From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757404AbcK2OOT (ORCPT ); Tue, 29 Nov 2016 09:14:19 -0500 Received: from foss.arm.com ([217.140.101.70]:47552 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753830AbcK2OOM (ORCPT ); Tue, 29 Nov 2016 09:14:12 -0500 Subject: Re: [PATCH] clocksource/arm_global_timer: reconfigure clockevents after cpufreq change To: Thomas Gleixner , Alexander Kochetkov References: <1480421716-30782-1-git-send-email-al.kochet@gmail.com> <1480421716-30782-2-git-send-email-al.kochet@gmail.com> Cc: LKML , LAK , kernel@stlinux.com, Daniel Lezcano , Patrice Chotard , Mark Rutland From: Marc Zyngier X-Enigmail-Draft-Status: N1110 Organization: ARM Ltd Message-ID: <763a77c2-3d19-8d20-88df-27f0b8b80b8b@arm.com> Date: Tue, 29 Nov 2016 14:14:09 +0000 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Icedove/45.2.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 29/11/16 13:42, Thomas Gleixner wrote: > On Tue, 29 Nov 2016, Alexander Kochetkov wrote: > >> After a cpufreq transition, update the clockevent's frequency >> by fetching the new clock rate from the clock framework and >> reprogram the next clock event. > > The frequency change would not only affect the clockevent device, it also > would affect the clocksource. So the patch is incomplete, but see below. > >> The clock supplying the arm-global-timer on the rk3188 is coming >> from the the cpu clock itself and thus changes its rate everytime >> cpufreq adjusts the cpu frequency. > > That's broken and the clk framework should keep the CORE_PERI clock at a > constant rate by reprogramming the divider of the CPU clock. > >> Found by code review, real impact not known. Assume what actual >> HZ value will be different from expected on platforms using >> arm-global-timer as clockevent. > > Assumptions w/o real impact are a perfect reason not to apply that > patch. This want's a proper proof that the global timer really changes and > this hackery is required, which I seriously doubt. Well, let's not underestimate the "creativity" [1] of A5/A9 when it comes to the timer clocks, and it is a very sad fact that both the global timer and the local timers are clocked by PERIPHCLK, which is ticking at a fixed ratio N (N >= 2) of the main CPU clock (CLK): http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.ddi0407f/CIHGECHJ.html I'm not sure how feasible it is to change this ratio (the TRM seems to be very silent on the subject). So short of being able to reconfigure it on the fly, this will probably need some surgery similar to what we already do for the TWD (which this patch mimics). Thankfully, we don't see that anymore on moderately recent HW (anything since A15) and the advent of the arch timer, which is guaranteed to have a fixed frequency. Thanks, M. [1] I wanted to write something else, but common decency prevents me from being more explicit... -- Jazz is not dead. It just smells funny...