linux / linux / kernel / git / torvalds / linux / 79e453d49bd49ba1b576f89310cc565c9e4ca379 / . / lib / crc32.c

/* | |

* 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/slab.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) | |

#define tobe(x) __constant_cpu_to_be32(x) | |

#else | |

#define tole(x) (x) | |

#define tobe(x) (x) | |

#endif | |

#include "crc32table.h" | |

MODULE_AUTHOR("Matt Domsch <Matt_Domsch@dell.com>"); | |

MODULE_DESCRIPTION("Ethernet CRC32 calculations"); | |

MODULE_LICENSE("GPL"); | |

/** | |

* 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 __attribute_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 __attribute_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 __attribute_pure__ crc32_le(u32 crc, unsigned char const *p, size_t len) | |

{ | |

# if CRC_LE_BITS == 8 | |

const u32 *b =(u32 *)p; | |

const u32 *tab = crc32table_le; | |

# 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 | |

crc = __cpu_to_le32(crc); | |

/* Align it */ | |

if(unlikely(((long)b)&3 && len)){ | |

do { | |

u8 *p = (u8 *)b; | |

DO_CRC(*p++); | |

b = (void *)p; | |

} while ((--len) && ((long)b)&3 ); | |

} | |

if(likely(len >= 4)){ | |

/* load data 32 bits wide, xor data 32 bits wide. */ | |

size_t save_len = len & 3; | |

len = len >> 2; | |

--b; /* use pre increment below(*++b) for speed */ | |

do { | |

crc ^= *++b; | |

DO_CRC(0); | |

DO_CRC(0); | |

DO_CRC(0); | |

DO_CRC(0); | |

} while (--len); | |

b++; /* point to next byte(s) */ | |

len = save_len; | |

} | |

/* And the last few bytes */ | |

if(len){ | |

do { | |

u8 *p = (u8 *)b; | |

DO_CRC(*p++); | |

b = (void *)p; | |

} while (--len); | |

} | |

return __le32_to_cpu(crc); | |

#undef ENDIAN_SHIFT | |

#undef DO_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 __attribute_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 __attribute_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 __attribute_pure__ crc32_be(u32 crc, unsigned char const *p, size_t len) | |

{ | |

# if CRC_BE_BITS == 8 | |

const u32 *b =(u32 *)p; | |

const u32 *tab = crc32table_be; | |

# 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 | |

crc = __cpu_to_be32(crc); | |

/* Align it */ | |

if(unlikely(((long)b)&3 && len)){ | |

do { | |

u8 *p = (u8 *)b; | |

DO_CRC(*p++); | |

b = (u32 *)p; | |

} while ((--len) && ((long)b)&3 ); | |

} | |

if(likely(len >= 4)){ | |

/* load data 32 bits wide, xor data 32 bits wide. */ | |

size_t save_len = len & 3; | |

len = len >> 2; | |

--b; /* use pre increment below(*++b) for speed */ | |

do { | |

crc ^= *++b; | |

DO_CRC(0); | |

DO_CRC(0); | |

DO_CRC(0); | |

DO_CRC(0); | |

} while (--len); | |

b++; /* point to next byte(s) */ | |

len = save_len; | |

} | |

/* And the last few bytes */ | |

if(len){ | |

do { | |

u8 *p = (u8 *)b; | |

DO_CRC(*p++); | |

b = (void *)p; | |

} while (--len); | |

} | |

return __be32_to_cpu(crc); | |

#undef ENDIAN_SHIFT | |

#undef DO_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 | |

/** | |

* bitreverse - reverse the order of bits in a u32 value | |

* @x: value to be bit-reversed | |

*/ | |

u32 bitreverse(u32 x) | |

{ | |

x = (x >> 16) | (x << 16); | |

x = (x >> 8 & 0x00ff00ff) | (x << 8 & 0xff00ff00); | |

x = (x >> 4 & 0x0f0f0f0f) | (x << 4 & 0xf0f0f0f0); | |

x = (x >> 2 & 0x33333333) | (x << 2 & 0xcccccccc); | |

x = (x >> 1 & 0x55555555) | (x << 1 & 0xaaaaaaaa); | |

return x; | |

} | |

EXPORT_SYMBOL(crc32_le); | |

EXPORT_SYMBOL(crc32_be); | |

EXPORT_SYMBOL(bitreverse); | |

/* | |

* 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 multile 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 = *buf; | |

x = (x >> 4) | (x << 4); | |

x = (x >> 2 & 0x33) | (x << 2 & 0xcc); | |

x = (x >> 1 & 0x55) | (x << 1 & 0xaa); | |

*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 = bitreverse(init); | |

crc2 = bitreverse(crc1); | |

if (crc1 != bitreverse(crc2)) | |

printf("\nBit reversal fail: 0x%08x -> 0x%08x -> 0x%08x\n", | |

crc1, crc2, bitreverse(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 */ |