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
  11 static 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  */
  27 uint8_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 }
  43 EXPORT_SYMBOL_GPL(crc4);
  44
  45 MODULE_DESCRIPTION("CRC4 calculations");
  46 MODULE_LICENSE("GPL");