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