| Re: [chrony-dev] [RFC] Transparent fallback from NTP reference clock to RTC |
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- Subject: Re: [chrony-dev] [RFC] Transparent fallback from NTP reference clock to RTC
- From: Miroslav Lichvar <mlichvar@xxxxxxxxxx>
- Date: Mon, 7 Apr 2025 16:52:17 +0200
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On Mon, Mar 31, 2025 at 05:38:11PM +0200, Ahmad Fatoum wrote:
> > I.e. use it only for stabilization of the system clock, not as a time
> > source. That would work even when it's synchronizing to NTP servers.
> > When NTP is not reachable, it would activate the local reference as
> > configured by the local directive. I guess something would be needed
> > for the case when it's started without NTP.
>
> Do you have a suggestion how to go about it?
Try it with this patch:
--- a/refclock.c
+++ b/refclock.c
@@ -732,7 +732,13 @@
SST_Stats stats = SRC_GetSourcestats(inst->source);
if (SST_Samples(stats) < SST_GetMinSamples(stats)) {
+#if 0
UTI_ZeroTimespec(ref);
+#else
+ LCL_ReadCookedTime(ref, NULL);
+ *freq = 0.0;
+ *offset = 0.0;
+#endif
return;
}
That should force the first adjustment to accumulate the "local"
refclock's frequency offset and track it closely until a time source
is selected. I suspect there could be numerical errors accumulating
over long periods of time, but insignificant when compared to a real
time source like RTC.
It could be a new option, if I can get a name for it.
> I also think that even if
> local works for us, we still need to address the synchronization of the
> RTC to system time when the system is NTP-synchronized.
>
> Currently, chrony disallows rtc reference clock to co-exist with
> rtcsync, but maybe we should allow it and enable the rtcsync only when
> NTP is reachable and the RTC is local and not activated?
rtcsync cannot work. That's the kernel doing changes at random times
unknown to chronyd. I think it could work with rtcfile if it shared
the descriptor and timestamps with the refclock driver.
--
Miroslav Lichvar
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