Alexandre,
On Sun, Dec 06 2020 at 22:46, Thomas Gleixner wrote:
> Miroslav ran into a situation where the periodic RTC synchronization almost
> never was able to hit the time window for the update. That happens due the
> usage of delayed_work and the properties of the timer wheel.
>
> While that particular problem is halfways simple to fix this started to
> unearth other problems with that code particularly with rtc_set_npt_time()
> but expanded into other things as well.
>
> 1) The update offset for rtc-cmos is off by a full second
>
> 2) The readout of MC146818 (rtc-cmos and arch code) is broken and can
> return garbage.
>
> 2) Alexandre questioned the approach in general and wants to get rid of
> it. Of course there are better methods to do that and it can be
> completely done in user space.
>
> Unfortunately it's not that simple as this would be a user visible
> change, so making it at least halfways correct.
>
> 3) Alexandre requested to move that code into the NTP code as this is not
> really RTC functionality and just usage of the RTC API.
>
> 4) The update offset itself was questioned, but the time between the
> write and the next seconds increment in the RTC is fundamentaly a
> hardware property. The transport time, which is pretty irrelevant for
> direct accessible RTCs (rtc-cmos), but relevant for RTC behind i2c/SPI
> needs to be added on top.
>
> It's undebated that this transport time cannot be correctly estimated,
> but right now it's 500ms which is far off. The correct transport time
> can be calibrated, a halfways correct value supplied via DT, but
> that's an orthogonal problem.
>
> The following series addresses the above:
>
> 1) Fix the readout function of MC146818
> 2) Fix the rtc-cmos offset
> 3) Reduce the default transport time
>
> 4) Switch the NTP periodic sync code to a hrtimer/work combo
>
> 5) Move rtc_set_npt_time() to the ntp code
> 6) Make the update offset more intuitive
> 7) Simplify the whole machinery
any opinion on this?
Thanks,
tglx
