Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753788Ab1FBNEj (ORCPT ); Thu, 2 Jun 2011 09:04:39 -0400 Received: from caramon.arm.linux.org.uk ([78.32.30.218]:59885 "EHLO caramon.arm.linux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751035Ab1FBNEh (ORCPT ); Thu, 2 Jun 2011 09:04:37 -0400 Date: Thu, 2 Jun 2011 14:04:06 +0100 From: Russell King - ARM Linux To: Santosh Shilimkar Cc: Mattias Wallin , Thomas Gleixner , Lee Jones , "linux-kernel@vger.kernel.org" , "linux-arm-kernel@lists.infradead.org" , john stultz Subject: Re: [PATCHv2 0/3] clocksource: add db8500 PRCMU timer Message-ID: <20110602130406.GW3660@n2100.arm.linux.org.uk> References: <1307007271-1004-1-git-send-email-mattias.wallin@stericsson.com> <20110602094622.GS3660@n2100.arm.linux.org.uk> <4DE7637B.3060901@stericsson.com> <20110602110137.GU3660@n2100.arm.linux.org.uk> <4DE77D9A.6090301@stericsson.com> <4DE788B2.5060308@ti.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4DE788B2.5060308@ti.com> User-Agent: Mutt/1.5.19 (2009-01-05) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1590 Lines: 33 On Thu, Jun 02, 2011 at 06:27:22PM +0530, Santosh Shilimkar wrote: > Earlier this year at ELC SFO, I had a discussion with > John and Thomas on how to have a high-resolution clock-source > and a low-resolution clock-source working together to cover > the low power scenario and still manage to get the highest > timer resolution. > The idea was to do dynamic switching of clock-source > which initially looked simple. Here the idea was to > have this working for suspend and as well as cupidle. I don't think you can do this, because you'll lose precision whenever you switch from the high resolution clocksource to the low resolution clocksource. While you can quickly update the current time of day from the highres one, the lowres may or may not be a long way from its next tick rollover. So you lose the precision whenever you switch. However, over a suspend/resume cycle, the precision loss is normally very small compared to the time which you have been suspended, so the %age error also becomes very small. With cpuidle, it becomes a completely different matter. Here, the %age error is much larger because of the smaller sleep periods, and chances are we can't wait for the low-res timer to change. So if you're using cpuidle, you really need a clocksource which runs continuously, even in whatever states cpuidle drops you into. -- 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/