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authorRichard Cochran <richardcochran@gmail.com>2014-12-21 19:47:06 +0100
committerDavid S. Miller <davem@davemloft.net>2014-12-30 18:29:27 -0500
commit2eebdde6528a722fbf8e2cffcf7aa52cbb4c2de0 (patch)
treec4b176d1e40eecb2caa00952eae95ecb2b2a046b /kernel
parentf25a30be359d0535fb1c7c1619cabb0ad17cfbf1 (diff)
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.c31
1 files changed, 24 insertions, 7 deletions
diff --git a/kernel/time/timecounter.c b/kernel/time/timecounter.c
index 59a1ec3a57cb..4687b3104bae 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;