[Linux-aus] Ubuntu 20.04 resume time problem

[NeilBrown]

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".

NeilBrown




> 
> Before someone says "oh what about the time change at the start/end of DST", 
> for a laptop it's not a big deal as it will probably be in sleep mode then. As 
> timedatectl supports this feature I think it's not unreasonable to expect it 
> to work.
> 
> -- 
> My Main Blog         http://etbe.coker.com.au/
> My Documents Blog    http://doc.coker.com.au/
> 
> 
> 
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