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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 4D96FC433EF for ; Sat, 12 Feb 2022 22:12:26 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229604AbiBLWM1 (ORCPT ); Sat, 12 Feb 2022 17:12:27 -0500 Received: from mxb-00190b01.gslb.pphosted.com ([23.128.96.19]:36470 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230283AbiBLWM0 (ORCPT ); Sat, 12 Feb 2022 17:12:26 -0500 X-Greylist: delayed 400 seconds by postgrey-1.37 at lindbergh.monkeyblade.net; Sat, 12 Feb 2022 14:12:22 PST Received: from mx-out.tlen.pl (mx-out.tlen.pl [193.222.135.142]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 82FEE60A84 for ; Sat, 12 Feb 2022 14:12:22 -0800 (PST) Received: (wp-smtpd smtp.tlen.pl 18600 invoked from network); 12 Feb 2022 23:05:38 +0100 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=o2.pl; s=1024a; t=1644703539; bh=AnQN+FJyRGopKtB+HTr7nThFXwjuArnFpHKHUUQNYoY=; h=From:To:Cc:Subject; b=r1TDnwTmPuDEihAsAdSvWAbdrgAuAZ6lf4OSZ/Y3JhQJcTbr3KbTzQu+/IsYxqcwC Y8zoZXDd1OmT5Qx1VJmD1j4gzeEgXhoIvYutCc4aNg5s+Sjut04B26/ay3xqtRr/xL ueakEzF45e8lnB18Ho3IWp6NDSwUG09I64hMKuO0= Received: from aaem217.neoplus.adsl.tpnet.pl (HELO localhost.localdomain) (mat.jonczyk@o2.pl@[83.4.116.217]) (envelope-sender ) by smtp.tlen.pl (WP-SMTPD) with SMTP for ; 12 Feb 2022 23:05:38 +0100 From: =?UTF-8?q?Mateusz=20Jo=C5=84czyk?= To: linux-rtc@vger.kernel.org, linux-kernel@vger.kernel.org Cc: =?UTF-8?q?Mateusz=20Jo=C5=84czyk?= , Alessandro Zummo , Alexandre Belloni Subject: [PATCH] rtc-mc146818-lib: wait longer for UIP to clear Date: Sat, 12 Feb 2022 23:04:54 +0100 Message-Id: <20220212220454.566548-1-mat.jonczyk@o2.pl> X-Mailer: git-send-email 2.25.1 MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-WP-MailID: 803afe6153e8175b7ef9efb8cb1be6a5 X-WP-AV: skaner antywirusowy Poczty o2 X-WP-SPAM: NO 0000000 [gZP0] Precedence: bulk List-ID: X-Mailing-List: linux-rtc@vger.kernel.org Before reading date / time from the CMOS RTC, we wait for the UIP (Update in progress) bit to clear --- so that the values are correct and consistent. To avoid a hang, there is a time limit after which we give up waiting. Increase the time limit from 10 to 20ms in case there are RTCs out there that are much slower than expected. Note: This may cause problems with hpet_rtc_interrupt() if the CMOS RTC breaks down while the system is running and RTC update interrupt / RTC alarm interrupt happens to be enabled (which should be rare). hpet_rtc_interrupt() is executed usually 64 times per second and after this patch it may take up to 20ms to complete (always hitting RTC read timeout) - which may constantly occupy one CPU. This looks very unlikely, though. Also fix "then" -> "than" in a comment. Signed-off-by: Mateusz Jończyk Cc: Alessandro Zummo Cc: Alexandre Belloni Fixes: ec5895c0f2d8 ("rtc: mc146818-lib: extract mc146818_avoid_UIP") --- This comment from Mr Alexandre Belloni got me thinking and is why I am submitting this patch: > We'll probably get some breakage later on because many RTCs using this > driver are not adhering to the spec. (See: https://lore.kernel.org/linux-rtc/277177e7-46a0-522c-297c-ad3ee0c15793@o2.pl/T/ ) Googling for dmesg messages that indicate problems with reading from RTC (such as "unable to read current time" - using quotation marks to force exact phrase search) produces no results apart from kernel code and patches. Any problems would happen rarely on affected systems, though. drivers/rtc/rtc-mc146818-lib.c | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-) diff --git a/drivers/rtc/rtc-mc146818-lib.c b/drivers/rtc/rtc-mc146818-lib.c index ae9f131b43c0..29ceec9875f4 100644 --- a/drivers/rtc/rtc-mc146818-lib.c +++ b/drivers/rtc/rtc-mc146818-lib.c @@ -21,7 +21,7 @@ bool mc146818_avoid_UIP(void (*callback)(unsigned char seconds, void *param), unsigned long flags; unsigned char seconds; - for (i = 0; i < 10; i++) { + for (i = 0; i < 20; i++) { spin_lock_irqsave(&rtc_lock, flags); /* @@ -79,8 +79,8 @@ bool mc146818_avoid_UIP(void (*callback)(unsigned char seconds, void *param), EXPORT_SYMBOL_GPL(mc146818_avoid_UIP); /* - * If the UIP (Update-in-progress) bit of the RTC is set for more then - * 10ms, the RTC is apparently broken or not present. + * If the UIP (Update-in-progress) bit of the RTC is set for more than + * 20ms, the RTC is apparently broken or not present. */ bool mc146818_does_rtc_work(void) { base-commit: dfd42facf1e4ada021b939b4e19c935dcdd55566 -- 2.25.1