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
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.
19 * This source code is licensed under the GNU General Public License,
20 * Version 2. See the file COPYING for more details.
23 #include <linux/crc32.h>
24 #include <linux/kernel.h>
25 #include <linux/module.h>
26 #include <linux/config.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"
33 #define tole(x) __constant_cpu_to_le32(x)
34 #define tobe(x) __constant_cpu_to_be32(x)
39 #include "crc32table.h"
41 #if __GNUC__ >= 3 /* 2.x has "attribute", but only 3.0 has "pure */
42 #define attribute(x) __attribute__(x)
48 MODULE_AUTHOR("Matt Domsch <Matt_Domsch@dell.com>");
49 MODULE_DESCRIPTION("Ethernet CRC32 calculations");
50 MODULE_LICENSE("GPL");
54 * In fact, the table-based code will work in this case, but it can be
55 * simplified by inlining the table in ?: form.
59 * crc32_le() - Calculate bitwise little-endian Ethernet AUTODIN II CRC32
60 * @crc - seed value for computation. ~0 for Ethernet, sometimes 0 for
61 * other uses, or the previous crc32 value if computing incrementally.
62 * @p - pointer to buffer over which CRC is run
63 * @len - length of buffer @p
66 u32 attribute((pure)) crc32_le(u32 crc, unsigned char const *p, size_t len)
71 for (i = 0; i < 8; i++)
72 crc = (crc >> 1) ^ ((crc & 1) ? CRCPOLY_LE : 0);
76 #else /* Table-based approach */
79 * crc32_le() - Calculate bitwise little-endian Ethernet AUTODIN II CRC32
80 * @crc - seed value for computation. ~0 for Ethernet, sometimes 0 for
81 * other uses, or the previous crc32 value if computing incrementally.
82 * @p - pointer to buffer over which CRC is run
83 * @len - length of buffer @p
86 u32 attribute((pure)) crc32_le(u32 crc, unsigned char const *p, size_t len)
89 const u32 *b =(u32 *)p;
90 const u32 *tab = crc32table_le;
92 # ifdef __LITTLE_ENDIAN
93 # define DO_CRC(x) crc = tab[ (crc ^ (x)) & 255 ] ^ (crc>>8)
95 # define DO_CRC(x) crc = tab[ ((crc >> 24) ^ (x)) & 255] ^ (crc<<8)
98 crc = __cpu_to_le32(crc);
100 if(unlikely(((long)b)&3 && len)){
105 } while ((--len) && ((long)b)&3 );
107 if(likely(len >= 4)){
108 /* load data 32 bits wide, xor data 32 bits wide. */
109 size_t save_len = len & 3;
111 --b; /* use pre increment below(*++b) for speed */
119 b++; /* point to next byte(s) */
122 /* And the last few bytes */
131 return __le32_to_cpu(crc);
135 # elif CRC_LE_BITS == 4
138 crc = (crc >> 4) ^ crc32table_le[crc & 15];
139 crc = (crc >> 4) ^ crc32table_le[crc & 15];
142 # elif CRC_LE_BITS == 2
145 crc = (crc >> 2) ^ crc32table_le[crc & 3];
146 crc = (crc >> 2) ^ crc32table_le[crc & 3];
147 crc = (crc >> 2) ^ crc32table_le[crc & 3];
148 crc = (crc >> 2) ^ crc32table_le[crc & 3];
157 * In fact, the table-based code will work in this case, but it can be
158 * simplified by inlining the table in ?: form.
162 * crc32_be() - Calculate bitwise big-endian Ethernet AUTODIN II CRC32
163 * @crc - seed value for computation. ~0 for Ethernet, sometimes 0 for
164 * other uses, or the previous crc32 value if computing incrementally.
165 * @p - pointer to buffer over which CRC is run
166 * @len - length of buffer @p
169 u32 attribute((pure)) crc32_be(u32 crc, unsigned char const *p, size_t len)
174 for (i = 0; i < 8; i++)
176 (crc << 1) ^ ((crc & 0x80000000) ? CRCPOLY_BE :
182 #else /* Table-based approach */
184 * crc32_be() - Calculate bitwise big-endian Ethernet AUTODIN II CRC32
185 * @crc - seed value for computation. ~0 for Ethernet, sometimes 0 for
186 * other uses, or the previous crc32 value if computing incrementally.
187 * @p - pointer to buffer over which CRC is run
188 * @len - length of buffer @p
191 u32 attribute((pure)) crc32_be(u32 crc, unsigned char const *p, size_t len)
193 # if CRC_BE_BITS == 8
194 const u32 *b =(u32 *)p;
195 const u32 *tab = crc32table_be;
197 # ifdef __LITTLE_ENDIAN
198 # define DO_CRC(x) crc = tab[ (crc ^ (x)) & 255 ] ^ (crc>>8)
200 # define DO_CRC(x) crc = tab[ ((crc >> 24) ^ (x)) & 255] ^ (crc<<8)
203 crc = __cpu_to_be32(crc);
205 if(unlikely(((long)b)&3 && len)){
210 } while ((--len) && ((long)b)&3 );
212 if(likely(len >= 4)){
213 /* load data 32 bits wide, xor data 32 bits wide. */
214 size_t save_len = len & 3;
216 --b; /* use pre increment below(*++b) for speed */
224 b++; /* point to next byte(s) */
227 /* And the last few bytes */
235 return __be32_to_cpu(crc);
239 # elif CRC_BE_BITS == 4
242 crc = (crc << 4) ^ crc32table_be[crc >> 28];
243 crc = (crc << 4) ^ crc32table_be[crc >> 28];
246 # elif CRC_BE_BITS == 2
249 crc = (crc << 2) ^ crc32table_be[crc >> 30];
250 crc = (crc << 2) ^ crc32table_be[crc >> 30];
251 crc = (crc << 2) ^ crc32table_be[crc >> 30];
252 crc = (crc << 2) ^ crc32table_be[crc >> 30];
259 u32 bitreverse(u32 x)
261 x = (x >> 16) | (x << 16);
262 x = (x >> 8 & 0x00ff00ff) | (x << 8 & 0xff00ff00);
263 x = (x >> 4 & 0x0f0f0f0f) | (x << 4 & 0xf0f0f0f0);
264 x = (x >> 2 & 0x33333333) | (x << 2 & 0xcccccccc);
265 x = (x >> 1 & 0x55555555) | (x << 1 & 0xaaaaaaaa);
270 /* To ensure that this file is pulled in from lib/lib.a if it's
271 configured in but nothing in-kernel uses it, we export its
272 symbols from kernel/ksyms.c in the CONFIG_CRC32=y case.
273 Otherwise (either modular or pulled in by the makefile magic)
274 we export them from here. */
275 EXPORT_SYMBOL(crc32_le);
276 EXPORT_SYMBOL(crc32_be);
277 EXPORT_SYMBOL(bitreverse);
281 * A brief CRC tutorial.
283 * A CRC is a long-division remainder. You add the CRC to the message,
284 * and the whole thing (message+CRC) is a multiple of the given
285 * CRC polynomial. To check the CRC, you can either check that the
286 * CRC matches the recomputed value, *or* you can check that the
287 * remainder computed on the message+CRC is 0. This latter approach
288 * is used by a lot of hardware implementations, and is why so many
289 * protocols put the end-of-frame flag after the CRC.
291 * It's actually the same long division you learned in school, except that
292 * - We're working in binary, so the digits are only 0 and 1, and
293 * - When dividing polynomials, there are no carries. Rather than add and
294 * subtract, we just xor. Thus, we tend to get a bit sloppy about
295 * the difference between adding and subtracting.
297 * A 32-bit CRC polynomial is actually 33 bits long. But since it's
298 * 33 bits long, bit 32 is always going to be set, so usually the CRC
299 * is written in hex with the most significant bit omitted. (If you're
300 * familiar with the IEEE 754 floating-point format, it's the same idea.)
302 * Note that a CRC is computed over a string of *bits*, so you have
303 * to decide on the endianness of the bits within each byte. To get
304 * the best error-detecting properties, this should correspond to the
305 * order they're actually sent. For example, standard RS-232 serial is
306 * little-endian; the most significant bit (sometimes used for parity)
307 * is sent last. And when appending a CRC word to a message, you should
308 * do it in the right order, matching the endianness.
310 * Just like with ordinary division, the remainder is always smaller than
311 * the divisor (the CRC polynomial) you're dividing by. Each step of the
312 * division, you take one more digit (bit) of the dividend and append it
313 * to the current remainder. Then you figure out the appropriate multiple
314 * of the divisor to subtract to being the remainder back into range.
315 * In binary, it's easy - it has to be either 0 or 1, and to make the
316 * XOR cancel, it's just a copy of bit 32 of the remainder.
318 * When computing a CRC, we don't care about the quotient, so we can
319 * throw the quotient bit away, but subtract the appropriate multiple of
320 * the polynomial from the remainder and we're back to where we started,
321 * ready to process the next bit.
323 * A big-endian CRC written this way would be coded like:
324 * for (i = 0; i < input_bits; i++) {
325 * multiple = remainder & 0x80000000 ? CRCPOLY : 0;
326 * remainder = (remainder << 1 | next_input_bit()) ^ multiple;
328 * Notice how, to get at bit 32 of the shifted remainder, we look
329 * at bit 31 of the remainder *before* shifting it.
331 * But also notice how the next_input_bit() bits we're shifting into
332 * the remainder don't actually affect any decision-making until
333 * 32 bits later. Thus, the first 32 cycles of this are pretty boring.
334 * Also, to add the CRC to a message, we need a 32-bit-long hole for it at
335 * the end, so we have to add 32 extra cycles shifting in zeros at the
336 * end of every message,
338 * So the standard trick is to rearrage merging in the next_input_bit()
339 * until the moment it's needed. Then the first 32 cycles can be precomputed,
340 * and merging in the final 32 zero bits to make room for the CRC can be
342 * This changes the code to:
343 * for (i = 0; i < input_bits; i++) {
344 * remainder ^= next_input_bit() << 31;
345 * multiple = (remainder & 0x80000000) ? CRCPOLY : 0;
346 * remainder = (remainder << 1) ^ multiple;
348 * With this optimization, the little-endian code is simpler:
349 * for (i = 0; i < input_bits; i++) {
350 * remainder ^= next_input_bit();
351 * multiple = (remainder & 1) ? CRCPOLY : 0;
352 * remainder = (remainder >> 1) ^ multiple;
355 * Note that the other details of endianness have been hidden in CRCPOLY
356 * (which must be bit-reversed) and next_input_bit().
358 * However, as long as next_input_bit is returning the bits in a sensible
359 * order, we can actually do the merging 8 or more bits at a time rather
360 * than one bit at a time:
361 * for (i = 0; i < input_bytes; i++) {
362 * remainder ^= next_input_byte() << 24;
363 * for (j = 0; j < 8; j++) {
364 * multiple = (remainder & 0x80000000) ? CRCPOLY : 0;
365 * remainder = (remainder << 1) ^ multiple;
368 * Or in little-endian:
369 * for (i = 0; i < input_bytes; i++) {
370 * remainder ^= next_input_byte();
371 * for (j = 0; j < 8; j++) {
372 * multiple = (remainder & 1) ? CRCPOLY : 0;
373 * remainder = (remainder << 1) ^ multiple;
376 * If the input is a multiple of 32 bits, you can even XOR in a 32-bit
377 * word at a time and increase the inner loop count to 32.
379 * You can also mix and match the two loop styles, for example doing the
380 * bulk of a message byte-at-a-time and adding bit-at-a-time processing
381 * for any fractional bytes at the end.
383 * The only remaining optimization is to the byte-at-a-time table method.
384 * Here, rather than just shifting one bit of the remainder to decide
385 * in the correct multiple to subtract, we can shift a byte at a time.
386 * This produces a 40-bit (rather than a 33-bit) intermediate remainder,
387 * but again the multiple of the polynomial to subtract depends only on
388 * the high bits, the high 8 bits in this case.
390 * The multile we need in that case is the low 32 bits of a 40-bit
391 * value whose high 8 bits are given, and which is a multiple of the
392 * generator polynomial. This is simply the CRC-32 of the given
395 * Two more details: normally, appending zero bits to a message which
396 * is already a multiple of a polynomial produces a larger multiple of that
397 * polynomial. To enable a CRC to detect this condition, it's common to
398 * invert the CRC before appending it. This makes the remainder of the
399 * message+crc come out not as zero, but some fixed non-zero value.
401 * The same problem applies to zero bits prepended to the message, and
402 * a similar solution is used. Instead of starting with a remainder of
403 * 0, an initial remainder of all ones is used. As long as you start
404 * the same way on decoding, it doesn't make a difference.
412 #if 0 /*Not used at present */
414 buf_dump(char const *prefix, unsigned char const *buf, size_t len)
416 fputs(prefix, stdout);
418 printf(" %02x", *buf++);
424 static void bytereverse(unsigned char *buf, size_t len)
427 unsigned char x = *buf;
428 x = (x >> 4) | (x << 4);
429 x = (x >> 2 & 0x33) | (x << 2 & 0xcc);
430 x = (x >> 1 & 0x55) | (x << 1 & 0xaa);
435 static void random_garbage(unsigned char *buf, size_t len)
438 *buf++ = (unsigned char) random();
441 #if 0 /* Not used at present */
442 static void store_le(u32 x, unsigned char *buf)
444 buf[0] = (unsigned char) x;
445 buf[1] = (unsigned char) (x >> 8);
446 buf[2] = (unsigned char) (x >> 16);
447 buf[3] = (unsigned char) (x >> 24);
451 static void store_be(u32 x, unsigned char *buf)
453 buf[0] = (unsigned char) (x >> 24);
454 buf[1] = (unsigned char) (x >> 16);
455 buf[2] = (unsigned char) (x >> 8);
456 buf[3] = (unsigned char) x;
460 * This checks that CRC(buf + CRC(buf)) = 0, and that
461 * CRC commutes with bit-reversal. This has the side effect
462 * of bytewise bit-reversing the input buffer, and returns
463 * the CRC of the reversed buffer.
465 static u32 test_step(u32 init, unsigned char *buf, size_t len)
470 crc1 = crc32_be(init, buf, len);
471 store_be(crc1, buf + len);
472 crc2 = crc32_be(init, buf, len + 4);
474 printf("\nCRC cancellation fail: 0x%08x should be 0\n",
477 for (i = 0; i <= len + 4; i++) {
478 crc2 = crc32_be(init, buf, i);
479 crc2 = crc32_be(crc2, buf + i, len + 4 - i);
481 printf("\nCRC split fail: 0x%08x\n", crc2);
484 /* Now swap it around for the other test */
486 bytereverse(buf, len + 4);
487 init = bitreverse(init);
488 crc2 = bitreverse(crc1);
489 if (crc1 != bitreverse(crc2))
490 printf("\nBit reversal fail: 0x%08x -> %0x08x -> 0x%08x\n",
491 crc1, crc2, bitreverse(crc2));
492 crc1 = crc32_le(init, buf, len);
494 printf("\nCRC endianness fail: 0x%08x != 0x%08x\n", crc1,
496 crc2 = crc32_le(init, buf, len + 4);
498 printf("\nCRC cancellation fail: 0x%08x should be 0\n",
501 for (i = 0; i <= len + 4; i++) {
502 crc2 = crc32_le(init, buf, i);
503 crc2 = crc32_le(crc2, buf + i, len + 4 - i);
505 printf("\nCRC split fail: 0x%08x\n", crc2);
517 unsigned char buf1[SIZE + 4];
518 unsigned char buf2[SIZE + 4];
519 unsigned char buf3[SIZE + 4];
521 u32 crc1, crc2, crc3;
523 for (i = 0; i <= SIZE; i++) {
524 printf("\rTesting length %d...", i);
526 random_garbage(buf1, i);
527 random_garbage(buf2, i);
528 for (j = 0; j < i; j++)
529 buf3[j] = buf1[j] ^ buf2[j];
531 crc1 = test_step(INIT1, buf1, i);
532 crc2 = test_step(INIT2, buf2, i);
533 /* Now check that CRC(buf1 ^ buf2) = CRC(buf1) ^ CRC(buf2) */
534 crc3 = test_step(INIT1 ^ INIT2, buf3, i);
535 if (crc3 != (crc1 ^ crc2))
536 printf("CRC XOR fail: 0x%08x != 0x%08x ^ 0x%08x\n",
539 printf("\nAll test complete. No failures expected.\n");
543 #endif /* UNITTEST */