From: Alexandre Belloni <alexandre.belloni@bootlin.com>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: LKML <linux-kernel@vger.kernel.org>,
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, 9 Dec 2020 05:01:55 +0100 [thread overview]
Message-ID: <20201209040155.GB1245199@piout.net> (raw)
In-Reply-To: <87sg8f24zf.fsf@nanos.tec.linutronix.de>
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
next prev parent reply other threads:[~2020-12-09 4:02 UTC|newest]
Thread overview: 44+ 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
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
2021-01-31 10:59 ` 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
2021-02-03 13:00 ` Mickaël Salaün
2020-12-06 21:46 ` [patch 2/8] rtc: mc146818: Reduce spinlock section in mc146818_set_time() 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
2020-12-06 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
2020-12-06 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
2020-12-29 19:41 ` 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
2020-12-06 21:46 ` [patch 7/8] ntp: Make the RTC sync offset less obscure 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
2020-12-09 0:33 ` [patch 0/8] ntp/rtc: Fixes and cleanups Thomas Gleixner
2020-12-09 4:01 ` Alexandre Belloni [this message]
2020-12-09 12:39 ` Thomas Gleixner
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20201209040155.GB1245199@piout.net \
--to=alexandre.belloni@bootlin.com \
--cc=a.zummo@towertech.it \
--cc=jgg@ziepe.ca \
--cc=john.stultz@linaro.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-rtc@vger.kernel.org \
--cc=mlichvar@redhat.com \
--cc=peterz@infradead.org \
--cc=prarit@redhat.com \
--cc=tglx@linutronix.de \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).