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Re: New kernel interface for sys_tz and timewarp?
- From: Alexandre Belloni <alexandre dot belloni at bootlin dot com>
- To: Karel Zak <kzak at redhat dot com>
- Cc: Lennart Poettering <mzxreary at 0pointer dot de>, Arnd Bergmann <arnd at arndb dot de>, "Theodore Y. Ts'o" <tytso at mit dot edu>, Linus Torvalds <torvalds at linux-foundation dot org>, Linux Kernel Mailing List <linux-kernel at vger dot kernel dot org>, Thomas Gleixner <tglx at linutronix dot de>, John Stultz <john dot stultz at linaro dot org>, Stephen Boyd <sboyd at kernel dot org>, Florian Weimer <fweimer at redhat dot com>, "H. Peter Anvin" <hpa at zytor dot com>, Palmer Dabbelt <palmer at dabbelt dot com>, Alistair Francis <alistair dot francis at wdc dot com>, GNU C Library <libc-alpha at sourceware dot org>, OGAWA Hirofumi <hirofumi at mail dot parknet dot co dot jp>
- Date: Tue, 20 Aug 2019 20:58:30 +0200
- Subject: Re: New kernel interface for sys_tz and timewarp?
- References: <CAK8P3a0VxM1BkjY1D2FfHi6L-ho_NH3v3+gBu45EfpjLF5NU5w@mail.gmail.com> <CAHk-=wiO2CWONDBud4nxoPgUJN1JEewFWhHa5wAqY8G5rrTXRQ@mail.gmail.com> <20190814000622.GB20365@mit.edu> <CAK8P3a1CXRETxn6Gh_cOxM3rZ-wUwVDu-7=yEwjqOY=uEdC6OQ@mail.gmail.com> <20190814090936.GB10516@gardel-login> <20190814093208.GG3600@piout.net> <20190819110903.if3dzhvfnlqutn6s@ws.net.home>
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