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.
24 #include <linux/crc32.h>
25 #include <linux/kernel.h>
26 #include <linux/module.h>
27 #include <linux/compiler.h>
29 #include <linux/types.h>
31 #include <asm/byteorder.h>
34 #include <linux/slab.h>
35 #include <linux/init.h>
36 #include <asm/atomic.h>
38 #include "crc32defs.h"
42 #define tole(x) cpu_to_le32(x)
43 #define tobe(x) cpu_to_be32(x)
48 #include "crc32table.h"
50 MODULE_AUTHOR("Matt Domsch <Matt_Domsch@dell.com>");
51 MODULE_DESCRIPTION("Ethernet CRC32 calculations");
52 MODULE_LICENSE("GPL");
55 * crc32_le() - Calculate bitwise little-endian Ethernet AUTODIN II CRC32
56 * @crc: seed value for computation. ~0 for Ethernet, sometimes 0 for
57 * other uses, or the previous crc32 value if computing incrementally.
58 * @p: pointer to buffer over which CRC is run
59 * @len: length of buffer @p
61 u32 crc32_le(u32 crc, unsigned char const *p, size_t len);
65 * In fact, the table-based code will work in this case, but it can be
66 * simplified by inlining the table in ?: form.
69 u32 crc32_le(u32 crc, unsigned char const *p, size_t len)
74 for (i = 0; i < 8; i++)
75 crc = (crc >> 1) ^ ((crc & 1) ? CRCPOLY_LE : 0);
79 #else /* Table-based approach */
81 u32 crc32_le(u32 crc, unsigned char const *p, size_t len)
84 const u32 *b =(u32 *)p;
85 const u32 *tab = crc32table_le;
87 # ifdef __LITTLE_ENDIAN
88 # define DO_CRC(x) crc = tab[ (crc ^ (x)) & 255 ] ^ (crc>>8)
90 # define DO_CRC(x) crc = tab[ ((crc >> 24) ^ (x)) & 255] ^ (crc<<8)
92 /* printf("Crc32_le crc=%x\n",crc); */
93 crc = __cpu_to_le32(crc);
95 if((((long)b)&3 && len)){
100 } while ((--len) && ((long)b)&3 );
103 /* load data 32 bits wide, xor data 32 bits wide. */
104 size_t save_len = len & 3;
106 --b; /* use pre increment below(*++b) for speed */
114 b++; /* point to next byte(s) */
117 /* And the last few bytes */
126 return __le32_to_cpu(crc);
130 # elif CRC_LE_BITS == 4
133 crc = (crc >> 4) ^ crc32table_le[crc & 15];
134 crc = (crc >> 4) ^ crc32table_le[crc & 15];
137 # elif CRC_LE_BITS == 2
140 crc = (crc >> 2) ^ crc32table_le[crc & 3];
141 crc = (crc >> 2) ^ crc32table_le[crc & 3];
142 crc = (crc >> 2) ^ crc32table_le[crc & 3];
143 crc = (crc >> 2) ^ crc32table_le[crc & 3];
151 * crc32_be() - Calculate bitwise big-endian Ethernet AUTODIN II CRC32
152 * @crc: seed value for computation. ~0 for Ethernet, sometimes 0 for
153 * other uses, or the previous crc32 value if computing incrementally.
154 * @p: pointer to buffer over which CRC is run
155 * @len: length of buffer @p
157 u32 __attribute_pure__ crc32_be(u32 crc, unsigned char const *p, size_t len);
161 * In fact, the table-based code will work in this case, but it can be
162 * simplified by inlining the table in ?: form.
165 u32 __attribute_pure__ crc32_be(u32 crc, unsigned char const *p, size_t len)
170 for (i = 0; i < 8; i++)
172 (crc << 1) ^ ((crc & 0x80000000) ? CRCPOLY_BE :
178 #else /* Table-based approach */
179 u32 __attribute_pure__ crc32_be(u32 crc, unsigned char const *p, size_t len)
181 # if CRC_BE_BITS == 8
182 const u32 *b =(u32 *)p;
183 const u32 *tab = crc32table_be;
185 # ifdef __LITTLE_ENDIAN
186 # define DO_CRC(x) crc = tab[ (crc ^ (x)) & 255 ] ^ (crc>>8)
188 # define DO_CRC(x) crc = tab[ ((crc >> 24) ^ (x)) & 255] ^ (crc<<8)
191 crc = __cpu_to_be32(crc);
193 if(unlikely(((long)b)&3 && len)){
198 } while ((--len) && ((long)b)&3 );
200 if(likely(len >= 4)){
201 /* load data 32 bits wide, xor data 32 bits wide. */
202 size_t save_len = len & 3;
204 --b; /* use pre increment below(*++b) for speed */
212 b++; /* point to next byte(s) */
215 /* And the last few bytes */
223 return __be32_to_cpu(crc);
227 # elif CRC_BE_BITS == 4
230 crc = (crc << 4) ^ crc32table_be[crc >> 28];
231 crc = (crc << 4) ^ crc32table_be[crc >> 28];
234 # elif CRC_BE_BITS == 2
237 crc = (crc << 2) ^ crc32table_be[crc >> 30];
238 crc = (crc << 2) ^ crc32table_be[crc >> 30];
239 crc = (crc << 2) ^ crc32table_be[crc >> 30];
240 crc = (crc << 2) ^ crc32table_be[crc >> 30];
247 EXPORT_SYMBOL(crc32_le);
248 EXPORT_SYMBOL(crc32_be);
251 * A brief CRC tutorial.
253 * A CRC is a long-division remainder. You add the CRC to the message,
254 * and the whole thing (message+CRC) is a multiple of the given
255 * CRC polynomial. To check the CRC, you can either check that the
256 * CRC matches the recomputed value, *or* you can check that the
257 * remainder computed on the message+CRC is 0. This latter approach
258 * is used by a lot of hardware implementations, and is why so many
259 * protocols put the end-of-frame flag after the CRC.
261 * It's actually the same long division you learned in school, except that
262 * - We're working in binary, so the digits are only 0 and 1, and
263 * - When dividing polynomials, there are no carries. Rather than add and
264 * subtract, we just xor. Thus, we tend to get a bit sloppy about
265 * the difference between adding and subtracting.
267 * A 32-bit CRC polynomial is actually 33 bits long. But since it's
268 * 33 bits long, bit 32 is always going to be set, so usually the CRC
269 * is written in hex with the most significant bit omitted. (If you're
270 * familiar with the IEEE 754 floating-point format, it's the same idea.)
272 * Note that a CRC is computed over a string of *bits*, so you have
273 * to decide on the endianness of the bits within each byte. To get
274 * the best error-detecting properties, this should correspond to the
275 * order they're actually sent. For example, standard RS-232 serial is
276 * little-endian; the most significant bit (sometimes used for parity)
277 * is sent last. And when appending a CRC word to a message, you should
278 * do it in the right order, matching the endianness.
280 * Just like with ordinary division, the remainder is always smaller than
281 * the divisor (the CRC polynomial) you're dividing by. Each step of the
282 * division, you take one more digit (bit) of the dividend and append it
283 * to the current remainder. Then you figure out the appropriate multiple
284 * of the divisor to subtract to being the remainder back into range.
285 * In binary, it's easy - it has to be either 0 or 1, and to make the
286 * XOR cancel, it's just a copy of bit 32 of the remainder.
288 * When computing a CRC, we don't care about the quotient, so we can
289 * throw the quotient bit away, but subtract the appropriate multiple of
290 * the polynomial from the remainder and we're back to where we started,
291 * ready to process the next bit.
293 * A big-endian CRC written this way would be coded like:
294 * for (i = 0; i < input_bits; i++) {
295 * multiple = remainder & 0x80000000 ? CRCPOLY : 0;
296 * remainder = (remainder << 1 | next_input_bit()) ^ multiple;
298 * Notice how, to get at bit 32 of the shifted remainder, we look
299 * at bit 31 of the remainder *before* shifting it.
301 * But also notice how the next_input_bit() bits we're shifting into
302 * the remainder don't actually affect any decision-making until
303 * 32 bits later. Thus, the first 32 cycles of this are pretty boring.
304 * Also, to add the CRC to a message, we need a 32-bit-long hole for it at
305 * the end, so we have to add 32 extra cycles shifting in zeros at the
306 * end of every message,
308 * So the standard trick is to rearrage merging in the next_input_bit()
309 * until the moment it's needed. Then the first 32 cycles can be precomputed,
310 * and merging in the final 32 zero bits to make room for the CRC can be
312 * This changes the code to:
313 * for (i = 0; i < input_bits; i++) {
314 * remainder ^= next_input_bit() << 31;
315 * multiple = (remainder & 0x80000000) ? CRCPOLY : 0;
316 * remainder = (remainder << 1) ^ multiple;
318 * With this optimization, the little-endian code is simpler:
319 * for (i = 0; i < input_bits; i++) {
320 * remainder ^= next_input_bit();
321 * multiple = (remainder & 1) ? CRCPOLY : 0;
322 * remainder = (remainder >> 1) ^ multiple;
325 * Note that the other details of endianness have been hidden in CRCPOLY
326 * (which must be bit-reversed) and next_input_bit().
328 * However, as long as next_input_bit is returning the bits in a sensible
329 * order, we can actually do the merging 8 or more bits at a time rather
330 * than one bit at a time:
331 * for (i = 0; i < input_bytes; i++) {
332 * remainder ^= next_input_byte() << 24;
333 * for (j = 0; j < 8; j++) {
334 * multiple = (remainder & 0x80000000) ? CRCPOLY : 0;
335 * remainder = (remainder << 1) ^ multiple;
338 * Or in little-endian:
339 * for (i = 0; i < input_bytes; i++) {
340 * remainder ^= next_input_byte();
341 * for (j = 0; j < 8; j++) {
342 * multiple = (remainder & 1) ? CRCPOLY : 0;
343 * remainder = (remainder << 1) ^ multiple;
346 * If the input is a multiple of 32 bits, you can even XOR in a 32-bit
347 * word at a time and increase the inner loop count to 32.
349 * You can also mix and match the two loop styles, for example doing the
350 * bulk of a message byte-at-a-time and adding bit-at-a-time processing
351 * for any fractional bytes at the end.
353 * The only remaining optimization is to the byte-at-a-time table method.
354 * Here, rather than just shifting one bit of the remainder to decide
355 * in the correct multiple to subtract, we can shift a byte at a time.
356 * This produces a 40-bit (rather than a 33-bit) intermediate remainder,
357 * but again the multiple of the polynomial to subtract depends only on
358 * the high bits, the high 8 bits in this case.
360 * The multile we need in that case is the low 32 bits of a 40-bit
361 * value whose high 8 bits are given, and which is a multiple of the
362 * generator polynomial. This is simply the CRC-32 of the given
365 * Two more details: normally, appending zero bits to a message which
366 * is already a multiple of a polynomial produces a larger multiple of that
367 * polynomial. To enable a CRC to detect this condition, it's common to
368 * invert the CRC before appending it. This makes the remainder of the
369 * message+crc come out not as zero, but some fixed non-zero value.
371 * The same problem applies to zero bits prepended to the message, and
372 * a similar solution is used. Instead of starting with a remainder of
373 * 0, an initial remainder of all ones is used. As long as you start
374 * the same way on decoding, it doesn't make a difference.
384 buf_dump(char const *prefix, unsigned char const *buf, size_t len)
386 fputs(prefix, stdout);
388 printf(" %02x", *buf++);
394 static void bytereverse(unsigned char *buf, size_t len)
397 unsigned char x = bitrev8(*buf);
402 static void random_garbage(unsigned char *buf, size_t len)
405 *buf++ = (unsigned char) random();
409 static void store_le(u32 x, unsigned char *buf)
411 buf[0] = (unsigned char) x;
412 buf[1] = (unsigned char) (x >> 8);
413 buf[2] = (unsigned char) (x >> 16);
414 buf[3] = (unsigned char) (x >> 24);
418 static void store_be(u32 x, unsigned char *buf)
420 buf[0] = (unsigned char) (x >> 24);
421 buf[1] = (unsigned char) (x >> 16);
422 buf[2] = (unsigned char) (x >> 8);
423 buf[3] = (unsigned char) x;
427 * This checks that CRC(buf + CRC(buf)) = 0, and that
428 * CRC commutes with bit-reversal. This has the side effect
429 * of bytewise bit-reversing the input buffer, and returns
430 * the CRC of the reversed buffer.
432 static u32 test_step(u32 init, unsigned char *buf, size_t len)
437 crc1 = crc32_be(init, buf, len);
438 store_be(crc1, buf + len);
439 crc2 = crc32_be(init, buf, len + 4);
441 printf("\nCRC cancellation fail: 0x%08x should be 0\n",
444 for (i = 0; i <= len + 4; i++) {
445 crc2 = crc32_be(init, buf, i);
446 crc2 = crc32_be(crc2, buf + i, len + 4 - i);
448 printf("\nCRC split fail: 0x%08x\n", crc2);
451 /* Now swap it around for the other test */
453 bytereverse(buf, len + 4);
454 init = bitrev32(init);
455 crc2 = bitrev32(crc1);
456 if (crc1 != bitrev32(crc2))
457 printf("\nBit reversal fail: 0x%08x -> 0x%08x -> 0x%08x\n",
458 crc1, crc2, bitrev32(crc2));
459 crc1 = crc32_le(init, buf, len);
461 printf("\nCRC endianness fail: 0x%08x != 0x%08x\n", crc1,
463 crc2 = crc32_le(init, buf, len + 4);
465 printf("\nCRC cancellation fail: 0x%08x should be 0\n",
468 for (i = 0; i <= len + 4; i++) {
469 crc2 = crc32_le(init, buf, i);
470 crc2 = crc32_le(crc2, buf + i, len + 4 - i);
472 printf("\nCRC split fail: 0x%08x\n", crc2);
484 unsigned char buf1[SIZE + 4];
485 unsigned char buf2[SIZE + 4];
486 unsigned char buf3[SIZE + 4];
488 u32 crc1, crc2, crc3;
490 for (i = 0; i <= SIZE; i++) {
491 printf("\rTesting length %d...", i);
493 random_garbage(buf1, i);
494 random_garbage(buf2, i);
495 for (j = 0; j < i; j++)
496 buf3[j] = buf1[j] ^ buf2[j];
498 crc1 = test_step(INIT1, buf1, i);
499 crc2 = test_step(INIT2, buf2, i);
500 /* Now check that CRC(buf1 ^ buf2) = CRC(buf1) ^ CRC(buf2) */
501 crc3 = test_step(INIT1 ^ INIT2, buf3, i);
502 if (crc3 != (crc1 ^ crc2))
503 printf("CRC XOR fail: 0x%08x != 0x%08x ^ 0x%08x\n",
506 printf("\nAll test complete. No failures expected.\n");
510 #endif /* UNITTEST */