From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id DA4FCC3A589 for ; Tue, 20 Aug 2019 18:58:35 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id B2C9F2332A for ; Tue, 20 Aug 2019 18:58:35 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730623AbfHTS6e (ORCPT ); Tue, 20 Aug 2019 14:58:34 -0400 Received: from relay4-d.mail.gandi.net ([217.70.183.196]:60437 "EHLO relay4-d.mail.gandi.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729231AbfHTS6e (ORCPT ); Tue, 20 Aug 2019 14:58:34 -0400 X-Originating-IP: 90.65.161.137 Received: from localhost (lfbn-1-1545-137.w90-65.abo.wanadoo.fr [90.65.161.137]) (Authenticated sender: alexandre.belloni@bootlin.com) by relay4-d.mail.gandi.net (Postfix) with ESMTPSA id 7A583E000A; Tue, 20 Aug 2019 18:58:30 +0000 (UTC) Date: Tue, 20 Aug 2019 20:58:30 +0200 From: Alexandre Belloni To: Karel Zak Cc: Lennart Poettering , Arnd Bergmann , "Theodore Y. Ts'o" , Linus Torvalds , Linux Kernel Mailing List , Thomas Gleixner , John Stultz , Stephen Boyd , Florian Weimer , "H. Peter Anvin" , Palmer Dabbelt , Alistair Francis , GNU C Library , OGAWA Hirofumi Subject: Re: New kernel interface for sys_tz and timewarp? Message-ID: <20190820185830.GQ3545@piout.net> References: <20190814000622.GB20365@mit.edu> <20190814090936.GB10516@gardel-login> <20190814093208.GG3600@piout.net> <20190819110903.if3dzhvfnlqutn6s@ws.net.home> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190819110903.if3dzhvfnlqutn6s@ws.net.home> User-Agent: Mutt/1.12.1 (2019-06-15) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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