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Diffstat (limited to 'partx/crc32.c')
| -rw-r--r-- | partx/crc32.c | 393 |
1 files changed, 393 insertions, 0 deletions
diff --git a/partx/crc32.c b/partx/crc32.c new file mode 100644 index 000000000..42d803d14 --- /dev/null +++ b/partx/crc32.c @@ -0,0 +1,393 @@ +/* + * 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(); +} |