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

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Date:   Mon, 10 Jun 2019 12:02:50 +0100
From:   Sudeep Holla <sudeep.holla@....com>
To:     Ulf Hansson <ulf.hansson@...aro.org>
Cc:     Lorenzo Pieralisi <Lorenzo.Pieralisi@....com>,
        Mark Rutland <mark.rutland@....com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        "Raju P . L . S . S . S . N" <rplsssn@...eaurora.org>,
        Amit Kucheria <amit.kucheria@...aro.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Stephen Boyd <sboyd@...nel.org>,
        Niklas Cassel <niklas.cassel@...aro.org>,
        Tony Lindgren <tony@...mide.com>,
        Kevin Hilman <khilman@...nel.org>,
        Lina Iyer <ilina@...eaurora.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Souvik Chakravarty <souvik.chakravarty@....com>,
        Sudeep Holla <sudeep.holla@....com>,
        Linux PM <linux-pm@...r.kernel.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 15/18] drivers: firmware: psci: Support CPU hotplug for
 the hierarchical model

On Mon, Jun 10, 2019 at 12:21:47PM +0200, Ulf Hansson wrote:
> On Fri, 7 Jun 2019 at 17:31, Sudeep Holla <sudeep.holla@....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?
>

Sorry for my bad choice of names. I just wrote names as I understand
rather than looking for exact match. But yes, I meant generic place
where such ref-counting is done currently for other things.

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

OK, I need to understand that then.

> >
> > 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.

Sure, I understand your intent. I have just mentioned my thoughts/comments
as I reviewed.

--
Regards,
Sudeep