From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752805Ab3AVBHY (ORCPT ); Mon, 21 Jan 2013 20:07:24 -0500 Received: from mail-vc0-f175.google.com ([209.85.220.175]:58057 "EHLO mail-vc0-f175.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752678Ab3AVBHW (ORCPT ); Mon, 21 Jan 2013 20:07:22 -0500 MIME-Version: 1.0 In-Reply-To: <50FDE2AE.8030608@linaro.org> References: <201301212041.17951.arnd@arndb.de> <50FDAC5F.4040605@linaro.org> <50FDC2DD.7090406@linaro.org> <50FDE2AE.8030608@linaro.org> From: Matt Sealey Date: Mon, 21 Jan 2013 19:06:59 -0600 Message-ID: Subject: Re: One of these things (CONFIG_HZ) is not like the others.. To: John Stultz Cc: Arnd Bergmann , Linux ARM Kernel ML , LKML , Peter Zijlstra , Ingo Molnar , Russell King - ARM Linux Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Jan 21, 2013 at 6:51 PM, John Stultz wrote: > On 01/21/2013 02:54 PM, Matt Sealey wrote: >> >> On Mon, Jan 21, 2013 at 4:36 PM, John Stultz >> wrote: >>> >>> On 01/21/2013 01:14 PM, Matt Sealey wrote: > > As far as jiffies rating, from jiffies.c: > .rating = 1, /* lowest valid rating*/ > > So I'm not sure what you mean by "the debug on the kernel log is telling me > it has a higher resolution". Oh, it is just if I actually don't run setup_sched_clock on my platform, it gives a little message (with #define DEBUG 1 in sched_clock.c) about who setup the last sched_clock. Since you only get one chance, and I was fiddling with setup_sched_clock being probed from multiple possible timers from device tree (i.MX3 has a crapload of valid timers, which one you use right now is basically forced by the not-quite-fully-DT-only code and some funky iomap tricks). And what I got was, if I use the real hardware timer, it runs at 66MHz and says it has 15ns resolution and wraps every 500 seconds or so. The jiffies timer says it's 750MHz, with a 2ns resoluton.. you get the drift. The generic reporting of how "good" the sched_clock source is kind of glosses over the quality rating of the clock source and at first glance (if you're not paying that much attention), it is a little bit misleading.. > Yes, in the case I was remembering, the 60HZ was driven by the electrical > line. While I have your attention, what would be the minimum "good" speed to run the sched_clock or delay timer implementation from? My rudimentary scribblings in my notebook give me a value of "don't bother" with less than 10KHz based on HZ=100, so I'm wondering if a direct 32.768KHz clock would do (i.MX osc clock input if I can supply it to one of the above myriad timers) since this would be low-power compared to a 66MHz one (by a couple mA anyway). I also have a bunch of questions about the delay timer requirements.. I might mail you personally.. or would you prefer on-list? -- Matt Sealey Product Development Analyst, Genesi USA, Inc.