linux-kernel - Re: Time keeping while suspended in the presence of persistent clock drift

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Date:   Wed, 15 Dec 2021 15:10:37 -0800
From:   John Stultz <john.stultz@...aro.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Alexandre Belloni <alexandre.belloni@...tlin.com>,
        Joel Daniels <jdaniels@...t.com>,
        Stephen Boyd <sboyd@...nel.org>, linux-kernel@...r.kernel.org,
        Alessandro Zummo <a.zummo@...ertech.it>,
        linux-rtc@...r.kernel.org, x86@...nel.org
Subject: Re: Time keeping while suspended in the presence of persistent clock drift

On Wed, Dec 15, 2021 at 2:33 PM Thomas Gleixner <tglx@...utronix.de> wrote:
>
> On Wed, Dec 15 2021 at 14:02, John Stultz wrote:
> > On Wed, Dec 15, 2021 at 1:32 PM Alexandre Belloni
> >> I'd rather lean towards the timekeeping code doing that. The RTC
> >
> > Heh, touche'!  :)
> >
> >> subsystem doesn't know which RTC has to be used.
> >
> > Though the RTC layer *is* the one that tracks which RTC is used, via
> > the logic in drivers/rtc/class.c, and the timekeeping core already has
> > adjtimex for timekeeping corrections, so if we're correcting
> > underlying RTCs it seems such tuning would best be done in the RTC
> > layer.
> >
> > Though how the persistent_clock interface ties into such corrections
> > would be a separate thing.
>
> Might be the final trigger to get rid of that leftover from the last
> millenium?
>

Yeah. Simplifying probably helps for consistency and maintainability.
(on top of the rtc and persistent clock, we also have the nonstop
clocksources that keep running through suspend and can be used. :)

It's just that window after resume but before the sleep time injection
where time would be incorrect always made me uncomfortable, so it was
nice to have some correct way to avoid that, even if all hardware
couldn't utilize it.  But as I'm less involved here, maybe someone
else can simplify things and live with that worry. :)

thanks
-john