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author | 2014-12-21 19:47:06 +0100 | |
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committer | 2014-12-30 18:29:27 -0500 | |
commit | 2eebdde6528a722fbf8e2cffcf7aa52cbb4c2de0 (patch) | |
tree | c4b176d1e40eecb2caa00952eae95ecb2b2a046b /include/linux/timecounter.h | |
parent | net: cpts: convert to timecounter adjtime. (diff) | |
download | linux-2eebdde6528a722fbf8e2cffcf7aa52cbb4c2de0.tar.gz linux-2eebdde6528a722fbf8e2cffcf7aa52cbb4c2de0.tar.bz2 linux-2eebdde6528a722fbf8e2cffcf7aa52cbb4c2de0.zip |
timecounter: keep track of accumulated fractional nanoseconds
The current timecounter implementation will drop a variable amount
of resolution, depending on the magnitude of the time delta. In
other words, reading the clock too often or too close to a time
stamp conversion will introduce errors into the time values. This
patch fixes the issue by introducing a fractional nanosecond field
that accumulates the low order bits.
Reported-by: Janusz Użycki <j.uzycki@elproma.com.pl>
Signed-off-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Diffstat (limited to 'include/linux/timecounter.h')
-rw-r--r-- | include/linux/timecounter.h | 19 |
1 files changed, 12 insertions, 7 deletions
diff --git a/include/linux/timecounter.h b/include/linux/timecounter.h index af3dfa4e90f0..74f45496e6d1 100644 --- a/include/linux/timecounter.h +++ b/include/linux/timecounter.h @@ -55,27 +55,32 @@ struct cyclecounter { * @cycle_last: most recent cycle counter value seen by * timecounter_read() * @nsec: continuously increasing count + * @mask: bit mask for maintaining the 'frac' field + * @frac: accumulated fractional nanoseconds */ struct timecounter { const struct cyclecounter *cc; cycle_t cycle_last; u64 nsec; + u64 mask; + u64 frac; }; /** * cyclecounter_cyc2ns - converts cycle counter cycles to nanoseconds * @cc: Pointer to cycle counter. * @cycles: Cycles - * - * XXX - This could use some mult_lxl_ll() asm optimization. Same code - * as in cyc2ns, but with unsigned result. + * @mask: bit mask for maintaining the 'frac' field + * @frac: pointer to storage for the fractional nanoseconds. */ static inline u64 cyclecounter_cyc2ns(const struct cyclecounter *cc, - cycle_t cycles) + cycle_t cycles, u64 mask, u64 *frac) { - u64 ret = (u64)cycles; - ret = (ret * cc->mult) >> cc->shift; - return ret; + u64 ns = (u64) cycles; + + ns = (ns * cc->mult) + *frac; + *frac = ns & mask; + return ns >> cc->shift; } /** |