Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id ; Mon, 5 Nov 2001 18:13:12 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id ; Mon, 5 Nov 2001 18:13:03 -0500 Received: from modem-2114.lynx.dialup.pol.co.uk ([217.135.200.66]:17416 "EHLO Mail.MemAlpha.cx") by vger.kernel.org with ESMTP id ; Mon, 5 Nov 2001 18:12:44 -0500 Posted-Date: Mon, 5 Nov 2001 23:08:48 GMT Date: Mon, 5 Nov 2001 23:08:47 +0000 (GMT) From: Riley Williams Reply-To: Riley Williams To: Pavel Machek cc: Linux Kernel Subject: Re: PROBLEM: Linux updates RTC secretly when clock synchronizes In-Reply-To: <20011102121602.A45@toy.ucw.cz> Message-ID: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Hi Pavel. >> PROBLEM: Linux updates RTC secretly when clock synchronizes. >> >> Please CC replies etc to Ian Maclaine-cross . >> >> When /usr/sbin/ntpd synchronizes the Linux kernel (or system) clock >> using the Network Time Protocol the kernel time is accurate to a few >> milliseconds. Linux then sets the Real Time (or Hardware or CMOS) >> Clock to this time at approximately 11 minute intervals. Typical >> RTCs drift less than 10 s/day so rebooting causes only millisecond >> errors. >> >> Linux currently does not record the 11 minute updates to a log file. >> Clock programs (like hwclock) cannot correct RTC drift at boot time >> without knowing when the RTC was last set. If NTP service is available >> after a long shutdown, ntpd may step the time. Worse after a longer >> shutdown ntpd may drop out or even synchronize to the wrong timezone. >> The workarounds are clumsy. >> >> Please find following my small patch for >> linux/arch/i386/kernel/time.c which adds a KERN_NOTICE of each 11 >> minute update to the RTC. This is just for i386 machines at present. >> A script can search the logs for the last set time of the RTC and >> update /etc/adjtime. Hwclock can then correct the RTC for drift and >> set the kernel clock. > That seems as very wrong solution. > What about just making kernel only _read_ system clock, and never > set it? That looks way cleaner to me. It is cleaner. However, I feel that the RTC code should printk (at least as KERN_DEBUG if not as KERN_NOTICE) whenever the RTC is written to. It's too important a subsystem to be left hidden like it currently is. Best wishes from Riley. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/