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/export.h> 8 #include <linux/module.h> 9 10 static const uint8_t crc4_tab[] = { 11 0x0, 0x7, 0xe, 0x9, 0xb, 0xc, 0x5, 0x2, 12 0x1, 0x6, 0xf, 0x8, 0xa, 0xd, 0x4, 0x3, 13 }; 14 15 /** 16 * crc4 - calculate the 4-bit crc of a value. 17 * @c: starting crc4 18 * @x: value to checksum 19 * @bits: number of bits in @x to checksum 20 * 21 * Returns the crc4 value of @x, using polynomial 0b10111. 22 * 23 * The @x value is treated as left-aligned, and bits above @bits are ignored 24 * in the crc calculations. 25 */ 26 uint8_t crc4(uint8_t c, uint64_t x, int bits) 27 { 28 int i; 29 30 /* mask off anything above the top bit */ 31 x &= (1ull << bits) - 1; 32 33 /* Align to 4-bits */ 34 bits = (bits + 3) & ~0x3; 35 36 /* Calculate crc4 over four-bit nibbles, starting at the MSbit */ 37 for (i = bits - 4; i >= 0; i -= 4) 38 c = crc4_tab[c ^ ((x >> i) & 0xf)]; 39 40 return c; 41 } 42 EXPORT_SYMBOL_GPL(crc4); 43 44 MODULE_DESCRIPTION("CRC4 calculations"); 45 MODULE_LICENSE("GPL"); 46