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Date:   Tue, 27 Aug 2019 18:31:34 +0200
From:   Alexandre Belloni <alexandre.belloni@bootlin.com>
To:     Arnd Bergmann <arnd@arndb.de>
Cc:     Karel Zak <kzak@redhat.com>,
        Lennart Poettering <mzxreary@0pointer.de>,
        "Theodore Y. Ts'o" <tytso@mit.edu>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        John Stultz <john.stultz@linaro.org>,
        Stephen Boyd <sboyd@kernel.org>,
        Florian Weimer <fweimer@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Alistair Francis <alistair.francis@wdc.com>,
        GNU C Library <libc-alpha@sourceware.org>,
        OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Subject: Re: New kernel interface for sys_tz and timewarp?
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On 27/08/2019 18:27:32+0200, Arnd Bergmann wrote:
> On Tue, Aug 20, 2019 at 8:58 PM Alexandre Belloni
> <alexandre.belloni@bootlin.com> wrote:
> >
> > 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@arndb.de) wrote:
> > > > >
> > > > > > - glibc stops passing the caller timezone argument to the kernel
> > > > > > - the distro kernel disables CONFIG_RTC_HCTOSYS,
> > > > > >   CONFIG_RTC_SYSTOHC  and CONFIG_GENERIC_CMOS_UPDATE
> > > > >
> > > > > 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.
> 
> Would you use the regular RTC_SET_TIME ioctl for that, or
> add a new RTC_SYS_TO_HC command that takes an explicit
> offset? It sounds to me that the synchronization bit (actually
> waiting for the right moment to update the rtc registers) is better
> done in the kernel, while the decision about the offset and when
> to call into the driver is better done in user space.
> 

The existing ioctls are fine to do that, see:
https://git.kernel.org/pub/scm/linux/kernel/git/abelloni/rtc-tools.git/tree/rtc-sync.c

-- 
Alexandre Belloni, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com