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From:   Arnd Bergmann <arnd@arndb.de>
Date:   Thu, 15 Aug 2019 15:22:45 +0200
Message-ID: <CAK8P3a0VQ1F5xnRNmfeg-rAWbKb64u_8xfQjFNahNRoAHTMJ3g@mail.gmail.com>
Subject: Re: New kernel interface for sys_tz and timewarp?
To:     "H. Peter Anvin" <hpa@zytor.com>
Cc:     David Laight <David.Laight@aculab.com>,
        "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>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Alistair Francis <alistair.francis@wdc.com>,
        GNU C Library <libc-alpha@sourceware.org>,
        Karel Zak <kzak@redhat.com>,
        Lennart Poettering <lennart@poettering.net>,
        OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
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On Wed, Aug 14, 2019 at 6:48 PM <hpa@zytor.com> wrote:
>
> I believe Windows 10 changed the default RTC to UTC, although perhaps
> only if running under UEFI.

I looked at the efi rtc driver now, and found two things:

- The EFI get_time() call passes down timezone information, so we know what
   UTC is, and can just ignore the timezone. This is good.

- The RTC_DRV_EFI depends on !X86 as of commit 7efe665903d0 ("rtc:
  Disable EFI rtc for x86"). This unfortunately means we always fall back to
  either the rtc-cmos driver or the x86 specific read_persistent_clock64()
  implementation even when the EFI RTC is reliable.

If 64-bit Windows relies on a working EFI RTC implementation, we could
decide to leave the driver enabled on 64-bit and only disable it for
32-bit EFI. That way, future distros would no longer have to worry about
the localtime hack, at least the ones that have dropped support for
32-bit x86 kernels.

        Arnd