linux-kernel - RE: New kernel interface for sys

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Message-ID: <0089C4CC-DD85-48A1-869B-A9D71852BEC7@zytor.com>
Date:   Wed, 14 Aug 2019 09:47:39 -0700
From:   hpa@...or.com
To:     David Laight <David.Laight@...LAB.COM>,
        "'Theodore Y. Ts'o'" <tytso@....edu>,
        Linus Torvalds <torvalds@...ux-foundation.org>
CC:     Arnd Bergmann <arnd@...db.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        John Stultz <john.stultz@...aro.org>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        Stephen Boyd <sboyd@...nel.org>,
        Florian Weimer <fweimer@...hat.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Alistair Francis <alistair.francis@....com>,
        GNU C Library <libc-alpha@...rceware.org>,
        Karel Zak <kzak@...hat.com>,
        Lennart Poettering <lennart@...ttering.net>,
        OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
Subject: RE: New kernel interface for sys_tz and timewarp?

On August 14, 2019 9:26:36 AM PDT, David Laight <David.Laight@...LAB.COM> wrote:
>From: Theodore Y. Ts'o
>> Sent: 14 August 2019 01:06
>> On Tue, Aug 13, 2019 at 10:30:34AM -0700, Linus Torvalds wrote:
>> >
>> > I suspect the only actual _valid_ use in the kernel for a time zone
>> > setting is likely for RTC clock setting, but even that isn't really
>> > "global", as much as "per RTC".
>> 
>> As I recall (and I may or may not have been original for the original
>> sys_tz; it was many years ago, and my memories of 1992 are a bit
>> fuzzy) the only reason why we added it was because x86 systems that
>> were dual-booting with Windows had a RTC which ticked localtime, and
>> originally, the system time was fetched from the RTC in early boot,
>> and then when the timezone was set, the time would be warped so it
>> would be correct.
>
>x86 systems are very likely to have the RTC set by the bios config.
>In which case it will almost certainly get set to local time.
>It is certainly the default for windows installs - I don't even know
>if you have any other option.
>
>The 'real fun' (tm) happens when a dual boot system changes from
>winter to summer time.
>ISTR that it is quite easy to get both (or more) OS to change the
>RTC by an hour (I went home an hour early one year).
>Although the x86 RTC chip has a bit defined for 'summertime', nothing
>sets it (at least when I looked).
>
>	David
>
>-
>Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes,
>MK1 1PT, UK
>Registration No: 1397386 (Wales)

I believe Windows 10 changed the default RTC to UTC, although perhaps only if running under UEFI.
-- 
Sent from my Android device with K-9 Mail. Please excuse my brevity.