From mboxrd@z Thu Jan 1 00:00:00 1970 From: Lina Iyer Subject: Re: [PATCH RFC v1 7/8] drivers: qcom: cpu_pd: Handle cpu hotplug in the domain Date: Fri, 12 Oct 2018 10:16:53 -0600 Message-ID: <20181012161653.GH2371@codeaurora.org> References: <1539206455-29342-1-git-send-email-rplsssn@codeaurora.org> <1539206455-29342-8-git-send-email-rplsssn@codeaurora.org> <20181011112013.GC32752@e107155-lin> <20181011160053.GA2371@codeaurora.org> <20181011161927.GC28583@e107155-lin> <20181011165822.GB2371@codeaurora.org> <20181011173733.GA26447@e107155-lin> <20181011210609.GD2371@codeaurora.org> <20181012150429.GH3401@e107155-lin> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Return-path: Content-Disposition: inline In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org To: Ulf Hansson Cc: Sudeep Holla , "Raju P.L.S.S.S.N" , Andy Gross , David Brown , "Rafael J. Wysocki" , Kevin Hilman , linux-arm-msm , linux-soc@vger.kernel.org, Rajendra Nayak , Bjorn Andersson , Linux Kernel Mailing List , Linux PM , DTML , Stephen Boyd , Evan Green , Doug Anderson , Matthias Kaehlcke , Lorenzo Pieralisi List-Id: devicetree@vger.kernel.org On Fri, Oct 12 2018 at 09:46 -0600, Ulf Hansson wrote: >On 12 October 2018 at 17:04, Sudeep Holla wrote: >> On Thu, Oct 11, 2018 at 03:06:09PM -0600, Lina Iyer wrote: >>> On Thu, Oct 11 2018 at 11:37 -0600, Sudeep Holla wrote: >> [...] >> >>> > >>> > Is DDR managed by Linux ? I assumed it was handled by higher exception >>> > levels. Can you give examples of resources used by CPU in this context. >>> > When CPU can be powered on or woken up without Linux intervention, the >>> > same holds true for CPU power down or sleep states. I still see no reason >>> > other than the firmware has no support to talk to RPMH. >>> > >>> DDR, shared clocks, regulators etc. Imagine you are running something on >>> the screen and CPUs enter low power mode, while the CPUs were active, >>> there was a need for bunch of display resources, and things the app may >>> have requested resources, while the CPU powered down the requests may >>> not be needed the full extent as when the CPU was running, so they can >>> voted down to a lower state of in some cases turn off the resources >>> completely. What the driver voted for is dependent on the runtime state >>> and the usecase currently active. The 'sleep' state value is also >>> determined by the driver/framework. >>> >> >> Why does CPU going down says that another (screen - supposedly shared) >> resource needs to be relinquished ? Shouldn't display decide that on it's >> own ? I have no idea why screen/display is brought into this discussion. >> CPU can just say: hey I am going down and I don't need my resource. >> How can it say: hey I am going down and display or screen also doesn't >> need the resource. On a multi-cluster, how will the last CPU on one know >> that it needs to act on behalf of the shared resource instead of another >> cluster. > >Apologize for sidetracking the discussion, just want to fold in a few comments. > No, this is perfect to warp the whole thing around. Thanks Ulf. >This is becoming a complicated story. May I suggest we pick the GIC as >an example instead? > >Let's assume the simple case, we have one cluster and when the cluster >becomes powered off, the GIC needs to be re-configured and wakeups >must be routed through some "always on" external logic. > >The PSCI spec mentions nothing about how to manage this and not the >rest of the SoC topology for that matter. Hence if the GIC is managed >by Linux - then Linux also needs to take actions before cluster power >down and after cluster power up. So, if PSCI FW can't deal with GIC, >how would manage it? > >> >> I think we are mixing the system sleep states with CPU idle here. >> If it's system sleeps states, the we need to deal it in some system ops >> when it's the last CPU in the system and not the cluster/power domain. > >What is really a system sleep state? One could consider it just being >another idles state, having heaver residency targets and greater >enter/exit latency values, couldn't you? > While I explained the driver's side of the story, for the CPUs, system sleep is a deeper low power mode that ties in with OSI. Thanks, Lina >In the end, there is no reason to keep things powered on, unless they >are being in used (or soon to be used), that is main point. > >We are also working on S2I at Linaro. We strive towards being able to >show the same power numbers as for S2R, but then we need to get these >cluster-idle things right. > >[...] > >Have a nice weekend! > >Kind regards >Uffe