Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S965038AbdGTJwr (ORCPT ); Thu, 20 Jul 2017 05:52:47 -0400 Received: from mail-io0-f178.google.com ([209.85.223.178]:34874 "EHLO mail-io0-f178.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S965022AbdGTJwn (ORCPT ); Thu, 20 Jul 2017 05:52:43 -0400 MIME-Version: 1.0 In-Reply-To: <20170720071846.GK352@vireshk-i7> References: <20170720071846.GK352@vireshk-i7> From: "Rafael J. Wysocki" Date: Thu, 20 Jul 2017 11:52:41 +0200 X-Google-Sender-Auth: 6uL92ptuJkGys9reToWWmlZU7-A Message-ID: Subject: Re: cpuidle and cpufreq coupling? To: Viresh Kumar Cc: "Rafael J. Wysocki" , Florian Fainelli , Linux Kernel Mailing List , Linux PM , "Rafael J. Wysocki" , Markus Mayer , Daniel Lezcano , Peter Zijlstra 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: 1801 Lines: 44 On Thu, Jul 20, 2017 at 9:18 AM, Viresh Kumar wrote: > On 20-07-17, 01:17, Rafael J. Wysocki wrote: >> On Thu, Jul 20, 2017 at 12:54 AM, Florian Fainelli wrote: >> > Hi, >> > >> > We have a particular ARM CPU design that is drawing quite a lot of >> > current upon exit from WFI, and it does so in a way even before the >> > first instruction out of WFI is executed. That means we cannot influence >> > directly the exit from WFI other than by changing the state in which it >> > would be previously entered because of this "dead" time during which the >> > internal logic needs to ramp up back where it left. >> > >> > A naive approach to solving this problem because we have CPU frequency >> > scaling available would be to do the following: >> > >> > - just before entering WFI, switch to a low frequency OPP >> > - enter WFI >> > - upon exit from WFI, ramp up the frequency back to e.g: highest OPP >> > >> > Some of the parts that I am not exactly clear on would be: >> > >> > - would that qualify as a cpuidle governor of some kind that ties in >> > which cpufreq? >> > - would using cpufreq_driver_fast_switch() be an appropriate API to use >> > from outside >> >> Generally, the idle driver is expected to manipulate OPPs as suitable >> for it at the low level. > > Does any idle driver do it today ? > > I am not sure, but I haven't heard anyone from ARM doing it. Though I > may have completely missed it :) You may not, but that's what is recommended. Had you attended PM sessions at the LPC and similar, you might have heard about it ... > So, that must call into cpufreq (somehow) and look for a low power > OPP? It should know what OPP to use and then coordinate with cpufreq so they don't go against each other (on shared policies).