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=-3.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED 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 CD990C433C1 for ; Tue, 30 Mar 2021 00:03:00 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 8FFE461990 for ; Tue, 30 Mar 2021 00:03:00 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230347AbhC3AC3 convert rfc822-to-8bit (ORCPT ); Mon, 29 Mar 2021 20:02:29 -0400 Received: from relay11.mail.gandi.net ([217.70.178.231]:38497 "EHLO relay11.mail.gandi.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229911AbhC3ACH (ORCPT ); Mon, 29 Mar 2021 20:02:07 -0400 Received: from localhost (lfbn-lyo-1-1676-55.w90-65.abo.wanadoo.fr [90.65.108.55]) (Authenticated sender: alexandre.belloni@bootlin.com) by relay11.mail.gandi.net (Postfix) with ESMTPSA id EBE6F100002; Tue, 30 Mar 2021 00:02:05 +0000 (UTC) Date: Tue, 30 Mar 2021 02:02:05 +0200 From: Alexandre Belloni To: Lukasz Stelmach Cc: Alessandro Zummo , linux-rtc@vger.kernel.org, =?utf-8?Q?Bart=C5=82omiej_=C5=BBolnierkiewicz?= , Marek Szyprowski , linux-kernel@vger.kernel.org Subject: Re: [PATCH] rtc: ds1307: set uie_unsupported if no interrupt is available Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: 8BIT In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-rtc@vger.kernel.org On 16/03/2021 19:04:14+0100, Lukasz Stelmach wrote: > OK, you are right. The problem seems to be elsewhere. > > How about this scnario? We call rtc_update_irq_enable(). We read rtc > with __rtc_read_time() and calculate the alarm time. We get through > rtc_timer_enqueue() and down to __rtc_set_alarm(). We loose the race > condition (I can do it, I've got really slow connection to DS3231) and > we return -ETIME from __rtc_set_alarm() > > if (scheduled <= now) > return -ETIME; > > and 0 from rtc_timer_enqueue() and the very same zero from > rtc_update_irq_enable(). The caller of ioctl() thinks they can expect > interrupts when, in fact, they won't receive any. > > The really weird stuff happens in rtc_timer_do_work(). For the timer to > be dequeued __rtc_set_alarm() needs to return EINVAL three times in a > row. In my setup this doesn't happen and the code keeps running loops > around "reporogram" and "again" labels. > > With my patch we never risk the above race condition between > __rtc_read_time() in rtc_update_irq_enable() and the one in > __rtc_set_alarm(), because we know rtc doesn't support alarms before we > start the race. In fact there is another race between __rtc_read_time() > and actually setting the alarm in the chip. > > IMHO the solution is to introduce RTC_HAS_ALARM flag for struct > rtc_device and check it at the very beginning of __rtc_set_alarm() the > same way it is being done in ds1337_set_alarm(). What are your thoughts? > I did introduce RTC_FEATURE_ALARM for that in v5.12. I'm sending patches that are not well tested but should solve your issue. -- Alexandre Belloni, co-owner and COO, Bootlin Embedded Linux and Kernel engineering https://bootlin.com