```   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");