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 Date 22 May 2011 11:25:03 -0400 From "George Spelvin" <> Subject Re: [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 compensatefor the final byte inversion.  You'll notice that it's equal tocrc8_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 withthe x^7 coefficient in bit 0, and the x^0 coefficient in bit 7.  (And thex^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 */voidcrc_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;	}}voidcrc_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:voidcrc_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;	}}voidcrc_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 bitsof the state variable.  To match the current crc7 code's result, itmust be right-shifted one bit.  (That would actually simplify all thecurrent users of the code, namely drivers/mmc/host/mmc_spi.c anddrivers/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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