crc4.c 1003 B

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  1. // SPDX-License-Identifier: GPL-2.0-only
  2. /*
  3. * crc4.c - simple crc-4 calculations.
  4. */
  5. #include <linux/crc4.h>
  6. #include <linux/module.h>
  7. static const uint8_t crc4_tab[] = {
  8. 0x0, 0x7, 0xe, 0x9, 0xb, 0xc, 0x5, 0x2,
  9. 0x1, 0x6, 0xf, 0x8, 0xa, 0xd, 0x4, 0x3,
  10. };
  11. /**
  12. * crc4 - calculate the 4-bit crc of a value.
  13. * @c: starting crc4
  14. * @x: value to checksum
  15. * @bits: number of bits in @x to checksum
  16. *
  17. * Returns the crc4 value of @x, using polynomial 0b10111.
  18. *
  19. * The @x value is treated as left-aligned, and bits above @bits are ignored
  20. * in the crc calculations.
  21. */
  22. uint8_t crc4(uint8_t c, uint64_t x, int bits)
  23. {
  24. int i;
  25. /* mask off anything above the top bit */
  26. x &= (1ull << bits) - 1;
  27. /* Align to 4-bits */
  28. bits = (bits + 3) & ~0x3;
  29. /* Calculate crc4 over four-bit nibbles, starting at the MSbit */
  30. for (i = bits - 4; i >= 0; i -= 4)
  31. c = crc4_tab[c ^ ((x >> i) & 0xf)];
  32. return c;
  33. }
  34. EXPORT_SYMBOL_GPL(crc4);
  35. MODULE_DESCRIPTION("CRC4 calculations");
  36. MODULE_LICENSE("GPL");