xref: /openbmc/linux/lib/crc4.c (revision 550987be)
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 
11 static 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  */
27 uint8_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 }
43 EXPORT_SYMBOL_GPL(crc4);
44 
45 MODULE_DESCRIPTION("CRC4 calculations");
46 MODULE_LICENSE("GPL");
47