From mboxrd@z Thu Jan 1 00:00:00 1970 From: Ashwin Chaugule Subject: Re: [PATCH v4 2/2] CPPC: Add CPUFreq driver based on CPPC methods Date: Wed, 4 Feb 2015 10:18:41 -0500 Message-ID: References: <1422392638-31334-1-git-send-email-ashwin.chaugule@linaro.org> <1805804.escc39d3ai@vostro.rjw.lan> <3237680.21UHnh0sSv@vostro.rjw.lan> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Return-path: Received: from mail-we0-f177.google.com ([74.125.82.177]:49900 "EHLO mail-we0-f177.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S965873AbbBDPSn (ORCPT ); Wed, 4 Feb 2015 10:18:43 -0500 Received: by mail-we0-f177.google.com with SMTP id l61so2339068wev.8 for ; Wed, 04 Feb 2015 07:18:42 -0800 (PST) In-Reply-To: <3237680.21UHnh0sSv@vostro.rjw.lan> Sender: linux-pm-owner@vger.kernel.org List-Id: linux-pm@vger.kernel.org To: "Rafael J. Wysocki" Cc: "rwells@codeaurora.org" , Linda Knippers , "linux-pm@vger.kernel.org" , Catalin Marinas , Patch Tracking , Linaro ACPI Mailman List , Viresh Kumar , Jaswinder Singh , Mark Brown , Arnd Bergmann Hello, On 4 February 2015 at 09:33, Rafael J. Wysocki wrote: > On Tuesday, February 03, 2015 10:23:31 PM Ashwin Chaugule wrote: >> On 3 February 2015 at 17:33, Rafael J. Wysocki wrote: >> > On Tuesday, January 27, 2015 04:03:58 PM Ashwin Chaugule wrote: >> > >> > First question: How do we ensure that this won't interact negatively with >> > the existing ACPI cpufreq driver? In particular, what is there to allow >> > users to use the driver they want? >> >> The existing ACPI cpufreq driver isnt enabled on ARM. (At a minimum, >> it needs spec updates in the PSS sections for ARM.) For ARM64 servers, >> CPPC is the preferred choice. But you're right, if someone adds PSS >> support on ARM then we need a way to make these two mutually exclusive >> at runtime. > > Analogously, if _CPC is present on a non-ARM platform. Hm. Currently CPPC is available only under Kconfig.arm under cpufreq. My understanding is that X86 will use HWP (CPPC equivalent) through intel_pstate (using MSRs). I'm not aware of any other non-ARM platform that could use this as of yet, although in theory, any platform with ACPI support should be able to. >> On ARM64 servers, we'd want it the other way. i.e. if the acpi-cpufreq >> driver detects CPC then it skips its init. > > I'm not sure if we can do that in general, though. Curious to know why we couldn't check for _CPC in the acpi-cpufreq driver and bail if it is found? FWIW, the spec also states that if _CPC is present it supersedes _PSS and friends (section 8.4.5.1.10) > >> I'm open to any suggestions on how to handle this. > > Let me think about that a bit more. Much appreciated! Cheers, Ashwin