kernel / pub / scm / utils / util-linux / util-linux / dcd7eb37dbe4e3e09d4e3a51a6e89e8bfc37c009 / . / partx / crc32.c

/* | |

* 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) | |

{ | |

if (crc32table_le) 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) | |

{ | |

if (crc32table_be) 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(); | |

} |