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

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Date:   Wed, 15 Dec 2021 15:05:01 -0700
From:   "Joel Daniels" <jdaniels@...t.com>
To:     "Alexandre Belloni" <alexandre.belloni@...tlin.com>,
        "Thomas Gleixner" <tglx@...utronix.de>
Cc:     "John Stultz" <john.stultz@...aro.org>,
        "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


>> > I would like to provide a way for user space to inform the kernel
>> > that the persistent clock drifts so it can make a corresponding
>> > adjustment when resuming from a long suspend period.
>> >
>> > In my use case it would be enough for me to set this parameter on
>> > boot. In use cases with continuous network access, NTP daemons
>> > could be enhanced to periodically update this parameter with the
>> > daemon's best estimate of the persistent clock drift.
>> 
>> That needs some thought. The RTC people (cc'ed now) might have opionions
>> on that.
>> 
>
> The RTC subsystem already has two interfaces to correct the drift of an
> RTC. However, this is currently limited to RTC that have hardware
> support for this feature. I guess we could had software emulation of the
> feature to be able to correct for any RTCs  but this will raise many
> design questions, like how often the correction has to happen, what to
> do with RTC that have a counter that doesn't reset when setting their
> time, etc...
>
> I guess this would be able to solve your particular issue has you will
> need a mechanism to handle when you overshoot the regular correction
> timer.
>
> However, everything falls down once the machine is turned off, making
> the whole effort moot...

Today two mechanism are regularly used to correct for rtc drift while
the machine is powered off: the hwclock program and chronyd with the
"-s" option. They both rely on the RTC running at the same rate when
the machine is on or off. So I agree with you that trying to emulate
hardware RTC drift correction in software is not going to work well.