On Mon, 2017-02-27 at 11:18 -0800, Russ Allbery wrote:
> > Daniel Pocock <daniel@pocock.pro> writes:
>
> > I've observed a system that had a wildly incorrect hardware clock (when
> > it was first unboxed), I ran ntpdate to sync the kernel clock but after
> > a shutdown and startup again it had a wacky time again.
> > I came across the discussion about how the hardware clock is no longer
> > set at shutdown[1]
> > The system has ntpd running
> > Looking at the output of
> > adjtimex --print | grep status
> > the bit corresponding to 64 / STA_UNSYNC is 0
> > There is a time and date page on the wiki[2] and in the manual[3],
> > neither of them appears to have up to date information about the way it
> > works with systemd or how to troubleshoot issues like this.
>
> My understanding from reading a bit about this just now is that the short
> version is "install ntpd if you want this to happen."
>
> My impression is that ntpdate has been obsolete for years and upstream has
> been slowly trying to kill it. ntpd is the upstream-supported daemon, and
> it periodically asks the kernel to set the hardware clock.
The kernel actually does the periodic setting automatically, so long as
the NTP server reports that it's synchronised (by clearing STA_UNSYNC
in timex::status).
(The kernel will only set one RTC device, which is specified in the
build config. On systems that have multiple RTCs and only one of them
works (e.g. the one in the SoC doesn't have battery power but the one
in the PMIC does) this may not work properly. It may be fixable by
disabling the broken RTC in the device tree.)
> (And it
> supports various command-line options to make it act like ntpdate if you
> really want.)
>
> The much simpler systemd-timesyncd doesn't set the hardware clock for
> reasons that one may or may not agree with (I honestly haven't researched
> it in any depth),
It looks like it does iff the RTC is set to UTC:
/*
* An unset STA_UNSYNC will enable the kernel's 11-minute mode,
* which syncs the system time periodically to the RTC.
*
* In case the RTC runs in local time, never touch the RTC,
* we have no way to properly handle daylight saving changes and
* mobile devices moving between time zones.
*/
if (m->rtc_local_time)
tmx.status |= STA_UNSYNC;
> but you can just run ntpd instead if you care.
But ntpd is also known to have a large amount of code written without
as much regard for security as one would hope. It seems like an
unnecessary risk for most systems.
Ben.
> Alternately, if you really want to use a clock setting mechanism that
> doesn't ask the kernel to sync the hardware clock but you still want to
> set the hardware clock, you can add your own shutdown init script / unit
> to run hwclock --systohc (or even a cron job if you want).
>
--
Ben Hutchings
This sentence contradicts itself - no actually it doesn't.
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