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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