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author | Richard Cochran <richardcochran@gmail.com> | 2014-12-21 19:47:06 +0100 |
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committer | David S. Miller <davem@davemloft.net> | 2014-12-30 18:29:27 -0500 |
commit | 2eebdde6528a722fbf8e2cffcf7aa52cbb4c2de0 (patch) | |
tree | c4b176d1e40eecb2caa00952eae95ecb2b2a046b /kernel | |
parent | f25a30be359d0535fb1c7c1619cabb0ad17cfbf1 (diff) | |
download | op-kernel-dev-2eebdde6528a722fbf8e2cffcf7aa52cbb4c2de0.zip op-kernel-dev-2eebdde6528a722fbf8e2cffcf7aa52cbb4c2de0.tar.gz |
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 'kernel')
-rw-r--r-- | kernel/time/timecounter.c | 31 |
1 files changed, 24 insertions, 7 deletions
diff --git a/kernel/time/timecounter.c b/kernel/time/timecounter.c index 59a1ec3..4687b31 100644 --- a/kernel/time/timecounter.c +++ b/kernel/time/timecounter.c @@ -25,6 +25,8 @@ void timecounter_init(struct timecounter *tc, tc->cc = cc; tc->cycle_last = cc->read(cc); tc->nsec = start_tstamp; + tc->mask = (1ULL << cc->shift) - 1; + tc->frac = 0; } EXPORT_SYMBOL_GPL(timecounter_init); @@ -51,7 +53,8 @@ static u64 timecounter_read_delta(struct timecounter *tc) cycle_delta = (cycle_now - tc->cycle_last) & tc->cc->mask; /* convert to nanoseconds: */ - ns_offset = cyclecounter_cyc2ns(tc->cc, cycle_delta); + ns_offset = cyclecounter_cyc2ns(tc->cc, cycle_delta, + tc->mask, &tc->frac); /* update time stamp of timecounter_read_delta() call: */ tc->cycle_last = cycle_now; @@ -72,22 +75,36 @@ u64 timecounter_read(struct timecounter *tc) } EXPORT_SYMBOL_GPL(timecounter_read); +/* + * This is like cyclecounter_cyc2ns(), but it is used for computing a + * time previous to the time stored in the cycle counter. + */ +static u64 cc_cyc2ns_backwards(const struct cyclecounter *cc, + cycle_t cycles, u64 mask, u64 frac) +{ + u64 ns = (u64) cycles; + + ns = ((ns * cc->mult) - frac) >> cc->shift; + + return ns; +} + u64 timecounter_cyc2time(struct timecounter *tc, cycle_t cycle_tstamp) { - u64 cycle_delta = (cycle_tstamp - tc->cycle_last) & tc->cc->mask; - u64 nsec; + u64 delta = (cycle_tstamp - tc->cycle_last) & tc->cc->mask; + u64 nsec = tc->nsec, frac = tc->frac; /* * Instead of always treating cycle_tstamp as more recent * than tc->cycle_last, detect when it is too far in the * future and treat it as old time stamp instead. */ - if (cycle_delta > tc->cc->mask / 2) { - cycle_delta = (tc->cycle_last - cycle_tstamp) & tc->cc->mask; - nsec = tc->nsec - cyclecounter_cyc2ns(tc->cc, cycle_delta); + if (delta > tc->cc->mask / 2) { + delta = (tc->cycle_last - cycle_tstamp) & tc->cc->mask; + nsec -= cc_cyc2ns_backwards(tc->cc, delta, tc->mask, frac); } else { - nsec = cyclecounter_cyc2ns(tc->cc, cycle_delta) + tc->nsec; + nsec += cyclecounter_cyc2ns(tc->cc, delta, tc->mask, &frac); } return nsec; |