Re: New kernel interface for sys_tz and timewarp?

From: Alexandre Belloni
Date: Tue Aug 20 2019 - 14:58:36 EST

On 19/08/2019 13:09:03+0200, Karel Zak wrote:
> On Wed, Aug 14, 2019 at 11:32:08AM +0200, Alexandre Belloni wrote:
> > On 14/08/2019 11:09:36+0200, Lennart Poettering wrote:
> > > On Mi, 14.08.19 10:31, Arnd Bergmann (arnd@xxxxxxxx) wrote:
> > >
> > > > - glibc stops passing the caller timezone argument to the kernel
> > > > - the distro kernel disables CONFIG_RTC_HCTOSYS,
> > >
> > > What's the benefit of letting userspace do this? It sounds a lot more
> > > fragile to leave this syncing to userspace if the kernel can do this
> > > trivially on its own.
> Good point, why CONFIG_RTC_SYSTOHC has been added to the kernel?
> If I good remember than it's because synchronize userspace hwclock
> with rtc is pretty fragile and frustrating. We have improved this
> hwclock code many times and it will never be perfect. See for example
> hwclock --delay= option, sometimes hwclock has no clue about RTC behaviour.

With a bit of care, we can reliably set the rtc to the system time from
userspace. It takes a bit of time (up to 2 seconds) but it can be
reliably set with an accuracy of a few ms on a slow system and an rtc on
a slow bus or a few ns with a fast system and a fast bus.
I know I did say I would implement it in hwclock and I still didn't
(sorry) but we could do better than the --delay option.

> > It does it trivially and badly:
> >
> > - hctosys will always think the RTC is in UTC so if the RTC is in
> > local time, you will anyway have up to 12 hours difference until
> > userspace fixes that.
> Cannot we provide all necessary information for example on kernel
> command line, or/and as rtc module option?

We could but from a distro point of view, would that be convenient?

Alexandre Belloni, Bootlin
Embedded Linux and Kernel engineering