Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1750932AbbG1X3n (ORCPT ); Tue, 28 Jul 2015 19:29:43 -0400 Received: from mail-wi0-f175.google.com ([209.85.212.175]:33948 "EHLO mail-wi0-f175.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750722AbbG1X3m (ORCPT ); Tue, 28 Jul 2015 19:29:42 -0400 MIME-Version: 1.0 In-Reply-To: References: <1438032507.17734.46.camel@gmail.com> Date: Tue, 28 Jul 2015 16:29:40 -0700 X-Google-Sender-Auth: rYoDbIQGxDUhcSbkq-_PLkatrbI Message-ID: Subject: Re: [RFC PATCH 0/3] clocksource: exynos_mct: allow mct to use 64-bit counter from coprocessor From: Doug Anderson To: Alexey Klimov Cc: linux-samsung-soc , Daniel Lezcano , Chirantan Ekbote , Tarek Dakhran , =?UTF-8?Q?Krzysztof_Koz=C5=82owski?= , kgene , Thomas Gleixner , "linux-kernel@vger.kernel.org" , "linux-arm-kernel@lists.infradead.org" , Yury Norov , Alim Akhtar , Mark Rutland , Marc Zyngier Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2719 Lines: 71 Hi, On Tue, Jul 28, 2015 at 9:20 AM, Alexey Klimov wrote: > Hi Doug, > > On Tue, Jul 28, 2015 at 6:24 PM, Doug Anderson wrote: >> Alexey, >> >> On Mon, Jul 27, 2015 at 2:28 PM, Alexey Klimov wrote: >>> Hi all, >>> >>> year(s) ago it was discovered that MCT timer and ARM architectured >>> timer >>> are the same hardware with different interface. Here [1]. >>> >>> I followed mail-list discussions about removing MCT and using arch >>> timer for Exynos5-based SoCs but things aren't moving at least latest >>> upstream kernel on odroid-xu3 will use MCT as default timers. >>> Maybe the reason are some power-management related things that very >>> specific to Samsung. I don't know. >>> >>> >>> Idea of this draft patchset comes from Doug patches when he tried to >>> optimize read of 64-bit counter located in mmio. [2] >>> Why not using cp15 counter instead if possible? >> >> I hate to burst your bubble here, but... >> >> ...I think it would be a bad idea to use the cp15 counter on exynos >> 5422. According to Samsung, there are issues where using cp15 could >> sometimes return the wrong value, especially if you happen to read it >> while on an A7 instead of an A15. It will tend to work pretty well, >> but Samsung claimed that it might not work right on some CPUs or at >> some temperatures. > > I think this is right time when I can say "If only I had known about > it before!". Yeah, we spent a whole lot of time on this too before giving up after we found out it was not possible to use. > That's quite interesting especially about temperature issues. > Maybe it's worth to insert comments in driver/docs about this. Maybe not a bad idea. Depends if you think someone else will try to do this... ...and if they do, will they check mailing list traffic. >> Generally I'd say that if the arch timer works for you: use the arch >> timer. If the arch timer doesn't work for you, use the MCT. >> >> -Doug > > I possess no knowledge if there're any different hw revisions of > odroid-xu3 on the field > but looks like arch timer works fine for me on odroid-xu3 Exynos5422 > SoC with upstream kernel. Well, it looked like it worked for us too. In fact, I never saw a failure. All I know is that Samsung said: don't do this, it's not a good idea. The performance gain wasn't enough to go against that... That's when I started trying to optimize the MCT a little bit. ;) -Doug -- 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/