linux-kernel - Re: [patch 0/8] ntp/rtc: Fixes and cleanups

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Message-ID: <87sg8f24zf.fsf@nanos.tec.linutronix.de>
Date:   Wed, 09 Dec 2020 01:33:08 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     Alexandre Belloni <alexandre.belloni@...tlin.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Miroslav Lichvar <mlichvar@...hat.com>,
        John Stultz <john.stultz@...aro.org>,
        Prarit Bhargava <prarit@...hat.com>,
        Alessandro Zummo <a.zummo@...ertech.it>,
        linux-rtc@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>
Subject: Re: [patch 0/8] ntp/rtc: Fixes and cleanups

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