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
   9 static 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  */
  25 uint8_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 }
  41 EXPORT_SYMBOL_GPL(crc4);
  42
  43 MODULE_DESCRIPTION("CRC4 calculations");
  44 MODULE_LICENSE("GPL");