lkml.org 
[lkml]   [2011]   [May]   [22]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
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.


\
 
 \ /
  Last update: 2011-05-22 17:27    [from the cache]
©2003-2014 Jasper Spaans. Advertise on this site