LXR linux/lib/crc4.c

   1// SPDX-License-Identifier: GPL-2.0-only
   2/*
   3 * crc4.c - simple crc-4 calculations.
   4 */
   5
   6#include <linux/crc4.h>
   7#include <linux/module.h>
   8
   9static const uint8_t crc4_tab[] = {
  10        0x0, 0x7, 0xe, 0x9, 0xb, 0xc, 0x5, 0x2,
  11        0x1, 0x6, 0xf, 0x8, 0xa, 0xd, 0x4, 0x3,
  12};
  13
  14/**
  15 * crc4 - calculate the 4-bit crc of a value.
  16 * @c:    starting crc4
  17 * @x:    value to checksum
  18 * @bits: number of bits in @x to checksum
  19 *
  20 * Returns the crc4 value of @x, using polynomial 0b10111.
  21 *
  22 * The @x value is treated as left-aligned, and bits above @bits are ignored
  23 * in the crc calculations.
  24 */
  25uint8_t crc4(uint8_t c, uint64_t x, int bits)
  26{
  27        int i;
  28
  29        /* mask off anything above the top bit */
  30        x &= (1ull << bits) - 1;
  31
  32        /* Align to 4-bits */
  33        bits = (bits + 3) & ~0x3;
  34
  35        /* Calculate crc4 over four-bit nibbles, starting at the MSbit */
  36        for (i = bits - 4; i >= 0; i -= 4)
  37                c = crc4_tab[c ^ ((x >> i) & 0xf)];
  38
  39        return c;
  40}
  41EXPORT_SYMBOL_GPL(crc4);
  42
  43MODULE_DESCRIPTION("CRC4 calculations");
  44MODULE_LICENSE("GPL");
  45