Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752434AbbG1QU2 (ORCPT ); Tue, 28 Jul 2015 12:20:28 -0400 Received: from mail-wi0-f180.google.com ([209.85.212.180]:34940 "EHLO mail-wi0-f180.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751187AbbG1QU0 (ORCPT ); Tue, 28 Jul 2015 12:20:26 -0400 MIME-Version: 1.0 In-Reply-To: References: <1438032507.17734.46.camel@gmail.com> Date: Tue, 28 Jul 2015 19:20:25 +0300 Message-ID: Subject: Re: [RFC PATCH 0/3] clocksource: exynos_mct: allow mct to use 64-bit counter from coprocessor From: Alexey Klimov To: Doug Anderson 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@arm.com 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: 2438 Lines: 65 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!". That's quite interesting especially about temperature issues. Maybe it's worth to insert comments in driver/docs about this. > I've added Alim who provided this information to me (in the private > http://crosbug.com/p/29556). He may be able to elaborate more. Alim, could you please add details if any? > 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. Is it right direction to switch xu3 to arch timer? Do you have any info or ideas about enabling MCT G_TCON register to start arch timer and what's the best way to do it? Thanks. -- Best regards, Klimov Alexey -- 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/