| 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"); |
| 45 |