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 11:19:10 -0600 Message-ID: <20181012171910.GI2371@codeaurora.org> References: <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> <20181012160427.GG2371@codeaurora.org> <20181012170040.GA21057@e107155-lin> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Return-path: Content-Disposition: inline In-Reply-To: <20181012170040.GA21057@e107155-lin> Sender: linux-kernel-owner@vger.kernel.org To: Sudeep Holla Cc: "Raju P.L.S.S.S.N" , andy.gross@linaro.org, david.brown@linaro.org, rjw@rjwysocki.net, ulf.hansson@linaro.org, khilman@kernel.org, linux-arm-msm@vger.kernel.org, linux-soc@vger.kernel.org, rnayak@codeaurora.org, bjorn.andersson@linaro.org, linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org, devicetree@vger.kernel.org, sboyd@kernel.org, evgreen@chromium.org, dianders@chromium.org, mka@chromium.org, Lorenzo Pieralisi List-Id: devicetree@vger.kernel.org On Fri, Oct 12 2018 at 11:01 -0600, Sudeep Holla wrote: >On Fri, Oct 12, 2018 at 10:04:27AM -0600, Lina Iyer wrote: >> On Fri, Oct 12 2018 at 09:04 -0600, Sudeep Holla wrote: > >[...] > >Yes all these are fine but with multiple power-domains/cluster, it's >hard to determine the first CPU. You may be able to identify it within >the power domain but not system wide. So this doesn't scale with large >systems(e.g. 4 - 8 clusters with 16 CPUs). > We would probably not worry too much about power savings in a msec scale, if we have that big a system. The driver is a platform specific driver, primarily intended for a mobile class CPU and usage. In fact, we haven't done this for QC's server class CPUs. >> > 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. >> > >> I think the confusion for you is system sleep vs suspend. System sleep >> here (probably more of a QC terminology), refers to powering down the >> entire SoC for very small durations, while not actually suspended. The >> drivers are unaware that this is happening. No hotplug happens and the >> interrupts are not migrated during system sleep. When all the CPUs go >> into cpuidle, the system sleep state is activated and the resource >> requirements are lowered. The resources are brought back to their >> previous active values before we exit cpuidle on any CPU. The drivers >> have no idea that this happened. We have been doing this on QCOM SoCs >> for a decade, so this is not something new for this SoC. Every QCOM SoC >> has been doing this, albeit differently because of their architecture. >> The newer ones do most of these transitions in hardware as opposed to an >> remote CPU. But this is the first time, we are upstreaming this :) >> > >Indeed, I know mobile platforms do such optimisations and I agree it may >save power. As I mentioned above it doesn't scale well with large systems >and also even with single power domains having multiple idle states where >only one state can do this system level idle but not all. As I mentioned >in the other email to Ulf, it's had to generalise this even with DT. >So it's better to have this dealt transparently in the firmware. > Good, then we are on agreement here. But this is how this platform is. It cannot be done in firmware and what we doing here is a Linux platform driver that cleans up nicely without having to piggy back on an external dependency. Thanks, Lina