From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932465AbXAGKC7 (ORCPT ); Sun, 7 Jan 2007 05:02:59 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S932466AbXAGKC7 (ORCPT ); Sun, 7 Jan 2007 05:02:59 -0500 Received: from mail.macqel.be ([194.78.208.39]:19546 "EHLO mail.macqel.be" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932465AbXAGKC6 (ORCPT ); Sun, 7 Jan 2007 05:02:58 -0500 Date: Sun, 7 Jan 2007 11:02:56 +0100 From: Philippe De Muyter To: David Brownell Cc: Linux Kernel list Subject: Re: RTC subsystem and fractions of seconds Message-ID: <20070107100256.GA24013@ingate.macqel.be> References: <200701051949.00662.david-b@pacbell.net> <20070106232633.GA8535@ingate.macqel.be> <200701061552.43654.david-b@pacbell.net> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <200701061552.43654.david-b@pacbell.net> User-Agent: Mutt/1.4.1i Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Sat, Jan 06, 2007 at 03:52:43PM -0800, David Brownell wrote: > On Saturday 06 January 2007 3:26 pm, Philippe De Muyter wrote: > > On Fri, Jan 05, 2007 at 07:49:00PM -0800, David Brownell wrote: > > > > Those rtc's actually have a 1/100th of second > > > > register. Should the generic rtc interface not support that? > > > > > > Are you implying a new userspace API, or just an in-kernel update? > > > > My only concern at the moment is initializing linux's timeofday from the rtc > > quickly and with a good precision. > > There will necessarily be a bit of fuzz there since it can take time to > get that RTC's mutex, and the task setting that time can be preempted. > Plus, there can also be delays at the I2C or SPI transaction level. > > > > The way it is done currently > > in drivers/rtc/hctosys.c is 0.5 sec off. We could obtain a much better > > precision by looping there until the next change (next second for old clocks, > > next 0.01 second for m41t81, maybe even better for other ones). > > Hmm ... "looping" fights against "quickly"; as would "wait for next > update IRQ" (on RTCs that support that). But it would improve precision, > at least in the sense of having the system clock and that RTC spending > less time with the lowest "seconds" digit disagreeing. > > This is something you could write a patch for, n'est-ce pas? That would require changing the interface provided by the rtc class, but I'll look at it. > > - Dave > > --