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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
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