linux/lib/crc32.c
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   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/init.h>
  29#include <linux/atomic.h>
  30#include "crc32defs.h"
  31#if CRC_LE_BITS == 8
  32# define tole(x) __constant_cpu_to_le32(x)
  33#else
  34# define tole(x) (x)
  35#endif
  36
  37#if CRC_BE_BITS == 8
  38# define tobe(x) __constant_cpu_to_be32(x)
  39#else
  40# define tobe(x) (x)
  41#endif
  42#include "crc32table.h"
  43
  44MODULE_AUTHOR("Matt Domsch <Matt_Domsch@dell.com>");
  45MODULE_DESCRIPTION("Ethernet CRC32 calculations");
  46MODULE_LICENSE("GPL");
  47
  48#if CRC_LE_BITS == 8 || CRC_BE_BITS == 8
  49
  50static inline u32
  51crc32_body(u32 crc, unsigned char const *buf, size_t len, const u32 (*tab)[256])
  52{
  53# ifdef __LITTLE_ENDIAN
  54#  define DO_CRC(x) crc = tab[0][(crc ^ (x)) & 255] ^ (crc >> 8)
  55#  define DO_CRC4 crc = tab[3][(crc) & 255] ^ \
  56                tab[2][(crc >> 8) & 255] ^ \
  57                tab[1][(crc >> 16) & 255] ^ \
  58                tab[0][(crc >> 24) & 255]
  59# else
  60#  define DO_CRC(x) crc = tab[0][((crc >> 24) ^ (x)) & 255] ^ (crc << 8)
  61#  define DO_CRC4 crc = tab[0][(crc) & 255] ^ \
  62                tab[1][(crc >> 8) & 255] ^ \
  63                tab[2][(crc >> 16) & 255] ^ \
  64                tab[3][(crc >> 24) & 255]
  65# endif
  66        const u32 *b;
  67        size_t    rem_len;
  68
  69        /* Align it */
  70        if (unlikely((long)buf & 3 && len)) {
  71                do {
  72                        DO_CRC(*buf++);
  73                } while ((--len) && ((long)buf)&3);
  74        }
  75        rem_len = len & 3;
  76        /* load data 32 bits wide, xor data 32 bits wide. */
  77        len = len >> 2;
  78        b = (const u32 *)buf;
  79        for (--b; len; --len) {
  80                crc ^= *++b; /* use pre increment for speed */
  81                DO_CRC4;
  82        }
  83        len = rem_len;
  84        /* And the last few bytes */
  85        if (len) {
  86                u8 *p = (u8 *)(b + 1) - 1;
  87                do {
  88                        DO_CRC(*++p); /* use pre increment for speed */
  89                } while (--len);
  90        }
  91        return crc;
  92#undef DO_CRC
  93#undef DO_CRC4
  94}
  95#endif
  96/**
  97 * crc32_le() - Calculate bitwise little-endian Ethernet AUTODIN II CRC32
  98 * @crc: seed value for computation.  ~0 for Ethernet, sometimes 0 for
  99 *      other uses, or the previous crc32 value if computing incrementally.
 100 * @p: pointer to buffer over which CRC is run
 101 * @len: length of buffer @p
 102 */
 103u32 __pure crc32_le(u32 crc, unsigned char const *p, size_t len);
 104
 105#if CRC_LE_BITS == 1
 106/*
 107 * In fact, the table-based code will work in this case, but it can be
 108 * simplified by inlining the table in ?: form.
 109 */
 110
 111u32 __pure crc32_le(u32 crc, unsigned char const *p, size_t len)
 112{
 113        int i;
 114        while (len--) {
 115                crc ^= *p++;
 116                for (i = 0; i < 8; i++)
 117                        crc = (crc >> 1) ^ ((crc & 1) ? CRCPOLY_LE : 0);
 118        }
 119        return crc;
 120}
 121#else                           /* Table-based approach */
 122
 123u32 __pure crc32_le(u32 crc, unsigned char const *p, size_t len)
 124{
 125# if CRC_LE_BITS == 8
 126        const u32      (*tab)[] = crc32table_le;
 127
 128        crc = __cpu_to_le32(crc);
 129        crc = crc32_body(crc, p, len, tab);
 130        return __le32_to_cpu(crc);
 131# elif CRC_LE_BITS == 4
 132        while (len--) {
 133                crc ^= *p++;
 134                crc = (crc >> 4) ^ crc32table_le[crc & 15];
 135                crc = (crc >> 4) ^ crc32table_le[crc & 15];
 136        }
 137        return crc;
 138# elif CRC_LE_BITS == 2
 139        while (len--) {
 140                crc ^= *p++;
 141                crc = (crc >> 2) ^ crc32table_le[crc & 3];
 142                crc = (crc >> 2) ^ crc32table_le[crc & 3];
 143                crc = (crc >> 2) ^ crc32table_le[crc & 3];
 144                crc = (crc >> 2) ^ crc32table_le[crc & 3];
 145        }
 146        return crc;
 147# endif
 148}
 149#endif
 150
 151/**
 152 * crc32_be() - Calculate bitwise big-endian Ethernet AUTODIN II CRC32
 153 * @crc: seed value for computation.  ~0 for Ethernet, sometimes 0 for
 154 *      other uses, or the previous crc32 value if computing incrementally.
 155 * @p: pointer to buffer over which CRC is run
 156 * @len: length of buffer @p
 157 */
 158u32 __pure crc32_be(u32 crc, unsigned char const *p, size_t len);
 159
 160#if CRC_BE_BITS == 1
 161/*
 162 * In fact, the table-based code will work in this case, but it can be
 163 * simplified by inlining the table in ?: form.
 164 */
 165
 166u32 __pure crc32_be(u32 crc, unsigned char const *p, size_t len)
 167{
 168        int i;
 169        while (len--) {
 170                crc ^= *p++ << 24;
 171                for (i = 0; i < 8; i++)
 172                        crc =
 173                            (crc << 1) ^ ((crc & 0x80000000) ? CRCPOLY_BE :
 174                                          0);
 175        }
 176        return crc;
 177}
 178
 179#else                           /* Table-based approach */
 180u32 __pure crc32_be(u32 crc, unsigned char const *p, size_t len)
 181{
 182# if CRC_BE_BITS == 8
 183        const u32      (*tab)[] = crc32table_be;
 184
 185        crc = __cpu_to_be32(crc);
 186        crc = crc32_body(crc, p, len, tab);
 187        return __be32_to_cpu(crc);
 188# elif CRC_BE_BITS == 4
 189        while (len--) {
 190                crc ^= *p++ << 24;
 191                crc = (crc << 4) ^ crc32table_be[crc >> 28];
 192                crc = (crc << 4) ^ crc32table_be[crc >> 28];
 193        }
 194        return crc;
 195# elif CRC_BE_BITS == 2
 196        while (len--) {
 197                crc ^= *p++ << 24;
 198                crc = (crc << 2) ^ crc32table_be[crc >> 30];
 199                crc = (crc << 2) ^ crc32table_be[crc >> 30];
 200                crc = (crc << 2) ^ crc32table_be[crc >> 30];
 201                crc = (crc << 2) ^ crc32table_be[crc >> 30];
 202        }
 203        return crc;
 204# endif
 205}
 206#endif
 207
 208EXPORT_SYMBOL(crc32_le);
 209EXPORT_SYMBOL(crc32_be);
 210
 211/*
 212 * A brief CRC tutorial.
 213 *
 214 * A CRC is a long-division remainder.  You add the CRC to the message,
 215 * and the whole thing (message+CRC) is a multiple of the given
 216 * CRC polynomial.  To check the CRC, you can either check that the
 217 * CRC matches the recomputed value, *or* you can check that the
 218 * remainder computed on the message+CRC is 0.  This latter approach
 219 * is used by a lot of hardware implementations, and is why so many
 220 * protocols put the end-of-frame flag after the CRC.
 221 *
 222 * It's actually the same long division you learned in school, except that
 223 * - We're working in binary, so the digits are only 0 and 1, and
 224 * - When dividing polynomials, there are no carries.  Rather than add and
 225 *   subtract, we just xor.  Thus, we tend to get a bit sloppy about
 226 *   the difference between adding and subtracting.
 227 *
 228 * A 32-bit CRC polynomial is actually 33 bits long.  But since it's
 229 * 33 bits long, bit 32 is always going to be set, so usually the CRC
 230 * is written in hex with the most significant bit omitted.  (If you're
 231 * familiar with the IEEE 754 floating-point format, it's the same idea.)
 232 *
 233 * Note that a CRC is computed over a string of *bits*, so you have
 234 * to decide on the endianness of the bits within each byte.  To get
 235 * the best error-detecting properties, this should correspond to the
 236 * order they're actually sent.  For example, standard RS-232 serial is
 237 * little-endian; the most significant bit (sometimes used for parity)
 238 * is sent last.  And when appending a CRC word to a message, you should
 239 * do it in the right order, matching the endianness.
 240 *
 241 * Just like with ordinary division, the remainder is always smaller than
 242 * the divisor (the CRC polynomial) you're dividing by.  Each step of the
 243 * division, you take one more digit (bit) of the dividend and append it
 244 * to the current remainder.  Then you figure out the appropriate multiple
 245 * of the divisor to subtract to being the remainder back into range.
 246 * In binary, it's easy - it has to be either 0 or 1, and to make the
 247 * XOR cancel, it's just a copy of bit 32 of the remainder.
 248 *
 249 * When computing a CRC, we don't care about the quotient, so we can
 250 * throw the quotient bit away, but subtract the appropriate multiple of
 251 * the polynomial from the remainder and we're back to where we started,
 252 * ready to process the next bit.
 253 *
 254 * A big-endian CRC written this way would be coded like:
 255 * for (i = 0; i < input_bits; i++) {
 256 *      multiple = remainder & 0x80000000 ? CRCPOLY : 0;
 257 *      remainder = (remainder << 1 | next_input_bit()) ^ multiple;
 258 * }
 259 * Notice how, to get at bit 32 of the shifted remainder, we look
 260 * at bit 31 of the remainder *before* shifting it.
 261 *
 262 * But also notice how the next_input_bit() bits we're shifting into
 263 * the remainder don't actually affect any decision-making until
 264 * 32 bits later.  Thus, the first 32 cycles of this are pretty boring.
 265 * Also, to add the CRC to a message, we need a 32-bit-long hole for it at
 266 * the end, so we have to add 32 extra cycles shifting in zeros at the
 267 * end of every message,
 268 *
 269 * So the standard trick is to rearrage merging in the next_input_bit()
 270 * until the moment it's needed.  Then the first 32 cycles can be precomputed,
 271 * and merging in the final 32 zero bits to make room for the CRC can be
 272 * skipped entirely.
 273 * This changes the code to:
 274 * for (i = 0; i < input_bits; i++) {
 275 *      remainder ^= next_input_bit() << 31;
 276 *      multiple = (remainder & 0x80000000) ? CRCPOLY : 0;
 277 *      remainder = (remainder << 1) ^ multiple;
 278 * }
 279 * With this optimization, the little-endian code is simpler:
 280 * for (i = 0; i < input_bits; i++) {
 281 *      remainder ^= next_input_bit();
 282 *      multiple = (remainder & 1) ? CRCPOLY : 0;
 283 *      remainder = (remainder >> 1) ^ multiple;
 284 * }
 285 *
 286 * Note that the other details of endianness have been hidden in CRCPOLY
 287 * (which must be bit-reversed) and next_input_bit().
 288 *
 289 * However, as long as next_input_bit is returning the bits in a sensible
 290 * order, we can actually do the merging 8 or more bits at a time rather
 291 * than one bit at a time:
 292 * for (i = 0; i < input_bytes; i++) {
 293 *      remainder ^= next_input_byte() << 24;
 294 *      for (j = 0; j < 8; j++) {
 295 *              multiple = (remainder & 0x80000000) ? CRCPOLY : 0;
 296 *              remainder = (remainder << 1) ^ multiple;
 297 *      }
 298 * }
 299 * Or in little-endian:
 300 * for (i = 0; i < input_bytes; i++) {
 301 *      remainder ^= next_input_byte();
 302 *      for (j = 0; j < 8; j++) {
 303 *              multiple = (remainder & 1) ? CRCPOLY : 0;
 304 *              remainder = (remainder << 1) ^ multiple;
 305 *      }
 306 * }
 307 * If the input is a multiple of 32 bits, you can even XOR in a 32-bit
 308 * word at a time and increase the inner loop count to 32.
 309 *
 310 * You can also mix and match the two loop styles, for example doing the
 311 * bulk of a message byte-at-a-time and adding bit-at-a-time processing
 312 * for any fractional bytes at the end.
 313 *
 314 * The only remaining optimization is to the byte-at-a-time table method.
 315 * Here, rather than just shifting one bit of the remainder to decide
 316 * in the correct multiple to subtract, we can shift a byte at a time.
 317 * This produces a 40-bit (rather than a 33-bit) intermediate remainder,
 318 * but again the multiple of the polynomial to subtract depends only on
 319 * the high bits, the high 8 bits in this case.  
 320 *
 321 * The multiple we need in that case is the low 32 bits of a 40-bit
 322 * value whose high 8 bits are given, and which is a multiple of the
 323 * generator polynomial.  This is simply the CRC-32 of the given
 324 * one-byte message.
 325 *
 326 * Two more details: normally, appending zero bits to a message which
 327 * is already a multiple of a polynomial produces a larger multiple of that
 328 * polynomial.  To enable a CRC to detect this condition, it's common to
 329 * invert the CRC before appending it.  This makes the remainder of the
 330 * message+crc come out not as zero, but some fixed non-zero value.
 331 *
 332 * The same problem applies to zero bits prepended to the message, and
 333 * a similar solution is used.  Instead of starting with a remainder of
 334 * 0, an initial remainder of all ones is used.  As long as you start
 335 * the same way on decoding, it doesn't make a difference.
 336 */
 337
 338#ifdef UNITTEST
 339
 340#include <stdlib.h>
 341#include <stdio.h>
 342
 343#if 0                           /*Not used at present */
 344static void
 345buf_dump(char const *prefix, unsigned char const *buf, size_t len)
 346{
 347        fputs(prefix, stdout);
 348        while (len--)
 349                printf(" %02x", *buf++);
 350        putchar('\n');
 351
 352}
 353#endif
 354
 355static void bytereverse(unsigned char *buf, size_t len)
 356{
 357        while (len--) {
 358                unsigned char x = bitrev8(*buf);
 359                *buf++ = x;
 360        }
 361}
 362
 363static void random_garbage(unsigned char *buf, size_t len)
 364{
 365        while (len--)
 366                *buf++ = (unsigned char) random();
 367}
 368
 369#if 0                           /* Not used at present */
 370static void store_le(u32 x, unsigned char *buf)
 371{
 372        buf[0] = (unsigned char) x;
 373        buf[1] = (unsigned char) (x >> 8);
 374        buf[2] = (unsigned char) (x >> 16);
 375        buf[3] = (unsigned char) (x >> 24);
 376}
 377#endif
 378
 379static void store_be(u32 x, unsigned char *buf)
 380{
 381        buf[0] = (unsigned char) (x >> 24);
 382        buf[1] = (unsigned char) (x >> 16);
 383        buf[2] = (unsigned char) (x >> 8);
 384        buf[3] = (unsigned char) x;
 385}
 386
 387/*
 388 * This checks that CRC(buf + CRC(buf)) = 0, and that
 389 * CRC commutes with bit-reversal.  This has the side effect
 390 * of bytewise bit-reversing the input buffer, and returns
 391 * the CRC of the reversed buffer.
 392 */
 393static u32 test_step(u32 init, unsigned char *buf, size_t len)
 394{
 395        u32 crc1, crc2;
 396        size_t i;
 397
 398        crc1 = crc32_be(init, buf, len);
 399        store_be(crc1, buf + len);
 400        crc2 = crc32_be(init, buf, len + 4);
 401        if (crc2)
 402                printf("\nCRC cancellation fail: 0x%08x should be 0\n",
 403                       crc2);
 404
 405        for (i = 0; i <= len + 4; i++) {
 406                crc2 = crc32_be(init, buf, i);
 407                crc2 = crc32_be(crc2, buf + i, len + 4 - i);
 408                if (crc2)
 409                        printf("\nCRC split fail: 0x%08x\n", crc2);
 410        }
 411
 412        /* Now swap it around for the other test */
 413
 414        bytereverse(buf, len + 4);
 415        init = bitrev32(init);
 416        crc2 = bitrev32(crc1);
 417        if (crc1 != bitrev32(crc2))
 418                printf("\nBit reversal fail: 0x%08x -> 0x%08x -> 0x%08x\n",
 419                       crc1, crc2, bitrev32(crc2));
 420        crc1 = crc32_le(init, buf, len);
 421        if (crc1 != crc2)
 422                printf("\nCRC endianness fail: 0x%08x != 0x%08x\n", crc1,
 423                       crc2);
 424        crc2 = crc32_le(init, buf, len + 4);
 425        if (crc2)
 426                printf("\nCRC cancellation fail: 0x%08x should be 0\n",
 427                       crc2);
 428
 429        for (i = 0; i <= len + 4; i++) {
 430                crc2 = crc32_le(init, buf, i);
 431                crc2 = crc32_le(crc2, buf + i, len + 4 - i);
 432                if (crc2)
 433                        printf("\nCRC split fail: 0x%08x\n", crc2);
 434        }
 435
 436        return crc1;
 437}
 438
 439#define SIZE 64
 440#define INIT1 0
 441#define INIT2 0
 442
 443int main(void)
 444{
 445        unsigned char buf1[SIZE + 4];
 446        unsigned char buf2[SIZE + 4];
 447        unsigned char buf3[SIZE + 4];
 448        int i, j;
 449        u32 crc1, crc2, crc3;
 450
 451        for (i = 0; i <= SIZE; i++) {
 452                printf("\rTesting length %d...", i);
 453                fflush(stdout);
 454                random_garbage(buf1, i);
 455                random_garbage(buf2, i);
 456                for (j = 0; j < i; j++)
 457                        buf3[j] = buf1[j] ^ buf2[j];
 458
 459                crc1 = test_step(INIT1, buf1, i);
 460                crc2 = test_step(INIT2, buf2, i);
 461                /* Now check that CRC(buf1 ^ buf2) = CRC(buf1) ^ CRC(buf2) */
 462                crc3 = test_step(INIT1 ^ INIT2, buf3, i);
 463                if (crc3 != (crc1 ^ crc2))
 464                        printf("CRC XOR fail: 0x%08x != 0x%08x ^ 0x%08x\n",
 465                               crc3, crc1, crc2);
 466        }
 467        printf("\nAll test complete.  No failures expected.\n");
 468        return 0;
 469}
 470
 471#endif                          /* UNITTEST */
 472
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