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");