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    SubjectRe: [patch 0/8] ntp/rtc: Fixes and cleanups
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    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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