xref: /openbmc/linux/lib/crc32.c (revision b6dcefde)
1 /*
2  * Oct 15, 2000 Matt Domsch <Matt_Domsch@dell.com>
3  * Nicer crc32 functions/docs submitted by linux@horizon.com.  Thanks!
4  * Code was from the public domain, copyright abandoned.  Code was
5  * subsequently included in the kernel, thus was re-licensed under the
6  * GNU GPL v2.
7  *
8  * Oct 12, 2000 Matt Domsch <Matt_Domsch@dell.com>
9  * Same crc32 function was used in 5 other places in the kernel.
10  * I made one version, and deleted the others.
11  * There are various incantations of crc32().  Some use a seed of 0 or ~0.
12  * Some xor at the end with ~0.  The generic crc32() function takes
13  * seed as an argument, and doesn't xor at the end.  Then individual
14  * users can do whatever they need.
15  *   drivers/net/smc9194.c uses seed ~0, doesn't xor with ~0.
16  *   fs/jffs2 uses seed 0, doesn't xor with ~0.
17  *   fs/partitions/efi.c uses seed ~0, xor's with ~0.
18  *
19  * This source code is licensed under the GNU General Public License,
20  * Version 2.  See the file COPYING for more details.
21  */
22 
23 #include <linux/crc32.h>
24 #include <linux/kernel.h>
25 #include <linux/module.h>
26 #include <linux/compiler.h>
27 #include <linux/types.h>
28 #include <linux/slab.h>
29 #include <linux/init.h>
30 #include <asm/atomic.h>
31 #include "crc32defs.h"
32 #if CRC_LE_BITS == 8
33 #define tole(x) __constant_cpu_to_le32(x)
34 #define tobe(x) __constant_cpu_to_be32(x)
35 #else
36 #define tole(x) (x)
37 #define tobe(x) (x)
38 #endif
39 #include "crc32table.h"
40 
41 MODULE_AUTHOR("Matt Domsch <Matt_Domsch@dell.com>");
42 MODULE_DESCRIPTION("Ethernet CRC32 calculations");
43 MODULE_LICENSE("GPL");
44 
45 #if CRC_LE_BITS == 8 || CRC_BE_BITS == 8
46 
47 static inline u32
48 crc32_body(u32 crc, unsigned char const *buf, size_t len, const u32 *tab)
49 {
50 # ifdef __LITTLE_ENDIAN
51 #  define DO_CRC(x) crc = tab[(crc ^ (x)) & 255 ] ^ (crc >> 8)
52 # else
53 #  define DO_CRC(x) crc = tab[((crc >> 24) ^ (x)) & 255] ^ (crc << 8)
54 # endif
55 	const u32 *b = (const u32 *)buf;
56 	size_t    rem_len;
57 
58 	/* Align it */
59 	if (unlikely((long)b & 3 && len)) {
60 		u8 *p = (u8 *)b;
61 		do {
62 			DO_CRC(*p++);
63 		} while ((--len) && ((long)p)&3);
64 		b = (u32 *)p;
65 	}
66 	rem_len = len & 3;
67 	/* load data 32 bits wide, xor data 32 bits wide. */
68 	len = len >> 2;
69 	for (--b; len; --len) {
70 		crc ^= *++b; /* use pre increment for speed */
71 		DO_CRC(0);
72 		DO_CRC(0);
73 		DO_CRC(0);
74 		DO_CRC(0);
75 	}
76 	len = rem_len;
77 	/* And the last few bytes */
78 	if (len) {
79 		u8 *p = (u8 *)(b + 1) - 1;
80 		do {
81 			DO_CRC(*++p); /* use pre increment for speed */
82 		} while (--len);
83 	}
84 	return crc;
85 }
86 #endif
87 /**
88  * crc32_le() - Calculate bitwise little-endian Ethernet AUTODIN II CRC32
89  * @crc: seed value for computation.  ~0 for Ethernet, sometimes 0 for
90  *	other uses, or the previous crc32 value if computing incrementally.
91  * @p: pointer to buffer over which CRC is run
92  * @len: length of buffer @p
93  */
94 u32 __pure crc32_le(u32 crc, unsigned char const *p, size_t len);
95 
96 #if CRC_LE_BITS == 1
97 /*
98  * In fact, the table-based code will work in this case, but it can be
99  * simplified by inlining the table in ?: form.
100  */
101 
102 u32 __pure crc32_le(u32 crc, unsigned char const *p, size_t len)
103 {
104 	int i;
105 	while (len--) {
106 		crc ^= *p++;
107 		for (i = 0; i < 8; i++)
108 			crc = (crc >> 1) ^ ((crc & 1) ? CRCPOLY_LE : 0);
109 	}
110 	return crc;
111 }
112 #else				/* Table-based approach */
113 
114 u32 __pure crc32_le(u32 crc, unsigned char const *p, size_t len)
115 {
116 # if CRC_LE_BITS == 8
117 	const u32      *tab = crc32table_le;
118 
119 	crc = __cpu_to_le32(crc);
120 	crc = crc32_body(crc, p, len, tab);
121 	return __le32_to_cpu(crc);
122 #undef ENDIAN_SHIFT
123 #undef DO_CRC
124 
125 # elif CRC_LE_BITS == 4
126 	while (len--) {
127 		crc ^= *p++;
128 		crc = (crc >> 4) ^ crc32table_le[crc & 15];
129 		crc = (crc >> 4) ^ crc32table_le[crc & 15];
130 	}
131 	return crc;
132 # elif CRC_LE_BITS == 2
133 	while (len--) {
134 		crc ^= *p++;
135 		crc = (crc >> 2) ^ crc32table_le[crc & 3];
136 		crc = (crc >> 2) ^ crc32table_le[crc & 3];
137 		crc = (crc >> 2) ^ crc32table_le[crc & 3];
138 		crc = (crc >> 2) ^ crc32table_le[crc & 3];
139 	}
140 	return crc;
141 # endif
142 }
143 #endif
144 
145 /**
146  * crc32_be() - Calculate bitwise big-endian Ethernet AUTODIN II CRC32
147  * @crc: seed value for computation.  ~0 for Ethernet, sometimes 0 for
148  *	other uses, or the previous crc32 value if computing incrementally.
149  * @p: pointer to buffer over which CRC is run
150  * @len: length of buffer @p
151  */
152 u32 __pure crc32_be(u32 crc, unsigned char const *p, size_t len);
153 
154 #if CRC_BE_BITS == 1
155 /*
156  * In fact, the table-based code will work in this case, but it can be
157  * simplified by inlining the table in ?: form.
158  */
159 
160 u32 __pure crc32_be(u32 crc, unsigned char const *p, size_t len)
161 {
162 	int i;
163 	while (len--) {
164 		crc ^= *p++ << 24;
165 		for (i = 0; i < 8; i++)
166 			crc =
167 			    (crc << 1) ^ ((crc & 0x80000000) ? CRCPOLY_BE :
168 					  0);
169 	}
170 	return crc;
171 }
172 
173 #else				/* Table-based approach */
174 u32 __pure crc32_be(u32 crc, unsigned char const *p, size_t len)
175 {
176 # if CRC_BE_BITS == 8
177 	const u32      *tab = crc32table_be;
178 
179 	crc = __cpu_to_be32(crc);
180 	crc = crc32_body(crc, p, len, tab);
181 	return __be32_to_cpu(crc);
182 #undef ENDIAN_SHIFT
183 #undef DO_CRC
184 
185 # elif CRC_BE_BITS == 4
186 	while (len--) {
187 		crc ^= *p++ << 24;
188 		crc = (crc << 4) ^ crc32table_be[crc >> 28];
189 		crc = (crc << 4) ^ crc32table_be[crc >> 28];
190 	}
191 	return crc;
192 # elif CRC_BE_BITS == 2
193 	while (len--) {
194 		crc ^= *p++ << 24;
195 		crc = (crc << 2) ^ crc32table_be[crc >> 30];
196 		crc = (crc << 2) ^ crc32table_be[crc >> 30];
197 		crc = (crc << 2) ^ crc32table_be[crc >> 30];
198 		crc = (crc << 2) ^ crc32table_be[crc >> 30];
199 	}
200 	return crc;
201 # endif
202 }
203 #endif
204 
205 EXPORT_SYMBOL(crc32_le);
206 EXPORT_SYMBOL(crc32_be);
207 
208 /*
209  * A brief CRC tutorial.
210  *
211  * A CRC is a long-division remainder.  You add the CRC to the message,
212  * and the whole thing (message+CRC) is a multiple of the given
213  * CRC polynomial.  To check the CRC, you can either check that the
214  * CRC matches the recomputed value, *or* you can check that the
215  * remainder computed on the message+CRC is 0.  This latter approach
216  * is used by a lot of hardware implementations, and is why so many
217  * protocols put the end-of-frame flag after the CRC.
218  *
219  * It's actually the same long division you learned in school, except that
220  * - We're working in binary, so the digits are only 0 and 1, and
221  * - When dividing polynomials, there are no carries.  Rather than add and
222  *   subtract, we just xor.  Thus, we tend to get a bit sloppy about
223  *   the difference between adding and subtracting.
224  *
225  * A 32-bit CRC polynomial is actually 33 bits long.  But since it's
226  * 33 bits long, bit 32 is always going to be set, so usually the CRC
227  * is written in hex with the most significant bit omitted.  (If you're
228  * familiar with the IEEE 754 floating-point format, it's the same idea.)
229  *
230  * Note that a CRC is computed over a string of *bits*, so you have
231  * to decide on the endianness of the bits within each byte.  To get
232  * the best error-detecting properties, this should correspond to the
233  * order they're actually sent.  For example, standard RS-232 serial is
234  * little-endian; the most significant bit (sometimes used for parity)
235  * is sent last.  And when appending a CRC word to a message, you should
236  * do it in the right order, matching the endianness.
237  *
238  * Just like with ordinary division, the remainder is always smaller than
239  * the divisor (the CRC polynomial) you're dividing by.  Each step of the
240  * division, you take one more digit (bit) of the dividend and append it
241  * to the current remainder.  Then you figure out the appropriate multiple
242  * of the divisor to subtract to being the remainder back into range.
243  * In binary, it's easy - it has to be either 0 or 1, and to make the
244  * XOR cancel, it's just a copy of bit 32 of the remainder.
245  *
246  * When computing a CRC, we don't care about the quotient, so we can
247  * throw the quotient bit away, but subtract the appropriate multiple of
248  * the polynomial from the remainder and we're back to where we started,
249  * ready to process the next bit.
250  *
251  * A big-endian CRC written this way would be coded like:
252  * for (i = 0; i < input_bits; i++) {
253  * 	multiple = remainder & 0x80000000 ? CRCPOLY : 0;
254  * 	remainder = (remainder << 1 | next_input_bit()) ^ multiple;
255  * }
256  * Notice how, to get at bit 32 of the shifted remainder, we look
257  * at bit 31 of the remainder *before* shifting it.
258  *
259  * But also notice how the next_input_bit() bits we're shifting into
260  * the remainder don't actually affect any decision-making until
261  * 32 bits later.  Thus, the first 32 cycles of this are pretty boring.
262  * Also, to add the CRC to a message, we need a 32-bit-long hole for it at
263  * the end, so we have to add 32 extra cycles shifting in zeros at the
264  * end of every message,
265  *
266  * So the standard trick is to rearrage merging in the next_input_bit()
267  * until the moment it's needed.  Then the first 32 cycles can be precomputed,
268  * and merging in the final 32 zero bits to make room for the CRC can be
269  * skipped entirely.
270  * This changes the code to:
271  * for (i = 0; i < input_bits; i++) {
272  *      remainder ^= next_input_bit() << 31;
273  * 	multiple = (remainder & 0x80000000) ? CRCPOLY : 0;
274  * 	remainder = (remainder << 1) ^ multiple;
275  * }
276  * With this optimization, the little-endian code is simpler:
277  * for (i = 0; i < input_bits; i++) {
278  *      remainder ^= next_input_bit();
279  * 	multiple = (remainder & 1) ? CRCPOLY : 0;
280  * 	remainder = (remainder >> 1) ^ multiple;
281  * }
282  *
283  * Note that the other details of endianness have been hidden in CRCPOLY
284  * (which must be bit-reversed) and next_input_bit().
285  *
286  * However, as long as next_input_bit is returning the bits in a sensible
287  * order, we can actually do the merging 8 or more bits at a time rather
288  * than one bit at a time:
289  * for (i = 0; i < input_bytes; i++) {
290  * 	remainder ^= next_input_byte() << 24;
291  * 	for (j = 0; j < 8; j++) {
292  * 		multiple = (remainder & 0x80000000) ? CRCPOLY : 0;
293  * 		remainder = (remainder << 1) ^ multiple;
294  * 	}
295  * }
296  * Or in little-endian:
297  * for (i = 0; i < input_bytes; i++) {
298  * 	remainder ^= next_input_byte();
299  * 	for (j = 0; j < 8; j++) {
300  * 		multiple = (remainder & 1) ? CRCPOLY : 0;
301  * 		remainder = (remainder << 1) ^ multiple;
302  * 	}
303  * }
304  * If the input is a multiple of 32 bits, you can even XOR in a 32-bit
305  * word at a time and increase the inner loop count to 32.
306  *
307  * You can also mix and match the two loop styles, for example doing the
308  * bulk of a message byte-at-a-time and adding bit-at-a-time processing
309  * for any fractional bytes at the end.
310  *
311  * The only remaining optimization is to the byte-at-a-time table method.
312  * Here, rather than just shifting one bit of the remainder to decide
313  * in the correct multiple to subtract, we can shift a byte at a time.
314  * This produces a 40-bit (rather than a 33-bit) intermediate remainder,
315  * but again the multiple of the polynomial to subtract depends only on
316  * the high bits, the high 8 bits in this case.
317  *
318  * The multiple we need in that case is the low 32 bits of a 40-bit
319  * value whose high 8 bits are given, and which is a multiple of the
320  * generator polynomial.  This is simply the CRC-32 of the given
321  * one-byte message.
322  *
323  * Two more details: normally, appending zero bits to a message which
324  * is already a multiple of a polynomial produces a larger multiple of that
325  * polynomial.  To enable a CRC to detect this condition, it's common to
326  * invert the CRC before appending it.  This makes the remainder of the
327  * message+crc come out not as zero, but some fixed non-zero value.
328  *
329  * The same problem applies to zero bits prepended to the message, and
330  * a similar solution is used.  Instead of starting with a remainder of
331  * 0, an initial remainder of all ones is used.  As long as you start
332  * the same way on decoding, it doesn't make a difference.
333  */
334 
335 #ifdef UNITTEST
336 
337 #include <stdlib.h>
338 #include <stdio.h>
339 
340 #if 0				/*Not used at present */
341 static void
342 buf_dump(char const *prefix, unsigned char const *buf, size_t len)
343 {
344 	fputs(prefix, stdout);
345 	while (len--)
346 		printf(" %02x", *buf++);
347 	putchar('\n');
348 
349 }
350 #endif
351 
352 static void bytereverse(unsigned char *buf, size_t len)
353 {
354 	while (len--) {
355 		unsigned char x = bitrev8(*buf);
356 		*buf++ = x;
357 	}
358 }
359 
360 static void random_garbage(unsigned char *buf, size_t len)
361 {
362 	while (len--)
363 		*buf++ = (unsigned char) random();
364 }
365 
366 #if 0				/* Not used at present */
367 static void store_le(u32 x, unsigned char *buf)
368 {
369 	buf[0] = (unsigned char) x;
370 	buf[1] = (unsigned char) (x >> 8);
371 	buf[2] = (unsigned char) (x >> 16);
372 	buf[3] = (unsigned char) (x >> 24);
373 }
374 #endif
375 
376 static void store_be(u32 x, unsigned char *buf)
377 {
378 	buf[0] = (unsigned char) (x >> 24);
379 	buf[1] = (unsigned char) (x >> 16);
380 	buf[2] = (unsigned char) (x >> 8);
381 	buf[3] = (unsigned char) x;
382 }
383 
384 /*
385  * This checks that CRC(buf + CRC(buf)) = 0, and that
386  * CRC commutes with bit-reversal.  This has the side effect
387  * of bytewise bit-reversing the input buffer, and returns
388  * the CRC of the reversed buffer.
389  */
390 static u32 test_step(u32 init, unsigned char *buf, size_t len)
391 {
392 	u32 crc1, crc2;
393 	size_t i;
394 
395 	crc1 = crc32_be(init, buf, len);
396 	store_be(crc1, buf + len);
397 	crc2 = crc32_be(init, buf, len + 4);
398 	if (crc2)
399 		printf("\nCRC cancellation fail: 0x%08x should be 0\n",
400 		       crc2);
401 
402 	for (i = 0; i <= len + 4; i++) {
403 		crc2 = crc32_be(init, buf, i);
404 		crc2 = crc32_be(crc2, buf + i, len + 4 - i);
405 		if (crc2)
406 			printf("\nCRC split fail: 0x%08x\n", crc2);
407 	}
408 
409 	/* Now swap it around for the other test */
410 
411 	bytereverse(buf, len + 4);
412 	init = bitrev32(init);
413 	crc2 = bitrev32(crc1);
414 	if (crc1 != bitrev32(crc2))
415 		printf("\nBit reversal fail: 0x%08x -> 0x%08x -> 0x%08x\n",
416 		       crc1, crc2, bitrev32(crc2));
417 	crc1 = crc32_le(init, buf, len);
418 	if (crc1 != crc2)
419 		printf("\nCRC endianness fail: 0x%08x != 0x%08x\n", crc1,
420 		       crc2);
421 	crc2 = crc32_le(init, buf, len + 4);
422 	if (crc2)
423 		printf("\nCRC cancellation fail: 0x%08x should be 0\n",
424 		       crc2);
425 
426 	for (i = 0; i <= len + 4; i++) {
427 		crc2 = crc32_le(init, buf, i);
428 		crc2 = crc32_le(crc2, buf + i, len + 4 - i);
429 		if (crc2)
430 			printf("\nCRC split fail: 0x%08x\n", crc2);
431 	}
432 
433 	return crc1;
434 }
435 
436 #define SIZE 64
437 #define INIT1 0
438 #define INIT2 0
439 
440 int main(void)
441 {
442 	unsigned char buf1[SIZE + 4];
443 	unsigned char buf2[SIZE + 4];
444 	unsigned char buf3[SIZE + 4];
445 	int i, j;
446 	u32 crc1, crc2, crc3;
447 
448 	for (i = 0; i <= SIZE; i++) {
449 		printf("\rTesting length %d...", i);
450 		fflush(stdout);
451 		random_garbage(buf1, i);
452 		random_garbage(buf2, i);
453 		for (j = 0; j < i; j++)
454 			buf3[j] = buf1[j] ^ buf2[j];
455 
456 		crc1 = test_step(INIT1, buf1, i);
457 		crc2 = test_step(INIT2, buf2, i);
458 		/* Now check that CRC(buf1 ^ buf2) = CRC(buf1) ^ CRC(buf2) */
459 		crc3 = test_step(INIT1 ^ INIT2, buf3, i);
460 		if (crc3 != (crc1 ^ crc2))
461 			printf("CRC XOR fail: 0x%08x != 0x%08x ^ 0x%08x\n",
462 			       crc3, crc1, crc2);
463 	}
464 	printf("\nAll test complete.  No failures expected.\n");
465 	return 0;
466 }
467 
468 #endif				/* UNITTEST */
469