Re: [chrony-dev] [RFC] Transparent fallback from NTP reference clock to RTC

[Bill Unruh <unruh@xxxxxxxxxxxxxx>]

Again perhaps you should tell use what it is you are really trying to do. You
seemed to intimate that you have and external RTC. Not suree what that is, but
why not replace it with and expernal GPS clock instead. Or is this supposed to
be operating at the bottom of the ocean? 
If you sync rtc from the system clock, why not just use the system clock
freerunning? Not at all clear why you want to go via the rtc since it can only
be queried to the nearest second anyway. But giving us more details of what
you are trying to do might alloow us to make better suggestions than we can
now.

On Mon, 31 Mar 2025, Ahmad Fatoum wrote:

> [CAUTION: Non-UBC Email]
>
> Hello Miroslav,
>
> Thanks for your feedback.
>
> On 3/25/25 14:46, Miroslav Lichvar wrote:
> > On Mon, Mar 24, 2025 at 04:26:31PM +0100, Ahmad Fatoum wrote:
>
> [snip]
>
> > >   - rtcsync when the RTC is a selected reference clock should be a no-op
> > >
> > >   - rtcsync when the RTC is _not_ the selected reference clock should
> > >     periodically program the time into the RTC like the kernel usually
> > >     does, once the drift exceeds a threshold
> >
> > This looks like a significant complication. I prefer to keeps things
> > simple if possible, avoiding fallbacks and switching between different
> > modes of operation.
>
> I am open to less complicated ways to achieve this that don't require us
> to restart chrony with different configs.
>
> > Have you considered using the local option for the RTC refclock?
>
> I was not aware for it and will try it out.
>
> > 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? 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?
>
> Thanks,
> Ahmad
>
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