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=-2.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_MUTT autolearn=ham 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 E79C6C43219 for ; Wed, 1 May 2019 20:30:34 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id C156920656 for ; Wed, 1 May 2019 20:30:34 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726096AbfEAUae (ORCPT ); Wed, 1 May 2019 16:30:34 -0400 Received: from relay10.mail.gandi.net ([217.70.178.230]:60931 "EHLO relay10.mail.gandi.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726077AbfEAUae (ORCPT ); Wed, 1 May 2019 16:30:34 -0400 Received: from localhost (lfbn-1-3034-80.w90-66.abo.wanadoo.fr [90.66.53.80]) (Authenticated sender: alexandre.belloni@bootlin.com) by relay10.mail.gandi.net (Postfix) with ESMTPSA id BE03B240008; Wed, 1 May 2019 20:30:23 +0000 (UTC) Date: Wed, 1 May 2019 22:30:23 +0200 From: Alexandre Belloni To: Trent Piepho Cc: "patrice.chotard@st.com" , "linux-rtc@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "linux-arm-kernel@lists.infradead.org" Subject: Re: [PATCH] rtc: st-lpc: remove unnecessary check Message-ID: <20190501203023.GL11339@piout.net> References: <20190430201834.12634-1-alexandre.belloni@bootlin.com> <1556663479.31309.36.camel@impinj.com> <20190501142513.GK11339@piout.net> <1556730703.31309.53.camel@impinj.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1556730703.31309.53.camel@impinj.com> User-Agent: Mutt/1.11.3 (2019-02-01) Sender: linux-rtc-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-rtc@vger.kernel.org On 01/05/2019 17:11:44+0000, Trent Piepho wrote: > > I can't believe you can possibly have more than one second between the > > check in the core and the check in the driver, it doesn't make much > > sense to check, even in the current state of the core. > > It's certainly possible to have multiple seconds pass. For an external > device over SPI or I2C, one has to wait for the bus to become free. > And on SPI that requires the kernel thread running the bus to be > scheduled. Just put in some real-time tasks and maybe a big transfer > to a flash chip and it could be a while before that happens. > > I don't think this device has that issue as I don't think it's > external. And ever for a device on an external bus, delays > 1 second > are unlikely. Possible, but unlikely. > > You can also get them when Linux is running under a hypervisor, i.e. a > Linux VM. But also something like an NMI and ACPI BIOS. If the Linux > guest is not scheduled to run for while anything that is supposed to be > based on real time, like the value returned by an RTC, will still > advance. It is possible that multiple seconds elapse from the guest > CPU executing one instruction to the next. > > But even ignoring that, does it require > 1 second to elapse. Can't it > happen when the clock ticks from one second to the next, which happens > effectively instantly? > > If the time from the check to the time when the alarm is set is 1 > microsecond, and the time this call to set the alarm is made is > randomly done and not synchronized to the RTC, then isn't there a 1 out > of 1 million chance (1 microsecond / 1 second), that the once per > second clock tick will hit our 1 us window? No, let's say you want Talarm == Tcurrent + 1, if the core check happens right before the next second, then you necessarily end up with Talarm == Tcurrent after the check. This means that you now have one second before the time read in st-lpc to avoid the alarm_secs -= now_secs; underflow. Obviously, in that case, you are likely to miss the alarm but this is as likely to happen with the check that is in the driver. This check doesn't provide anything but a false sense of security. -- Alexandre Belloni, Bootlin Embedded Linux and Kernel engineering https://bootlin.com