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

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Message-ID: <20201209040155.GB1245199@piout.net>
Date:   Wed, 9 Dec 2020 05:01:55 +0100
From:   Alexandre Belloni <alexandre.belloni@...tlin.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        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

On 09/12/2020 01:33:08+0100, Thomas Gleixner wrote:
> 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?
> 

This looks very good to me, however, I think the 10ms offset is a bit
too much. Do you mind waiting one or two days so I can get my test setup
back up?


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