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
