, 0x30, 0x3c, 0x69, 0x00, 0x55, 0x59, 0x0c,
67 0x99, 0xcc, 0xc0, 0x95, 0xfc, 0xa9, 0xa5, 0xf0,
68 0xf0, 0xa5, 0xa9, 0xfc, 0x95, 0xc0, 0xcc, 0x99,
69 0x95, 0xc0, 0xcc, 0x99, 0xf0, 0xa5, 0xa9, 0xfc,
70 0xfc, 0xa9, 0xa5, 0xf0, 0x99, 0xcc, 0xc0, 0x95,
71 0x00, 0x55, 0x59, 0x0c, 0x65, 0x30, 0x3c, 0x69,
72 0x69, 0x3c, 0x30, 0x65, 0x0c, 0x59, 0x55, 0x00,
73 };
74
75 /* Count the bits in an unsigned char or a U32 */
76
yaffs_CountBits(unsigned char x)77 static int yaffs_CountBits(unsigned char x)
78 {
79 int r = 0;
80 while (x) {
81 if (x & 1)
82 r++;
83 x >>= 1;
84 }
85 return r;
86 }
87
yaffs_CountBits32(unsigned x)88 static int yaffs_CountBits32(unsigned x)
89 {
90 int r = 0;
91 while (x) {
92 if (x & 1)
93 r++;
94 x >>= 1;
95 }
96 return r;
97 }
98
99 /* Calculate the ECC for a 256-byte block of data */
yaffs_ECCCalculate(const unsigned char * data,unsigned char * ecc)100 void yaffs_ECCCalculate(const unsigned char *data, unsigned char *ecc)
101 {
102 unsigned int i;
103
104 unsigned char col_parity = 0;
105 unsigned char line_parity = 0;
106 unsigned char line_parity_prime = 0;
107 unsigned char t;
108 unsigned char b;
109
110 for (i = 0; i < 256; i++) {
111 b = column_parity_table[*data++];
112 col_parity ^= b;
113
114 if (b & 0x01) // odd number of bits in the byte
115 {
116 line_parity ^= i;
117 line_parity_prime ^= ~i;
118 }
119
120 }
121
122 ecc[2] = (~col_parity) | 0x03;
123
124 t = 0;
125 if (line_parity & 0x80)
126 t |= 0x80;
127 if (line_parity_prime & 0x80)
128 t |= 0x40;
129 if (line_parity & 0x40)
130 t |= 0x20;
131 if (line_parity_prime & 0x40)
132 t |= 0x10;
133 if (line_parity & 0x20)
134 t |= 0x08;
135 if (line_parity_prime & 0x20)
136 t |= 0x04;
137 if (line_parity & 0x10)
138 t |= 0x02;
139 if (line_parity_prime & 0x10)
140 t |= 0x01;
141 ecc[1] = ~t;
142
143 t = 0;
144 if (line_parity & 0x08)
145 t |= 0x80;
146 if (line_parity_prime & 0x08)
147 t |= 0x40;
148 if (line_parity & 0x04)
149 t |= 0x20;
150 if (line_parity_prime & 0x04)
151 t |= 0x10;
152 if (line_parity & 0x02)
153 t |= 0x08;
154 if (line_parity_prime & 0x02)
155 t |= 0x04;
156 if (line_parity & 0x01)
157 t |= 0x02;
158 if (line_parity_prime & 0x01)
159 t |= 0x01;
160 ecc[0] = ~t;
161
162 #ifdef CONFIG_YAFFS_ECC_WRONG_ORDER
163 // Swap the bytes into the wrong order
164 t = ecc[0];
165 ecc[0] = ecc[1];
166 ecc[1] = t;
167 #endif
168 }
169
170
171 /* Correct the ECC on a 256 byte block of data */
172
yaffs_ECCCorrect(unsigned char * data,unsigned char * read_ecc,const unsigned char * test_ecc)173 int yaffs_ECCCorrect(unsigned char *data, unsigned char *read_ecc,
174 const unsigned char *test_ecc)
175 {
176 unsigned char d0, d1, d2; /* deltas */
177
178 d0 = read_ecc[0] ^ test_ecc[0];
179 d1 = read_ecc[1] ^ test_ecc[1];
180 d2 = read_ecc[2] ^ test_ecc[2];
181
182 if ((d0 | d1 | d2) == 0)
183 return 0; /* no error */
184
185 if (((d0 ^ (d0 >> 1)) & 0x55) == 0x55 &&
186 ((d1 ^ (d1 >> 1)) & 0x55) == 0x55 &&
187 ((d2 ^ (d2 >> 1)) & 0x54) == 0x54) {
188 /* Single bit (recoverable) error in data */
189
190 unsigned byte;
191 unsigned bit;
192
193 #ifdef CONFIG_YAFFS_ECC_WRONG_ORDER
194 // swap the bytes to correct for the wrong order
195 unsigned char t;
196
197 t = d0;
198 d0 = d1;
199 d1 = t;
200 #endif
201
202 bit = byte = 0;
203
204 if (d1 & 0x80)
205 byte |= 0x80;
206 if (d1 & 0x20)
207 byte |= 0x40;
208 if (d1 & 0x08)
209 byte |= 0x20;
210 if (d1 & 0x02)
211 byte |= 0x10;
212 if (d0 & 0x80)
213 byte |= 0x08;
214 if (d0 & 0x20)
215 byte |= 0x04;
216 if (d0 & 0x08)
217 byte |= 0x02;
218 if (d0 & 0x02)
219 byte |= 0x01;
220
221 if (d2 & 0x80)
222 bit |= 0x04;
223 if (d2 & 0x20)
224 bit |= 0x02;
225 if (d2 & 0x08)
226 bit |= 0x01;
227
228 data[byte] ^= (1 << bit);
229
230 return 1; /* Corrected the error */
231 }
232
233 if ((yaffs_CountBits(d0) +
234 yaffs_CountBits(d1) +
235 yaffs_CountBits(d2)) == 1) {
236 /* Reccoverable error in ecc */
237
238 read_ecc[0] = test_ecc[0];
239 read_ecc[1] = test_ecc[1];
240 read_ecc[2] = test_ecc[2];
241
242 return 1; /* Corrected the error */
243 }
244
245 /* Unrecoverable error */
246
247 return -1;
248
249 }
250
251
252 /*
253 * ECCxxxOther does ECC calcs on arbitrary n bytes of data
254 */
yaffs_ECCCalculateOther(const unsigned char * data,unsigned nBytes,yaffs_ECCOther * eccOther)255 void yaffs_ECCCalculateOther(const unsigned char *data, unsigned nBytes,
256 yaffs_ECCOther * eccOther)
257 {
258 unsigned int i;
259
260 unsigned char col_parity = 0;
261 unsigned line_parity = 0;
262 unsigned line_parity_prime = 0;
263 unsigned char b;
264
265 for (i = 0; i < nBytes; i++) {
266 b = column_parity_table[*data++];
267 col_parity ^= b;
268
269 if (b & 0x01) {
270 /* odd number of bits in the byte */
271 line_parity ^= i;
272 line_parity_prime ^= ~i;
273 }
274
275 }
276
277 eccOther->colParity = (col_parity >> 2) & 0x3f;
278 eccOther->lineParity = line_parity;
279 eccOther->lineParityPrime = line_parity_prime;
280 }
281
yaffs_ECCCorrectOther(unsigned char * data,unsigned nBytes,yaffs_ECCOther * read_ecc,const yaffs_ECCOther * test_ecc)282 int yaffs_ECCCorrectOther(unsigned char *data, unsigned nBytes,
283 yaffs_ECCOther * read_ecc,
284 const yaffs_ECCOther * test_ecc)
285 {
286 unsigned char cDelta; /* column parity delta */
287 unsigned lDelta; /* line parity delta */
288 unsigned lDeltaPrime; /* line parity delta */
289 unsigned bit;
290
291 cDelta = read_ecc->colParity ^ test_ecc->colParity;
292 lDelta = read_ecc->lineParity ^ test_ecc->lineParity;
293 lDeltaPrime = read_ecc->lineParityPrime ^ test_ecc->lineParityPrime;
294
295 if ((cDelta | lDelta | lDeltaPrime) == 0)
296 return 0; /* no error */
297
298 if (lDelta == ~lDeltaPrime &&
299 (((cDelta ^ (cDelta >> 1)) & 0x15) == 0x15))
300 {
301 /* Single bit (recoverable) error in data */
302
303 bit = 0;
304
305 if (cDelta & 0x20)
306 bit |= 0x04;
307 if (cDelta & 0x08)
308 bit |= 0x02;
309 if (cDelta & 0x02)
310 bit |= 0x01;
311
312 if(lDelta >= nBytes)
313 return -1;
314
315 data[lDelta] ^= (1 << bit);
316
317 return 1; /* corrected */
318 }
319
320 if ((yaffs_CountBits32(lDelta) + yaffs_CountBits32(lDeltaPrime) +
321 yaffs_CountBits(cDelta)) == 1) {
322 /* Reccoverable error in ecc */
323
324 *read_ecc = *test_ecc;
325 return 1; /* corrected */
326 }
327
328 /* Unrecoverable error */
329
330 return -1;
331
332 }
333
334