ntp: Make periodic RTC update more reliable

The current code requires that the scheduled update of the RTC happens
in the closest tick to the half of the second. This seems to be
difficult to achieve reliably. The scheduled work may be missing the
target time by a tick or two and be constantly rescheduled every second.

Relax the limit to 10 ticks. As a typical RTC drifts in the 11-minute
update interval by several milliseconds, this shouldn't affect the
overall accuracy of the RTC much.

Signed-off-by: Miroslav Lichvar <mlichvar@redhat.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>

diff --git a/kernel/time/ntp.c b/kernel/time/ntp.c
index 8f5b3b98577b797663f8057762b6248d0fae1823..ab1fa7cb8912d3f682b82ad52520f82ec8dee0b8 100644 (file)
--- a/kernel/time/ntp.c
+++ b/kernel/time/ntp.c
@@ -475,6 +475,7 @@ static void sync_cmos_clock(struct work_struct *work)
         * called as close as possible to 500 ms before the new second starts.
         * This code is run on a timer.  If the clock is set, that timer
         * may not expire at the correct time.  Thus, we adjust...
+        * We want the clock to be within a couple of ticks from the target.
         */
        if (!ntp_synced()) {
                /*
@@ -485,7 +486,7 @@ static void sync_cmos_clock(struct work_struct *work)
        }
 
        getnstimeofday(&now);
-       if (abs(now.tv_nsec - (NSEC_PER_SEC / 2)) <= tick_nsec / 2) {
+       if (abs(now.tv_nsec - (NSEC_PER_SEC / 2)) <= tick_nsec * 5) {
                struct timespec adjust = now;
 
                fail = -ENODEV;