From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1032211Ab2COXXs (ORCPT ); Thu, 15 Mar 2012 19:23:48 -0400 Received: from kirsty.vergenet.net ([202.4.237.240]:34724 "EHLO kirsty.vergenet.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S965275Ab2COXXq (ORCPT ); Thu, 15 Mar 2012 19:23:46 -0400 Date: Fri, 16 Mar 2012 08:23:41 +0900 From: Simon Horman To: Mark Brown Cc: Liam Girdwood , linux-kernel@vger.kernel.org Subject: Re: [PATCH] regulator: notify sysfs when voltage is set Message-ID: <20120315232341.GC18690@verge.net.au> References: <1331813073-714-1-git-send-email-horms@verge.net.au> <20120315124631.GJ3138@opensource.wolfsonmicro.com> <20120315142657.GD3129@verge.net.au> <20120315144708.GM3138@opensource.wolfsonmicro.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20120315144708.GM3138@opensource.wolfsonmicro.com> Organisation: Horms Solutions Ltd. User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Mar 15, 2012 at 02:47:08PM +0000, Mark Brown wrote: > On Thu, Mar 15, 2012 at 11:26:58PM +0900, Simon Horman wrote: > > On Thu, Mar 15, 2012 at 12:46:31PM +0000, Mark Brown wrote: > > > > I notice that we don't generate similar events for cpufreq... what are > > > the performance implications from firing off udev (which isn't free) > > > every time we scale the CPU frequency? It feels like this might be > > > disruptive, especially with a governor like ondemand which responds to > > > system load. > > > that is a good point and to be honest not one that I had considered. For > > the use-case that I have in mind, which is basically to log voltage changes > > over time, it may be acceptable to rate-limit notifications somehow. But at > > that point I may be better off just polling. > > Hrm, if you're doing logging then we do already have tracepoints defined > for voltage changes - could you have your application work with those > instead of sysfs? One of the applications of tracepoints is flight > recorder style system monitoring. Thanks, that does sound like it would work for me. I'll look into it further. > > Out of interest, how often can ondemand potentially change the voltage? > > Depends on how slow it thinks frequency changes are. I'm not so worried > about the performance impact from that point of view, I'm more worried > about what happens when we lower the clock frequency and generate a > sysfs event causing userspace to wake up and start doing stuff. It'd be > bad if ondemand (or another dynamic governor) noticed that the system > got more busy again and responded by ramping the frequency... Ok, understood, that could be a bit nasty.