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    SubjectRe: [RFC] lib: crc8: add new library module providing crc8
    > True, a different init value likely results in a different good value.

    Actually, it doesn't. The good value is only non-zero to compensate
    for the final byte inversion. You'll notice that it's equal to
    crc8_table[0xff], where oxff is the XOR of the final result.

    This is a little-endian CRC, so x^8+x^7+x^6+x^4+x^2+1 is encoded with
    the x^7 coefficient in bit 0, and the x^0 coefficient in bit 7. (And the
    x^8 coefficient is implicit and suppressed.)

    You can fill in a CRC table for an arbitrary polynomial with

    #define POLY 0xAB /* 1 + x^2 + x^4 + x^6 + x^7 (+ x^8) */
    typedef uint8_t crc_type; /* Must be an unsigned type */

    void
    crc_init_le(crc_type table[256], crc_type poly)
    {
    int i, j;
    crc_type t = 1;

    table[0] = 0;

    for (i = 128; i; i >>= 1) {
    t = (t >> 1) ^ (t & 1 ? poly : 0);
    for (j = 0; j < 256; j += 2*i)
    table[i+j] = table[j] ^ t;
    }
    }

    void
    crc_le(crc_type const table[256], crc_type crc, u8 const *buf, size_t len)
    {
    while (len--)
    crc = (crc >> 8) ^ table[(crc ^ *buf++) & 0xff];
    return crc;
    }

    If you care, the corresponding code for a big-endian CRC
    (data and CRC transmitted msbit-first) is:

    void
    crc_init_be(crc_type table[256], crc_type poly)
    {
    int i, j;
    crc_type const msbit = ~(~(crc_type)0 >> 1); /* Must be unsigned */
    crc_type t = msbit;

    table[0] = 0;

    for (i = 1; i < 256; i *= 2) {
    t = (t << 1) ^ (t & msbit ? poly : 0);
    for (j = 0; j < i; j++)
    table[i+j] = table[j] ^ t;
    }
    }

    void
    crc_be(crc_type const table[256], crc_type crc, u8 const *buf, size_t len)
    {
    while (len--)
    crc = (crc << 8) ^ table[(crc >> (8*sizeof crc - 8)) ^ *buf++];
    return crc;
    }

    This uses a left-justified CRC; the CRC is in the most significant bits
    of the state variable. To match the current crc7 code's result, it
    must be right-shifted one bit. (That would actually simplify all the
    current users of the code, namely drivers/mmc/host/mmc_spi.c and
    drivers/net/wireless/wl12??/spi.c, as well as the computation loop.)

    The only other differences are the variable types and the actual polynomials.


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