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Date:   Tue, 20 Aug 2019 20:58:30 +0200
From:   Alexandre Belloni <alexandre.belloni@bootlin.com>
To:     Karel Zak <kzak@redhat.com>
Cc:     Lennart Poettering <mzxreary@0pointer.de>,
        Arnd Bergmann <arnd@arndb.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 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.

> > 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
https://bootlin.com