| // SPDX-License-Identifier: GPL-2.0 |
| /* |
| * Use slice-by-8, which is the fastest variant. |
| * |
| * Calculate checksum 8 bytes at a time with a clever slicing algorithm. |
| * This is the fastest algorithm, but comes with a 8KiB lookup table. |
| * Most modern processors have enough cache to hold this table without |
| * thrashing the cache. |
| * |
| * The Linux kernel uses this as the default implementation "unless you |
| * have a good reason not to". The reason why Kconfig urges you to pick |
| * SLICEBY8 is because people challenged the assertion that we should |
| * always use slice by 8, so Darrick wrote a crc microbenchmark utility |
| * and ran it on as many machines as he could get his hands on to show |
| * that sb8 was the fastest. |
| * |
| * Every 64-bit machine (and most of the 32-bit ones too) saw the best |
| * results with sb8. Any machine with more than 4K of cache saw better |
| * results. The spreadsheet still exists today[1]; note that |
| * 'crc32-kern-le' corresponds to the slice by 4 algorithm which is the |
| * default unless CRC_LE_BITS is defined explicitly. |
| * |
| * FWIW, there are a handful of board defconfigs in the kernel that |
| * don't pick sliceby8. These are all embedded 32-bit mips/ppc systems |
| * with very small cache sizes which experience cache thrashing with the |
| * slice by 8 algorithm, and therefore chose to pick defaults that are |
| * saner for their particular board configuration. For nearly all of |
| * XFS' perceived userbase (which we assume are 32 and 64-bit machines |
| * with sufficiently large CPU cache and largeish storage devices) slice |
| * by 8 is the right choice. |
| * |
| * [1] https://goo.gl/0LSzsG ("crc32c_bench") |
| */ |
| #define CRC_LE_BITS 64 |
| |
| /* |
| * This is the CRC32c polynomial, as outlined by Castagnoli. |
| * x^32+x^28+x^27+x^26+x^25+x^23+x^22+x^20+x^19+x^18+x^14+x^13+x^11+x^10+x^9+ |
| * x^8+x^6+x^0 |
| */ |
| #define CRC32C_POLY_LE 0x82F63B78 |
| |
| /* |
| * Little-endian CRC computation. Used with serial bit streams sent |
| * lsbit-first. Be sure to use cpu_to_le32() to append the computed CRC. |
| */ |
| #if CRC_LE_BITS > 64 || CRC_LE_BITS < 1 || CRC_LE_BITS == 16 || \ |
| CRC_LE_BITS & CRC_LE_BITS-1 |
| # error "CRC_LE_BITS must be one of {1, 2, 4, 8, 32, 64}" |
| #endif |
