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Date:   Wed, 14 Aug 2019 11:09:36 +0200
From:   Lennart Poettering <mzxreary@0pointer.de>
To:     Arnd Bergmann <arnd@arndb.de>
Cc:     "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>,
        Alexandre Belloni <alexandre.belloni@bootlin.com>,
        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>,
        Karel Zak <kzak@redhat.com>,
        OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Subject: Re: New kernel interface for sys_tz and timewarp?
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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.

IIRC there are uses in kernel that use CLOCK_REALTIME already before
userspace starts. e.g. iirc networking generally prefers
CLOCK_REALTIME timestamps over CLOCK_MONOTONIC timestamps
(i.e. SO_TIMESTAMP and friends are still CLOCK_REALTIME only so far,
unless I am missing something). If the kernel comes up with a
CLOCK_REALTIME that starts at 0 this is pretty annoying I
figure... Hence, so far I suggested to distros to continue turning on
the options above, and let the kernel do this on its own without
involving userspace in that.

Lennart

--
Lennart Poettering, Berlin