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timecounter: keep track of accumulated fractional nanoseconds

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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>
This commit is contained in:
Richard Cochran
2014-12-21 19:47:06 +01:00
committed by David S. Miller
parent f25a30be35
commit 2eebdde652
4 changed files with 40 additions and 17 deletions
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31
kernel/time/timecounter.c
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@@ -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;