1*40b0b3f8SThomas Gleixner // SPDX-License-Identifier: GPL-2.0-only
20cbaa448SJeremy Kerr /*
30cbaa448SJeremy Kerr * crc4.c - simple crc-4 calculations.
40cbaa448SJeremy Kerr */
50cbaa448SJeremy Kerr
60cbaa448SJeremy Kerr #include <linux/crc4.h>
70cbaa448SJeremy Kerr #include <linux/module.h>
80cbaa448SJeremy Kerr
90cbaa448SJeremy Kerr static const uint8_t crc4_tab[] = {
100cbaa448SJeremy Kerr 0x0, 0x7, 0xe, 0x9, 0xb, 0xc, 0x5, 0x2,
110cbaa448SJeremy Kerr 0x1, 0x6, 0xf, 0x8, 0xa, 0xd, 0x4, 0x3,
120cbaa448SJeremy Kerr };
130cbaa448SJeremy Kerr
140cbaa448SJeremy Kerr /**
150cbaa448SJeremy Kerr * crc4 - calculate the 4-bit crc of a value.
168a29896aSRandy Dunlap * @c: starting crc4
170cbaa448SJeremy Kerr * @x: value to checksum
180cbaa448SJeremy Kerr * @bits: number of bits in @x to checksum
190cbaa448SJeremy Kerr *
200cbaa448SJeremy Kerr * Returns the crc4 value of @x, using polynomial 0b10111.
210cbaa448SJeremy Kerr *
220cbaa448SJeremy Kerr * The @x value is treated as left-aligned, and bits above @bits are ignored
230cbaa448SJeremy Kerr * in the crc calculations.
240cbaa448SJeremy Kerr */
crc4(uint8_t c,uint64_t x,int bits)250cbaa448SJeremy Kerr uint8_t crc4(uint8_t c, uint64_t x, int bits)
260cbaa448SJeremy Kerr {
270cbaa448SJeremy Kerr int i;
280cbaa448SJeremy Kerr
290cbaa448SJeremy Kerr /* mask off anything above the top bit */
300cbaa448SJeremy Kerr x &= (1ull << bits) - 1;
310cbaa448SJeremy Kerr
320cbaa448SJeremy Kerr /* Align to 4-bits */
330cbaa448SJeremy Kerr bits = (bits + 3) & ~0x3;
340cbaa448SJeremy Kerr
350cbaa448SJeremy Kerr /* Calculate crc4 over four-bit nibbles, starting at the MSbit */
360cbaa448SJeremy Kerr for (i = bits - 4; i >= 0; i -= 4)
370cbaa448SJeremy Kerr c = crc4_tab[c ^ ((x >> i) & 0xf)];
380cbaa448SJeremy Kerr
390cbaa448SJeremy Kerr return c;
400cbaa448SJeremy Kerr }
410cbaa448SJeremy Kerr EXPORT_SYMBOL_GPL(crc4);
420cbaa448SJeremy Kerr
430cbaa448SJeremy Kerr MODULE_DESCRIPTION("CRC4 calculations");
440cbaa448SJeremy Kerr MODULE_LICENSE("GPL");
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