/* * Oct 15, 2000 Matt Domsch <Matt_Domsch@dell.com> * Nicer crc32 functions/docs submitted by linux@horizon.com. Thanks! * Code was from the public domain, copyright abandoned. Code was * subsequently included in the kernel, thus was re-licensed under the * GNU GPL v2. * * Oct 12, 2000 Matt Domsch <Matt_Domsch@dell.com> * Same crc32 function was used in 5 other places in the kernel. * I made one version, and deleted the others. * There are various incantations of crc32(). Some use a seed of 0 or ~0. * Some xor at the end with ~0. The generic crc32() function takes * seed as an argument, and doesn't xor at the end. Then individual * users can do whatever they need. * drivers/net/smc9194.c uses seed ~0, doesn't xor with ~0. * fs/jffs2 uses seed 0, doesn't xor with ~0. * fs/partitions/efi.c uses seed ~0, xor's with ~0. * * This source code is licensed under the GNU General Public License, * Version 2. See the file COPYING for more details. */ #include <linux/crc32.h> #include <linux/kernel.h> #include <linux/module.h> #include <linux/compiler.h> #include <linux/types.h> #include <linux/init.h> #include <asm/atomic.h> #include "crc32defs.h" #if CRC_LE_BITS == 8 # define tole(x) __constant_cpu_to_le32(x) #else # define tole(x) (x) #endif #if CRC_BE_BITS == 8 # define tobe(x) __constant_cpu_to_be32(x) #else # define tobe(x) (x) #endif #include "crc32table.h" MODULE_AUTHOR("Matt Domsch <Matt_Domsch@dell.com>"); MODULE_DESCRIPTION("Ethernet CRC32 calculations"); MODULE_LICENSE("GPL"); #if CRC_LE_BITS == 8 || CRC_BE_BITS == 8 static inline u32 crc32_body(u32 crc, unsigned char const *buf, size_t len, const u32 *tab) { # ifdef __LITTLE_ENDIAN # define DO_CRC(x) crc = tab[(crc ^ (x)) & 255 ] ^ (crc >> 8) # else # define DO_CRC(x) crc = tab[((crc >> 24) ^ (x)) & 255] ^ (crc << 8) # endif const u32 *b; size_t rem_len; /* Align it */ if (unlikely((long)buf & 3 && len)) { do { DO_CRC(*buf++); } while ((--len) && ((long)buf)&3); } rem_len = len & 3; /* load data 32 bits wide, xor data 32 bits wide. */ len = len >> 2; b = (const u32 *)buf; for (--b; len; --len) { crc ^= *++b; /* use pre increment for speed */ DO_CRC(0); DO_CRC(0); DO_CRC(0); DO_CRC(0); } len = rem_len; /* And the last few bytes */ if (len) { u8 *p = (u8 *)(b + 1) - 1; do { DO_CRC(*++p); /* use pre increment for speed */ } while (--len); } return crc; #undef DO_CRC } #endif /** * crc32_le() - Calculate bitwise little-endian Ethernet AUTODIN II CRC32 * @crc: seed value for computation. ~0 for Ethernet, sometimes 0 for * other uses, or the previous crc32 value if computing incrementally. * @p: pointer to buffer over which CRC is run * @len: length of buffer @p */ u32 __pure crc32_le(u32 crc, unsigned char const *p, size_t len); #if CRC_LE_BITS == 1 /* * In fact, the table-based code will work in this case, but it can be * simplified by inlining the table in ?: form. */ u32 __pure crc32_le(u32 crc, unsigned char const *p, size_t len) { int i; while (len--) { crc ^= *p++; for (i = 0; i < 8; i++) crc = (crc >> 1) ^ ((crc & 1) ? CRCPOLY_LE : 0); } return crc; } #else /* Table-based approach */ u32 __pure crc32_le(u32 crc, unsigned char const *p, size_t len) { # if CRC_LE_BITS == 8 const u32 *tab = crc32table_le; crc = __cpu_to_le32(crc); crc = crc32_body(crc, p, len, tab); return __le32_to_cpu(crc); # elif CRC_LE_BITS == 4 while (len--) { crc ^= *p++; crc = (crc >> 4) ^ crc32table_le[crc & 15]; crc = (crc >> 4) ^ crc32table_le[crc & 15]; } return crc; # elif CRC_LE_BITS == 2 while (len--) { crc ^= *p++; crc = (crc >> 2) ^ crc32table_le[crc & 3]; crc = (crc >> 2) ^ crc32table_le[crc & 3]; crc = (crc >> 2) ^ crc32table_le[crc & 3]; crc = (crc >> 2) ^ crc32table_le[crc & 3]; } return crc; # endif } #endif /** * crc32_be() - Calculate bitwise big-endian Ethernet AUTODIN II CRC32 * @crc: seed value for computation. ~0 for Ethernet, sometimes 0 for * other uses, or the previous crc32 value if computing incrementally. * @p: pointer to buffer over which CRC is run * @len: length of buffer @p */ u32 __pure crc32_be(u32 crc, unsigned char const *p, size_t len); #if CRC_BE_BITS == 1 /* * In fact, the table-based code will work in this case, but it can be * simplified by inlining the table in ?: form. */ u32 __pure crc32_be(u32 crc, unsigned char const *p, size_t len) { int i; while (len--) { crc ^= *p++ << 24; for (i = 0; i < 8; i++) crc = (crc << 1) ^ ((crc & 0x80000000) ? CRCPOLY_BE : 0); } return crc; } #else /* Table-based approach */ u32 __pure crc32_be(u32 crc, unsigned char const *p, size_t len) { # if CRC_BE_BITS == 8 const u32 *tab = crc32table_be; crc = __cpu_to_be32(crc); crc = crc32_body(crc, p, len, tab); return __be32_to_cpu(crc); # elif CRC_BE_BITS == 4 while (len--) { crc ^= *p++ << 24; crc = (crc << 4) ^ crc32table_be[crc >> 28]; crc = (crc << 4) ^ crc32table_be[crc >> 28]; } return crc; # elif CRC_BE_BITS == 2 while (len--) { crc ^= *p++ << 24; crc = (crc << 2) ^ crc32table_be[crc >> 30]; crc = (crc << 2) ^ crc32table_be[crc >> 30]; crc = (crc << 2) ^ crc32table_be[crc >> 30]; crc = (crc << 2) ^ crc32table_be[crc >> 30]; } return crc; # endif } #endif EXPORT_SYMBOL(crc32_le); EXPORT_SYMBOL(crc32_be); /* * A brief CRC tutorial. * * A CRC is a long-division remainder. You add the CRC to the message, * and the whole thing (message+CRC) is a multiple of the given * CRC polynomial. To check the CRC, you can either check that the * CRC matches the recomputed value, *or* you can check that the * remainder computed on the message+CRC is 0. This latter approach * is used by a lot of hardware implementations, and is why so many * protocols put the end-of-frame flag after the CRC. * * It's actually the same long division you learned in school, except that * - We're working in binary, so the digits are only 0 and 1, and * - When dividing polynomials, there are no carries. Rather than add and * subtract, we just xor. Thus, we tend to get a bit sloppy about * the difference between adding and subtracting. * * A 32-bit CRC polynomial is actually 33 bits long. But since it's * 33 bits long, bit 32 is always going to be set, so usually the CRC * is written in hex with the most significant bit omitted. (If you're * familiar with the IEEE 754 floating-point format, it's the same idea.) * * Note that a CRC is computed over a string of *bits*, so you have * to decide on the endianness of the bits within each byte. To get * the best error-detecting properties, this should correspond to the * order they're actually sent. For example, standard RS-232 serial is * little-endian; the most significant bit (sometimes used for parity) * is sent last. And when appending a CRC word to a message, you should * do it in the right order, matching the endianness. * * Just like with ordinary division, the remainder is always smaller than * the divisor (the CRC polynomial) you're dividing by. Each step of the * division, you take one more digit (bit) of the dividend and append it * to the current remainder. Then you figure out the appropriate multiple * of the divisor to subtract to being the remainder back into range. * In binary, it's easy - it has to be either 0 or 1, and to make the * XOR cancel, it's just a copy of bit 32 of the remainder. * * When computing a CRC, we don't care about the quotient, so we can * throw the quotient bit away, but subtract the appropriate multiple of * the polynomial from the remainder and we're back to where we started, * ready to process the next bit. * * A big-endian CRC written this way would be coded like: * for (i = 0; i < input_bits; i++) { * multiple = remainder & 0x80000000 ? CRCPOLY : 0; * remainder = (remainder << 1 | next_input_bit()) ^ multiple; * } * Notice how, to get at bit 32 of the shifted remainder, we look * at bit 31 of the remainder *before* shifting it. * * But also notice how the next_input_bit() bits we're shifting into * the remainder don't actually affect any decision-making until * 32 bits later. Thus, the first 32 cycles of this are pretty boring. * Also, to add the CRC to a message, we need a 32-bit-long hole for it at * the end, so we have to add 32 extra cycles shifting in zeros at the * end of every message, * * So the standard trick is to rearrage merging in the next_input_bit() * until the moment it's needed. Then the first 32 cycles can be precomputed, * and merging in the final 32 zero bits to make room for the CRC can be * skipped entirely. * This changes the code to: * for (i = 0; i < input_bits; i++) { * remainder ^= next_input_bit() << 31; * multiple = (remainder & 0x80000000) ? CRCPOLY : 0; * remainder = (remainder << 1) ^ multiple; * } * With this optimization, the little-endian code is simpler: * for (i = 0; i < input_bits; i++) { * remainder ^= next_input_bit(); * multiple = (remainder & 1) ? CRCPOLY : 0; * remainder = (remainder >> 1) ^ multiple; * } * * Note that the other details of endianness have been hidden in CRCPOLY * (which must be bit-reversed) and next_input_bit(). * * However, as long as next_input_bit is returning the bits in a sensible * order, we can actually do the merging 8 or more bits at a time rather * than one bit at a time: * for (i = 0; i < input_bytes; i++) { * remainder ^= next_input_byte() << 24; * for (j = 0; j < 8; j++) { * multiple = (remainder & 0x80000000) ? CRCPOLY : 0; * remainder = (remainder << 1) ^ multiple; * } * } * Or in little-endian: * for (i = 0; i < input_bytes; i++) { * remainder ^= next_input_byte(); * for (j = 0; j < 8; j++) { * multiple = (remainder & 1) ? CRCPOLY : 0; * remainder = (remainder << 1) ^ multiple; * } * } * If the input is a multiple of 32 bits, you can even XOR in a 32-bit * word at a time and increase the inner loop count to 32. * * You can also mix and match the two loop styles, for example doing the * bulk of a message byte-at-a-time and adding bit-at-a-time processing * for any fractional bytes at the end. * * The only remaining optimization is to the byte-at-a-time table method. * Here, rather than just shifting one bit of the remainder to decide * in the correct multiple to subtract, we can shift a byte at a time. * This produces a 40-bit (rather than a 33-bit) intermediate remainder, * but again the multiple of the polynomial to subtract depends only on * the high bits, the high 8 bits in this case. * * The multiple we need in that case is the low 32 bits of a 40-bit * value whose high 8 bits are given, and which is a multiple of the * generator polynomial. This is simply the CRC-32 of the given * one-byte message. * * Two more details: normally, appending zero bits to a message which * is already a multiple of a polynomial produces a larger multiple of that * polynomial. To enable a CRC to detect this condition, it's common to * invert the CRC before appending it. This makes the remainder of the * message+crc come out not as zero, but some fixed non-zero value. * * The same problem applies to zero bits prepended to the message, and * a similar solution is used. Instead of starting with a remainder of * 0, an initial remainder of all ones is used. As long as you start * the same way on decoding, it doesn't make a difference. */ #ifdef UNITTEST #include <stdlib.h> #include <stdio.h> #if 0 /*Not used at present */ static void buf_dump(char const *prefix, unsigned char const *buf, size_t len) { fputs(prefix, stdout); while (len--) printf(" %02x", *buf++); putchar('\n'); } #endif static void bytereverse(unsigned char *buf, size_t len) { while (len--) { unsigned char x = bitrev8(*buf); *buf++ = x; } } static void random_garbage(unsigned char *buf, size_t len) { while (len--) *buf++ = (unsigned char) random(); } #if 0 /* Not used at present */ static void store_le(u32 x, unsigned char *buf) { buf[0] = (unsigned char) x; buf[1] = (unsigned char) (x >> 8); buf[2] = (unsigned char) (x >> 16); buf[3] = (unsigned char) (x >> 24); } #endif static void store_be(u32 x, unsigned char *buf) { buf[0] = (unsigned char) (x >> 24); buf[1] = (unsigned char) (x >> 16); buf[2] = (unsigned char) (x >> 8); buf[3] = (unsigned char) x; } /* * This checks that CRC(buf + CRC(buf)) = 0, and that * CRC commutes with bit-reversal. This has the side effect * of bytewise bit-reversing the input buffer, and returns * the CRC of the reversed buffer. */ static u32 test_step(u32 init, unsigned char *buf, size_t len) { u32 crc1, crc2; size_t i; crc1 = crc32_be(init, buf, len); store_be(crc1, buf + len); crc2 = crc32_be(init, buf, len + 4); if (crc2) printf("\nCRC cancellation fail: 0x%08x should be 0\n", crc2); for (i = 0; i <= len + 4; i++) { crc2 = crc32_be(init, buf, i); crc2 = crc32_be(crc2, buf + i, len + 4 - i); if (crc2) printf("\nCRC split fail: 0x%08x\n", crc2); } /* Now swap it around for the other test */ bytereverse(buf, len + 4); init = bitrev32(init); crc2 = bitrev32(crc1); if (crc1 != bitrev32(crc2)) printf("\nBit reversal fail: 0x%08x -> 0x%08x -> 0x%08x\n", crc1, crc2, bitrev32(crc2)); crc1 = crc32_le(init, buf, len); if (crc1 != crc2) printf("\nCRC endianness fail: 0x%08x != 0x%08x\n", crc1, crc2); crc2 = crc32_le(init, buf, len + 4); if (crc2) printf("\nCRC cancellation fail: 0x%08x should be 0\n", crc2); for (i = 0; i <= len + 4; i++) { crc2 = crc32_le(init, buf, i); crc2 = crc32_le(crc2, buf + i, len + 4 - i); if (crc2) printf("\nCRC split fail: 0x%08x\n", crc2); } return crc1; } #define SIZE 64 #define INIT1 0 #define INIT2 0 int main(void) { unsigned char buf1[SIZE + 4]; unsigned char buf2[SIZE + 4]; unsigned char buf3[SIZE + 4]; int i, j; u32 crc1, crc2, crc3; for (i = 0; i <= SIZE; i++) { printf("\rTesting length %d...", i); fflush(stdout); random_garbage(buf1, i); random_garbage(buf2, i); for (j = 0; j < i; j++) buf3[j] = buf1[j] ^ buf2[j]; crc1 = test_step(INIT1, buf1, i); crc2 = test_step(INIT2, buf2, i); /* Now check that CRC(buf1 ^ buf2) = CRC(buf1) ^ CRC(buf2) */ crc3 = test_step(INIT1 ^ INIT2, buf3, i); if (crc3 != (crc1 ^ crc2)) printf("CRC XOR fail: 0x%08x != 0x%08x ^ 0x%08x\n", crc3, crc1, crc2); } printf("\nAll test complete. No failures expected.\n"); return 0; } #endif /* UNITTEST */