Linux-2.6.33.2/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");
#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 = (const u32 *)buf;
size_t rem_len;
/* Align it */
if (unlikely((long)b & 3 && len)) {
u8 *p = (u8 *)b;
do {
DO_CRC(*p++);
} while ((--len) && ((long)p)&3);
b = (u32 *)p;
}
rem_len = len & 3;
/* load data 32 bits wide, xor data 32 bits wide. */
len = len >> 2;
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;
}
#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);
#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 __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);
#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
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 */