the-tree/lib/crc4.c

0001 // SPDX-License-Identifier: GPL-2.0-only
0002 /*
0003  * crc4.c - simple crc-4 calculations.
0004  */
0005
0006 #include <linux/crc4.h>
0007 #include <linux/module.h>
0008
0009 static const uint8_t crc4_tab[] = {
0010     0x0, 0x7, 0xe, 0x9, 0xb, 0xc, 0x5, 0x2,
0011     0x1, 0x6, 0xf, 0x8, 0xa, 0xd, 0x4, 0x3,
0012 };
0013
0014 /**
0015  * crc4 - calculate the 4-bit crc of a value.
0016  * @c:    starting crc4
0017  * @x:    value to checksum
0018  * @bits: number of bits in @x to checksum
0019  *
0020  * Returns the crc4 value of @x, using polynomial 0b10111.
0021  *
0022  * The @x value is treated as left-aligned, and bits above @bits are ignored
0023  * in the crc calculations.
0024  */
0025 uint8_t crc4(uint8_t c, uint64_t x, int bits)
0026 {
0027     int i;
0028
0029     /* mask off anything above the top bit */
0030     x &= (1ull << bits) - 1;
0031
0032     /* Align to 4-bits */
0033     bits = (bits + 3) & ~0x3;
0034
0035     /* Calculate crc4 over four-bit nibbles, starting at the MSbit */
0036     for (i = bits - 4; i >= 0; i -= 4)
0037         c = crc4_tab[c ^ ((x >> i) & 0xf)];
0038
0039     return c;
0040 }
0041 EXPORT_SYMBOL_GPL(crc4);
0042
0043 MODULE_DESCRIPTION("CRC4 calculations");
0044 MODULE_LICENSE("GPL");