[Linux-aus] Ubuntu 20.04 resume time problem

[Russell Coker]

On Tuesday, 28 June 2022 13:30:57 AEST NeilBrown via linux-aus wrote:
> On Tue, 28 Jun 2022, Russell Coker wrote:
> > I have a laptop that is dual boot Ubuntu 20.04 and Windows. I've
> > configured it for "RTC in local TZ" with timedatectl so it can have the
> > same time as Windows (I'm aware that I could change Windows via regedit
> > to use UTC for the hwclock but I'm trying to work out the best option for
> > a corporate rollout).  The problem is that I resume from suspend and the
> > system immediately treats the RTC as if it was in UTC thus being out by
> > 10 hours (time zone +1000).
> > 
> > How does Ubuntu get the time from the RTC on resume? Any clues as to where
> > to look would be really appreciated. I've done Google searches and found
> > nothing helpful. Even if you just know which program does it that would
> > help a lot, I could read the source.
> 
> It isn't up to Ubuntu - the Linux kernel does (or should do) everything
> needed.
> Providing the kernel is built with CONFIG_PM_SLEEP and
> CONFIG_RTC_HCTOSYS_DEVICE, and providing you have RTC hardware with a
> suitable driver, then on suspend rtc_suspend() in drivers/rtc/class.c
> will take note of the value stored in the rtc, and on resume
> rtc_resume() will check what the difference was and update the clock.
> 
> cat /sys/class/rtc/rtc0/hctosys
> 
> should show "1" if everythings is configured properly.  In that case it
> should "just work".

It does show "1" and the kernel build options include those ones you specify, 
but unfortunately it doesn't "just work".

NTP the protocol allows fixing this with the systemd-timesyncd implementation, 
the ntpd implementation gets unhappy when the time is more than 20 minutes 
out.  With systemd-timesyncd it takes some time to get back to the right time, 
sometimes more than 10 minutes.

I have not seen this cause a SSL problem.  When this happens Kerberos is no 
more broken than usual (that's another issue I have to solve).

Yes I know that using the local time for the RTC has some downsides, but every 
option for dual boot has some downsides.

Thanks for the suggestions.

-- 
My Main Blog         http://etbe.coker.com.au/
My Documents Blog    http://doc.coker.com.au/