Messages in this thread | | | Date | Sun, 25 Feb 2018 01:34:58 +0100 | From | Alexandre Belloni <> | Subject | Re: 500 ms delay in time saved into RTC |
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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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