lib: Add crc4 module

#ifndef _LINUX_CRC4_H

#define _LINUX_CRC4_H

#include <linux/types.h>

extern uint8_t crc4(uint8_t c, uint64_t x, int bits);

#endif /* _LINUX_CRC4_H */

endchoice

config CRC4

	tristate "CRC4 functions"

	help

	  This option is provided for the case where no in-kernel-tree

	  modules require CRC4 functions, but a module built outside

	  the kernel tree does. Such modules that use library CRC4

	  functions require M here.

config CRC7

	tristate "CRC7 functions"

	help

obj-$(CONFIG_CRC_ITU_T)	+= crc-itu-t.o

obj-$(CONFIG_CRC32)	+= crc32.o

obj-$(CONFIG_CRC32_SELFTEST)	+= crc32test.o

obj-$(CONFIG_CRC4)	+= crc4.o

obj-$(CONFIG_CRC7)	+= crc7.o

obj-$(CONFIG_LIBCRC32C)	+= libcrc32c.o

obj-$(CONFIG_CRC8)	+= crc8.o

/*

 * crc4.c - simple crc-4 calculations.

 *

 * This source code is licensed under the GNU General Public License, Version

 * 2. See the file COPYING for more details.

 */

#include <linux/crc4.h>

#include <linux/module.h>

static const uint8_t crc4_tab[] = {

	0x0, 0x7, 0xe, 0x9, 0xb, 0xc, 0x5, 0x2,

	0x1, 0x6, 0xf, 0x8, 0xa, 0xd, 0x4, 0x3,

};

/**

 * crc4 - calculate the 4-bit crc of a value.

 * @crc:  starting crc4

 * @x:    value to checksum

 * @bits: number of bits in @x to checksum

 *

 * Returns the crc4 value of @x, using polynomial 0b10111.

 *

 * The @x value is treated as left-aligned, and bits above @bits are ignored

 * in the crc calculations.

 */

uint8_t crc4(uint8_t c, uint64_t x, int bits)

{

	int i;

	/* mask off anything above the top bit */

	x &= (1ull << bits) - 1;

	/* Align to 4-bits */

	bits = (bits + 3) & ~0x3;

	/* Calculate crc4 over four-bit nibbles, starting at the MSbit */

	for (i = bits - 4; i >= 0; i -= 4)

		c = crc4_tab[c ^ ((x >> i) & 0xf)];

	return c;

}

EXPORT_SYMBOL_GPL(crc4);

MODULE_DESCRIPTION("CRC4 calculations");

MODULE_LICENSE("GPL");