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    SubjectRe: 500 ms delay in time saved into RTC
    Hi,

    On 19/02/2018 at 12:03:28 +0100, Karel Zak wrote:
    > On Mon, Feb 19, 2018 at 08:11:05AM +0100, Rasmus Villemoes wrote:
    > > It's because util-linux's hwclock still assumes the world is x86. See
    > > this comment in the util-linux source code:
    > >
    > > /*
    > > * The Hardware Clock can only be set to any integer time plus one
    > > * half second. The integer time is required because there is no
    > > * interface to set or get a fractional second. The additional half
    > > * second is because the Hardware Clock updates to the following
    > > * second precisely 500 ms (not 1 second!) after you release the
    > > * divider reset (after setting the new time) - see description of
    > > * DV2, DV1, DV0 in Register A in the MC146818A data sheet (and note
    > >
    > > So if hwclock is asked to --systohc at time 01:02:03.x, it waits until
    > > the time is 01:02:03.5 to set the rtc to 01:02:03, or if that has
    > > already passed, waits until 01:02:04.5 and sets it to 01:02:04.
    > >
    > > On our ARM BSP we patch util-linux to have the "implicit fractional
    > > part" configurable, and trying to upstream something like that has been
    > > on my todo-list for quite a while. See
    > >
    > > https://raw.githubusercontent.com/oe-lite/base/master/recipes/util-linux/util-linux-2.29/hwclock-tweak-delay.patch
    > >
    > > for the patch we currently use (on top of that, we change the 0.5
    > > initializer to 0.0 to avoid having to always pass the --delay argument).
    > > Incidentally, it seems we're on the same util-linux version, so you
    > > should be able to try out that patch and see if it works for you.
    >
    > Would be possible to somehow detect what is the right default setting for
    > --delay? I mean for example detect architecture / clock HW, etc.
    >
    > I have no problem with --delay, but it's tuning for advanced users and
    > HW specific stuff. It would be nice to have something more portable.
    >

    This is what I'm using to synchronize the RTC to the system time:

    https://git.kernel.org/pub/scm/linux/kernel/git/abelloni/rtc-tools.git/tree/rtc-sync.c

    With sane RTCs, it manages to do so with a good precision, 10µs on a
    pcf85363 connected to a sama5d4 xplained, 96µs on my PC.

    It can still be improved and doesn't handle RTCs in localtime.

    I'm planning to integrate that in hwclock at some point i(hopefully
    soon) but I didn't have the time to dive too much in the code yet.

    --
    Alexandre Belloni, Bootlin (formerly Free Electrons)
    Embedded Linux and Kernel engineering
    https://bootlin.com

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