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

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Date:   Thu, 16 Dec 2021 00:36:08 +0100
From:   Alexandre Belloni <alexandre.belloni@...tlin.com>
To:     John Stultz <john.stultz@...aro.org>
Cc:     Joel Daniels <jdaniels@...t.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        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 15/12/2021 15:26:28-0800, John Stultz wrote:
> > Any method of measuring the drift is going to need to persist the
> > drift coefficient to disk so that it can set the system clock
> > correctly on boot. I think it would be best for the kernel to use this
> > same coefficient.
> 
> My initial thought was for the rtc class layer to do the estimation
> internally against the system time (assuming it was NTP corrected) to
> try establish a close enough correction factor as the system was up
> and running, but you're right that would be reset on every reboot, and
> with second granular RTCs accurate error calculations will take awhile
> (though polling for the second-edge might work well enough, but would
> be cpu heavy for a background task).
> 
> Maybe that's a good enough argument for having an ADJ ioctl added to
> the rtc chardev?
> 

Then why not got for the correction software emulation? that would avoid
duplicating interfaces and we'd avoid to use it on RTCs with hardware
support.

> But it also seems to suggest that if chronyd already has all this
> logic in userland, maybe having it calculate and call settimeofday()
> on resume from userland instead of pushing half of that logic into the
> kernel?

My suggestion would leave the correction calculation to userspace which
is definitively where it should stay.

> 
> > > Alternatively I'd go very simple and just put the correction factor in
> > > a boot argument.
> >
> > This works for my use case though it won't be useful to a general
> > distro. Would you have one argument being used regardless of where the
> > sleep injection was coming from or would you try to tie it to the
> > persistent clock and/or a specific RTC?
> 
> I agree it is an important thing to consider how to generalize this
> for common use (which is why I prefer the approach that works
> *without* any distro integration).
> 
> But it's also important to consider if the added complexity is
> *really* needed in the general case.
> 
> thanks
> -john

-- 
Alexandre Belloni, co-owner and COO, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com