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 * @crc: 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