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=-13.7 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=unavailable 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 41BA9C4167B for ; Fri, 11 Dec 2020 00:01:12 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id EC17B23EB0 for ; Fri, 11 Dec 2020 00:01:11 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2389149AbgLKAAp (ORCPT ); Thu, 10 Dec 2020 19:00:45 -0500 Received: from relay1-d.mail.gandi.net ([217.70.183.193]:31781 "EHLO relay1-d.mail.gandi.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2389587AbgLJX7w (ORCPT ); Thu, 10 Dec 2020 18:59:52 -0500 X-Originating-IP: 86.194.74.19 Received: from localhost (lfbn-lyo-1-997-19.w86-194.abo.wanadoo.fr [86.194.74.19]) (Authenticated sender: alexandre.belloni@bootlin.com) by relay1-d.mail.gandi.net (Postfix) with ESMTPSA id 0C1D1240006; Thu, 10 Dec 2020 23:59:08 +0000 (UTC) Date: Fri, 11 Dec 2020 00:59:08 +0100 From: Alexandre Belloni To: Thomas Gleixner Cc: LKML , Jason Gunthorpe , Miroslav Lichvar , John Stultz , Prarit Bhargava , Alessandro Zummo , linux-rtc@vger.kernel.org, Peter Zijlstra Subject: Re: [patch 4/8] rtc: core: Make the sync offset default more realistic Message-ID: <20201210235908.GA1781038@piout.net> References: <20201206214613.444124194@linutronix.de> <20201206220541.960333166@linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201206220541.960333166@linutronix.de> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi, On 06/12/2020 22:46:17+0100, Thomas Gleixner wrote: > The offset which is used to steer the start of an RTC synchronization > update via rtc_set_ntp_time() is huge. The math behind this is: > > tsched twrite(t2.tv_sec - 1) t2 (seconds increment) > > twrite - tsched is the transport time for the write to hit the device. > > t2 - twrite depends on the chip and is for most chips one second. > > The rtc_set_ntp_time() calculation of tsched is: > > tsched = t2 - 1sec - (t2 - twrite) > > The default for the sync offset is 500ms which means that twrite - tsched > is 500ms assumed that t2 - twrite is one second. > > This is 0.5 seconds off for RTCs which are directly accessible by IO writes > and probably for the majority of i2C/SPI based RTC off by an order of > magnitude. Set it to 10ms which should bring it closer to reality. > > The default can be adjusted by drivers (rtc_cmos does so) and could be > adjusted further by a calibration method which is an orthogonal problem. > > Signed-off-by: Thomas Gleixner > --- > drivers/rtc/class.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > --- a/drivers/rtc/class.c > +++ b/drivers/rtc/class.c > @@ -201,7 +201,7 @@ static struct rtc_device *rtc_allocate_d > device_initialize(&rtc->dev); > > /* Drivers can revise this default after allocating the device. */ > - rtc->set_offset_nsec = NSEC_PER_SEC / 2; > + rtc->set_offset_nsec = 10 * NSEC_PER_MSEC; I did retest, on a slow 100kHz i2c bus, with a fairly inconvenient RTC, The maximum offset to set the RTC was 4845533ns so I'd say 10ms is too large. Should we make that 5ms ? Apart from that, on the series, you can add my Acked-by: Alexandre Belloni -- Alexandre Belloni, Bootlin Embedded Linux and Kernel engineering https://bootlin.com