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-rw-r--r--drivers/md/bcache/util.h26
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);