/*
* crc32.c
* This code is in the public domain; copyright abandoned.
* Liability for non-performance of this code is limited to the amount
* you paid for it. Since it is distributed for free, your refund will
* be very very small. If it breaks, you get to keep both pieces.
*/
#include "crc32.h"
#if __GNUC__ >= 3 /* 2.x has "attribute", but only 3.0 has "pure */
#define attribute(x) __attribute__(x)
#else
#define attribute(x)
#endif
/*
* There are multiple 16-bit CRC polynomials in common use, but this is
* *the* standard CRC-32 polynomial, first popularized by Ethernet.
* x^32+x^26+x^23+x^22+x^16+x^12+x^11+x^10+x^8+x^7+x^5+x^4+x^2+x^1+x^0
*/
#define CRCPOLY_LE 0xedb88320
#define CRCPOLY_BE 0x04c11db7
/* How many bits at a time to use. Requires a table of 4<<CRC_xx_BITS bytes. */
/* For less performance-sensitive, use 4 */
#define CRC_LE_BITS 8
#define CRC_BE_BITS 8
/*
* Little-endian CRC computation. Used with serial bit streams sent
* lsbit-first. Be sure to use cpu_to_le32() to append the computed CRC.
*/
#if CRC_LE_BITS > 8 || CRC_LE_BITS < 1 || CRC_LE_BITS & CRC_LE_BITS-1
# error CRC_LE_BITS must be a power of 2 between 1 and 8
#endif
#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.
*/
#define crc32init_le()
#define crc32cleanup_le()
/**
* 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
*
*/
uint32_t attribute((pure)) crc32_le(uint32_t 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 */
static uint32_t *crc32table_le;
/**
* crc32init_le() - allocate and initialize LE table data
*
* crc is the crc of the byte i; other entries are filled in based on the
* fact that crctable[i^j] = crctable[i] ^ crctable[j].
*
*/
static int
crc32init_le(void)
{
unsigned i, j;
uint32_t crc = 1;
crc32table_le =
malloc((1 << CRC_LE_BITS) * sizeof(uint32_t));
if (!crc32table_le)
return 1;
crc32table_le[0] = 0;
for (i = 1 << (CRC_LE_BITS - 1); i; i >>= 1) {
crc = (crc >> 1) ^ ((crc & 1) ? CRCPOLY_LE : 0);
for (j = 0; j < 1 << CRC_LE_BITS; j += 2 * i)
crc32table_le[i + j] = crc ^ crc32table_le[j];
}
return 0;
}
/**
* crc32cleanup_le(): free LE table data
*/
static void
crc32cleanup_le(void)
{
free(crc32table_le);
crc32table_le = NULL;
}
/**
* 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
*
*/
uint32_t attribute((pure)) crc32_le(uint32_t crc, unsigned char const *p, size_t len)
{
while (len--) {
# if CRC_LE_BITS == 8
crc = (crc >> 8) ^ crc32table_le[(crc ^ *p++) & 255];
# elif CRC_LE_BITS == 4
crc ^= *p++;
crc = (crc >> 4) ^ crc32table_le[crc & 15];
crc = (crc >> 4) ^ crc32table_le[crc & 15];
# elif CRC_LE_BITS == 2
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];
# endif
}
return crc;
}
#endif
/*
* Big-endian CRC computation. Used with serial bit streams sent
* msbit-first. Be sure to use cpu_to_be32() to append the computed CRC.
*/
#if CRC_BE_BITS > 8 || CRC_BE_BITS < 1 || CRC_BE_BITS & CRC_BE_BITS-1
# error CRC_BE_BITS must be a power of 2 between 1 and 8
#endif
#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.
*/
#define crc32init_be()
#define crc32cleanup_be()
/**
* 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
*
*/
uint32_t attribute((pure)) crc32_be(uint32_t 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 */
static uint32_t *crc32table_be;
/**
* crc32init_be() - allocate and initialize BE table data
*/
static int
crc32init_be(void)
{
unsigned i, j;
uint32_t crc = 0x80000000;
crc32table_be =
malloc((1 << CRC_BE_BITS) * sizeof(uint32_t));
if (!crc32table_be)
return 1;
crc32table_be[0] = 0;
for (i = 1; i < 1 << CRC_BE_BITS; i <<= 1) {
crc = (crc << 1) ^ ((crc & 0x80000000) ? CRCPOLY_BE : 0);
for (j = 0; j < i; j++)
crc32table_be[i + j] = crc ^ crc32table_be[j];
}
return 0;
}
/**
* crc32cleanup_be(): free BE table data
*/
static void
crc32cleanup_be(void)
{
free(crc32table_be);
crc32table_be = NULL;
}
/**
* 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
*
*/
uint32_t attribute((pure)) crc32_be(uint32_t crc, unsigned char const *p, size_t len)
{
while (len--) {
# if CRC_BE_BITS == 8
crc = (crc << 8) ^ crc32table_be[(crc >> 24) ^ *p++];
# elif CRC_BE_BITS == 4
crc ^= *p++ << 24;
crc = (crc << 4) ^ crc32table_be[crc >> 28];
crc = (crc << 4) ^ crc32table_be[crc >> 28];
# elif CRC_BE_BITS == 2
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];
# endif
}
return crc;
}
#endif
/*
* 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.
*/
/**
* init_crc32(): generates CRC32 tables
*
* On successful initialization, use count is increased.
* This guarantees that the library functions will stay resident
* in memory, and prevents someone from 'rmmod crc32' while
* a driver that needs it is still loaded.
* This also greatly simplifies drivers, as there's no need
* to call an initialization/cleanup function from each driver.
* Since crc32.o is a library module, there's no requirement
* that the user can unload it.
*/
int
init_crc32(void)
{
int rc1, rc2, rc;
rc1 = crc32init_le();
rc2 = crc32init_be();
rc = rc1 || rc2;
return rc;
}
/**
* cleanup_crc32(): frees crc32 data when no longer needed
*/
void
cleanup_crc32(void)
{
crc32cleanup_le();
crc32cleanup_be();
}
