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Date:   Fri, 12 Oct 2018 18:25:00 +0100
From:   Sudeep Holla <sudeep.holla@arm.com>
To:     Lina Iyer <ilina@codeaurora.org>
Cc:     "Raju P.L.S.S.S.N" <rplsssn@codeaurora.org>, 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,
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        sboyd@kernel.org, evgreen@chromium.org, dianders@chromium.org,
        mka@chromium.org, Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>,
        Sudeep Holla <sudeep.holla@arm.com>
Subject: Re: [PATCH RFC v1 7/8] drivers: qcom: cpu_pd: Handle cpu hotplug in
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On Fri, Oct 12, 2018 at 11:19:10AM -0600, Lina Iyer wrote:
> 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.
>

OK, along as there's no attempt to make it generic and keep it platform
specific, I am not that bothered.

> > > > 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.

No worries.

> 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.
>
Yes Qcom always says it can't be done in firmware. Even PSCI was adopted 
after couple of years of pushback.

--
Regards,
Sudeep