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");