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From: Thomas Gleixner <tglx@linutronix.de>
To: LKML <linux-kernel@vger.kernel.org>
Cc: Alexandre Belloni <alexandre.belloni@bootlin.com>,
	Jason Gunthorpe <jgg@ziepe.ca>,
	Miroslav Lichvar <mlichvar@redhat.com>,
	John Stultz <john.stultz@linaro.org>,
	Prarit Bhargava <prarit@redhat.com>,
	Alessandro Zummo <a.zummo@towertech.it>,
	linux-rtc@vger.kernel.org, Peter Zijlstra <peterz@infradead.org>
Subject: Re: [patch 0/8] ntp/rtc: Fixes and cleanups
Date: Wed, 09 Dec 2020 01:33:08 +0100	[thread overview]
Message-ID: <87sg8f24zf.fsf@nanos.tec.linutronix.de> (raw)
In-Reply-To: <20201206214613.444124194@linutronix.de>

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

  parent reply	other threads:[~2020-12-09  0:33 UTC|newest]

Thread overview: 54+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-12-06 21:46 [patch 0/8] ntp/rtc: Fixes and cleanups Thomas Gleixner
2020-12-06 21:46 ` [patch 1/8] rtc: mc146818: Prevent reading garbage Thomas Gleixner
2020-12-11 10:07   ` [tip: timers/core] " tip-bot2 for Thomas Gleixner
2021-01-25 18:40   ` [patch 1/8] rtc: mc146818: Prevent reading garbage - bug Mickaël Salaün
2021-01-26 13:26     ` Thomas Gleixner
2021-01-26 14:17       ` Mickaël Salaün
2021-01-26 15:35         ` Thomas Gleixner
2021-01-26 17:02           ` [PATCH V2] rtc: mc146818: Detect and handle broken RTCs Thomas Gleixner
2021-01-26 18:00             ` Alexandre Belloni
2021-01-27  8:41             ` [tip: timers/urgent] " tip-bot2 for Thomas Gleixner
2021-01-31 10:59             ` [PATCH V2] " Dirk Gouders
2021-02-01 13:53               ` Serge Belyshev
2021-02-01 19:09                 ` [PATCH] rtc: mc146818: Dont test for bit 0-5 in Register D Thomas Gleixner
2021-02-01 19:24                   ` [PATCH V2] " Thomas Gleixner
2021-02-01 19:32                     ` Linus Torvalds
2021-02-01 19:40                       ` Thomas Gleixner
2021-02-11 23:09                         ` Maciej W. Rozycki
2021-02-01 19:38                     ` Alexandre Belloni
2021-02-01 20:09                     ` Borislav Petkov
2021-02-01 20:15                     ` Dirk Gouders
2021-02-02  4:22                     ` Len Brown
2021-02-02 19:40                     ` [tip: timers/urgent] " tip-bot2 for Thomas Gleixner
2021-02-03 13:00                     ` [PATCH V2] " Mickaël Salaün
2020-12-06 21:46 ` [patch 2/8] rtc: mc146818: Reduce spinlock section in mc146818_set_time() Thomas Gleixner
2020-12-11 10:07   ` [tip: timers/core] " tip-bot2 for Thomas Gleixner
2020-12-06 21:46 ` [patch 3/8] rtc: cmos: Make rtc_cmos sync offset correct Thomas Gleixner
2020-12-07 20:50   ` Jason Gunthorpe
2020-12-11 10:07   ` [tip: timers/core] " tip-bot2 for Thomas Gleixner
2020-12-06 21:46 ` [patch 4/8] rtc: core: Make the sync offset default more realistic Thomas Gleixner
2020-12-07 20:55   ` Jason Gunthorpe
2020-12-10 23:59   ` Alexandre Belloni
2020-12-11  0:23     ` Thomas Gleixner
2020-12-11  0:28       ` Alexandre Belloni
2020-12-11 10:07   ` [tip: timers/core] " tip-bot2 for Thomas Gleixner
2020-12-06 21:46 ` [patch 5/8] ntp: Make the RTC synchronization more reliable Thomas Gleixner
2020-12-07 12:47   ` Miroslav Lichvar
2020-12-07 20:59   ` Jason Gunthorpe
2020-12-11 10:07   ` [tip: timers/core] " tip-bot2 for Thomas Gleixner
2020-12-29 19:41   ` [patch 5/8] " Geert Uytterhoeven
2021-01-11 10:12     ` Thomas Gleixner
2021-01-11 10:40       ` Geert Uytterhoeven
2020-12-06 21:46 ` [patch 6/8] ntp, rtc: Move rtc_set_ntp_time() to ntp code Thomas Gleixner
2020-12-07 20:59   ` Jason Gunthorpe
2020-12-09  3:51     ` Alexandre Belloni
2020-12-11 10:07   ` [tip: timers/core] " tip-bot2 for Thomas Gleixner
2020-12-06 21:46 ` [patch 7/8] ntp: Make the RTC sync offset less obscure Thomas Gleixner
2020-12-11 10:07   ` [tip: timers/core] " tip-bot2 for Thomas Gleixner
2020-12-06 21:46 ` [patch 8/8] ntp: Consolidate the RTC update implementation Thomas Gleixner
2020-12-07 21:05   ` Jason Gunthorpe
2020-12-11  9:23     ` Thomas Gleixner
2020-12-11 10:07   ` [tip: timers/core] " tip-bot2 for Thomas Gleixner
2020-12-09  0:33 ` Thomas Gleixner [this message]
2020-12-09  4:01   ` [patch 0/8] ntp/rtc: Fixes and cleanups Alexandre Belloni
2020-12-09 12:39     ` Thomas Gleixner

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