diff options
Diffstat (limited to 'drivers/md/bcache/util.h')
| -rw-r--r-- | drivers/md/bcache/util.h | 26 |
1 files changed, 19 insertions, 7 deletions
diff --git a/drivers/md/bcache/util.h b/drivers/md/bcache/util.h index 00aab6abcfe4..1fbced94e4cc 100644 --- a/drivers/md/bcache/util.h +++ b/drivers/md/bcache/util.h @@ -560,17 +560,29 @@ static inline uint64_t bch_crc64_update(uint64_t crc, return crc; } -/* Does linear interpolation between powers of two */ +/* + * A stepwise-linear pseudo-exponential. This returns 1 << (x >> + * frac_bits), with the less-significant bits filled in by linear + * interpolation. + * + * This can also be interpreted as a floating-point number format, + * where the low frac_bits are the mantissa (with implicit leading + * 1 bit), and the more significant bits are the exponent. + * The return value is 1.mantissa * 2^exponent. + * + * The way this is used, fract_bits is 6 and the largest possible + * input is CONGESTED_MAX-1 = 1023 (exponent 16, mantissa 0x1.fc), + * so the maximum output is 0x1fc00. + */ static inline unsigned int fract_exp_two(unsigned int x, unsigned int fract_bits) { - unsigned int fract = x & ~(~0 << fract_bits); - - x >>= fract_bits; - x = 1 << x; - x += (x * fract) >> fract_bits; + unsigned int mantissa = 1 << fract_bits; /* Implicit bit */ - return x; + mantissa += x & (mantissa - 1); + x >>= fract_bits; /* The exponent */ + /* Largest intermediate value 0x7f0000 */ + return mantissa << x >> fract_bits; } void bch_bio_map(struct bio *bio, void *base); |