From:   Thomas Gleixner <tglx@linutronix.de>
To:     Joel Daniels <jdaniels@sent.com>,
        John Stultz <john.stultz@linaro.org>,
        Stephen Boyd <sboyd@kernel.org>
Cc:     linux-kernel@vger.kernel.org,
        Alessandro Zummo <a.zummo@towertech.it>,
        Alexandre Belloni <alexandre.belloni@bootlin.com>,
        linux-rtc@vger.kernel.org, x86@kernel.org
Subject: Re: Time keeping while suspended in the presence of persistent
 clock drift
In-Reply-To: <4bb238e1-e8fa-44e6-9f5e-d047d1d4a892@www.fastmail.com>
References: <5af5d2a5-767c-d313-3be6-cb6f426f1980@sent.com>
 <b074f506-2568-4506-9557-4a9bc9cbea83@www.fastmail.com>
 <87wnkbuuuz.ffs@tglx>
 <4bb238e1-e8fa-44e6-9f5e-d047d1d4a892@www.fastmail.com>
Date:   Tue, 14 Dec 2021 14:57:45 +0100
Message-ID: <8735mvthk6.ffs@tglx>
MIME-Version: 1.0
Content-Type: text/plain
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Joel,

On Mon, Dec 13 2021 at 06:39, Joel Daniels wrote:
> On Sat, 11 Dec 2021 14:36 +0100, Thomas Gleixner wrote:
>> Can you please verify that the problem persists with NTP enabled and
>> synchronized?
>
> Yes, I just verified that the problem still exists while
> synchronized to NTP.
...
>     $ chronyc tracking && echo && chronyc sources
>     [...]
>     Ref time (UTC)  : Mon Dec 13 13:30:52 2021
>     System time     : 5.597892284 seconds fast of NTP time

thanks for making sure that this is really a RTC issue on that machine.

> The "if" branch does not apply as I have no clock sources flagged as
> CLOCK_SOURCE_SUSPEND_NONSTOP but the "else if" branch does apply.

Which CPU is in that box?

> The kernel seems to believe that the time spent sleeping is exactly
> the difference of two calls to read_persistent_clock64 with no option
> to adjust for persistent clock drift.

The kernel does not believe. It relies on the accuracy of the CMOS clock
which is usually pretty good.

> 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.

Thanks,

        tglx