Re: [PATCH 15/18] drivers: firmware: psci: Support CPU hotplug for the hierarchical model

From: Ulf Hansson <ulf.hansson@linaro.org>
To: Sudeep Holla <sudeep.holla@arm.com>
Cc: Lorenzo Pieralisi <Lorenzo.Pieralisi@arm.com>,
	Mark Rutland <mark.rutland@arm.com>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	"Rafael J . Wysocki" <rjw@rjwysocki.net>,
	Daniel Lezcano <daniel.lezcano@linaro.org>,
	"Raju P . L . S . S . S . N" <rplsssn@codeaurora.org>,
	Amit Kucheria <amit.kucheria@linaro.org>,
	Bjorn Andersson <bjorn.andersson@linaro.org>,
	Stephen Boyd <sboyd@kernel.org>,
	Niklas Cassel <niklas.cassel@linaro.org>,
	Tony Lindgren <tony@atomide.com>,
	Kevin Hilman <khilman@kernel.org>,
	Lina Iyer <ilina@codeaurora.org>,
	Viresh Kumar <viresh.kumar@linaro.org>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Geert Uytterhoeven <geert+renesas@glider.be>,
	Souvik Chakravarty <souvik.chakravarty@arm.com>,
	Linux PM <linux-pm@vger.kernel.org>,
	linux-arm-msm <linux-arm-msm@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 15/18] drivers: firmware: psci: Support CPU hotplug for the hierarchical model
Date: Mon, 10 Jun 2019 12:21:47 +0200	[thread overview]
Message-ID: <CAPDyKFqooMww0B6vGL56BnG-L=13C3oWeDFx4v1cO9=W9QPwVQ@mail.gmail.com> (raw)
In-Reply-To: <20190607153114.GI15577@e107155-lin>

On Fri, 7 Jun 2019 at 17:31, Sudeep Holla <sudeep.holla@arm.com> wrote:
>
> On Mon, May 13, 2019 at 09:22:57PM +0200, Ulf Hansson wrote:
> > When the hierarchical CPU topology is used and when a CPU has been put
> > offline (hotplug), that same CPU prevents its PM domain and thus also
> > potential master PM domains, from being powered off. This is because genpd
> > observes the CPU's attached device as being active from a runtime PM point
> > of view.
> >
> > To deal with this, let's decrease the runtime PM usage count by calling
> > pm_runtime_put_sync_suspend() of the attached struct device when putting
> > the CPU offline. Consequentially, we must then increase the runtime PM
> > usage count, while putting the CPU online again.
> >
>
> Why is this firmware/driver specific ? Why can't this be dealt in core
> pm-domain ? I am concerned that if any other architectures or firmware
> method decides to use this feature, this need to be duplicated there.

What is the core pm-domain? Do you refer to the generic PM domain (genpd), no?

In such case, this is not the job of genpd, but rather the opposite
(to *monitor* the reference count).

>
> The way I see this is pure reference counting and is hardware/firmware/
> driver agnostic and can be made generic.

As stated in the another reply, I would rather start with having more
things driver specific rather than generic. Later on we can always
consider to move/split things, when there are more users.

In this particular case, the runtime PM reference counting is done on
the struct device*, that genpd returned via
dev_pm_domain_attach_by_name(). And because
dev_pm_domain_attach_by_name() is called from PSCI code, I decided to
keep this struct device* internal to PSCI.

Kind regards
Uffe