From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-5.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 4A24FC2BA83 for ; Wed, 12 Feb 2020 10:32:56 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 2AEF120848 for ; Wed, 12 Feb 2020 10:32:56 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727862AbgBLKcz (ORCPT ); Wed, 12 Feb 2020 05:32:55 -0500 Received: from foss.arm.com ([217.140.110.172]:58812 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725710AbgBLKcw (ORCPT ); Wed, 12 Feb 2020 05:32:52 -0500 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id C589330E; Wed, 12 Feb 2020 02:32:51 -0800 (PST) Received: from localhost (unknown [10.1.198.52]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 61A953F68F; Wed, 12 Feb 2020 02:32:51 -0800 (PST) Date: Wed, 12 Feb 2020 10:32:49 +0000 From: Ionela Voinescu To: Valentin Schneider Cc: catalin.marinas@arm.com, will@kernel.org, mark.rutland@arm.com, maz@kernel.org, suzuki.poulose@arm.com, sudeep.holla@arm.com, lukasz.luba@arm.com, rjw@rjwysocki.net, peterz@infradead.org, mingo@redhat.com, vincent.guittot@linaro.org, viresh.kumar@linaro.org, linux-arm-kernel@lists.infradead.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org Subject: Re: [PATCH v3 7/7] clocksource/drivers/arm_arch_timer: validate arch_timer_rate Message-ID: <20200212103249.GA19041@arm.com> References: <20200211184542.29585-1-ionela.voinescu@arm.com> <20200211184542.29585-8-ionela.voinescu@arm.com> <05e257b6-0a39-135d-8117-7883739538c3@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <05e257b6-0a39-135d-8117-7883739538c3@arm.com> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Valentin, On Wednesday 12 Feb 2020 at 09:30:34 (+0000), Valentin Schneider wrote: > On 11/02/2020 18:45, Ionela Voinescu wrote: > > From: Valentin Schneider > > > > Using an arch timer with a frequency of less than 1MHz can result in an > > incorrect functionality of the system which assumes a reasonable rate. > > > > One example is the use of activity monitors for frequency invariance > > which uses the rate of the arch timer as the known rate of the constant > > cycle counter in computing its ratio compared to the maximum frequency > > of a CPU. For arch timer frequencies less than 1MHz this ratio could > > end up being 0 which is an invalid value for its use. > > > > I'm being pedantic here (as usual), but I'd contrast this a bit more. The > activity monitor code checks by itself that the ratio doesn't end up being > 0, which is why we don't slam the brakes if the arch timer freq is < 1MHz. > > It's just a CNTFRQ sanity check that goes a bit beyond the 0 value check, > IMO. > I agree, but this part was just given as an example of functionality that relies on a reasonable arch timer rate. The AMU code checks for the ratio not to be 0 so it does not end up breaking frequency invariance. But if the ratio does end up being 0 due to the value of arch_time_rate, we bypass the use of activity monitors which I'd argue it's incorrect functionality by disabling a potential better source of information for frequency invariance. But I can rewrite this part for more clarity. > > Therefore, warn if the arch timer rate is below 1MHz which contravenes > > the recommended architecture interval of 1 to 50MHz. > > > > Signed-off-by: Ionela Voinescu > > Cc: Mark Rutland > > Cc: Marc Zyngier > > ISTR something somewhere that says the first signoff should be that of the > author of the patch, and seeing as I just provided an untested diff that > ought to be you :) Will do! Thanks, Ionela.