Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S935743AbdGTJXg (ORCPT ); Thu, 20 Jul 2017 05:23:36 -0400 Received: from foss.arm.com ([217.140.101.70]:50078 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933734AbdGTJXc (ORCPT ); Thu, 20 Jul 2017 05:23:32 -0400 Cc: Sudeep Holla , Florian Fainelli , Linux Kernel Mailing List , Linux PM , "Rafael J. Wysocki" , Markus Mayer , Daniel Lezcano Subject: Re: cpuidle and cpufreq coupling? To: Viresh Kumar , "Rafael J. Wysocki" References: <20170720071846.GK352@vireshk-i7> From: Sudeep Holla Organization: ARM Message-ID: <49e8479c-bc47-16fe-0bf9-8a4aba333909@arm.com> Date: Thu, 20 Jul 2017 10:23:27 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.2.1 MIME-Version: 1.0 In-Reply-To: <20170720071846.GK352@vireshk-i7> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1987 Lines: 57 On 20/07/17 08:18, 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 :) > It doesn't need to be in Linux. E.g. PSCI or any low lever driver can do that transparently. > So, that must call into cpufreq (somehow) and look for a low power > OPP? > That's seems hacky and NAK if it's PSCI platform. It's cleaner do such hacks/workarounds in platform specific PSCI firmware. > @Florian: It would be more tricky then we anticipate. We don't always > want to go to low OPP on idle, as we may get out of it very quickly > and changing OPP twice (before and after idle) in that scenario would > be a complete waste of time. Exactly. -- Regards, Sudeep