LXR linux/lib/crc4.c

   1/*
   2 * crc4.c - simple crc-4 calculations.
   3 *
   4 * This source code is licensed under the GNU General Public License, Version
   5 * 2. See the file COPYING for more details.
   6 */
   7
   8#include <linux/crc4.h>
   9#include <linux/module.h>
  10
  11static const uint8_t crc4_tab[] = {
  12        0x0, 0x7, 0xe, 0x9, 0xb, 0xc, 0x5, 0x2,
  13        0x1, 0x6, 0xf, 0x8, 0xa, 0xd, 0x4, 0x3,
  14};
  15
  16/**
  17 * crc4 - calculate the 4-bit crc of a value.
  18 * @c:    starting crc4
  19 * @x:    value to checksum
  20 * @bits: number of bits in @x to checksum
  21 *
  22 * Returns the crc4 value of @x, using polynomial 0b10111.
  23 *
  24 * The @x value is treated as left-aligned, and bits above @bits are ignored
  25 * in the crc calculations.
  26 */
  27uint8_t crc4(uint8_t c, uint64_t x, int bits)
  28{
  29        int i;
  30
  31        /* mask off anything above the top bit */
  32        x &= (1ull << bits) - 1;
  33
  34        /* Align to 4-bits */
  35        bits = (bits + 3) & ~0x3;
  36
  37        /* Calculate crc4 over four-bit nibbles, starting at the MSbit */
  38        for (i = bits - 4; i >= 0; i -= 4)
  39                c = crc4_tab[c ^ ((x >> i) & 0xf)];
  40
  41        return c;
  42}
  43EXPORT_SYMBOL_GPL(crc4);
  44
  45MODULE_DESCRIPTION("CRC4 calculations");
  46MODULE_LICENSE("GPL");
  47