linux-kernel - Re: [PATCH v3 6/7] thermal/drivers/cpu_cooling: Introduce the cpu idle cooling driver

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Message-ID: <20180416123019.GA9341@e107981-ln.cambridge.arm.com>
Date:   Mon, 16 Apr 2018 13:30:20 +0100
From:   Lorenzo Pieralisi <lorenzo.pieralisi@....com>
To:     Daniel Lezcano <daniel.lezcano@...aro.org>
Cc:     Viresh Kumar <viresh.kumar@...aro.org>,
        Sudeep Holla <sudeep.holla@....com>, edubezval@...il.com,
        kevin.wangtao@...aro.org, leo.yan@...aro.org,
        vincent.guittot@...aro.org, linux-kernel@...r.kernel.org,
        javi.merino@...nel.org, rui.zhang@...el.com,
        daniel.thompson@...aro.org, linux-pm@...r.kernel.org,
        Amit Daniel Kachhap <amit.kachhap@...il.com>
Subject: Re: [PATCH v3 6/7] thermal/drivers/cpu_cooling: Introduce the cpu
 idle cooling driver

On Mon, Apr 16, 2018 at 02:10:30PM +0200, Daniel Lezcano wrote:
> On 16/04/2018 12:10, Viresh Kumar wrote:
> > On 16-04-18, 12:03, Daniel Lezcano wrote:
> >> On 16/04/2018 11:50, Viresh Kumar wrote:
> >>> On 16-04-18, 11:45, Daniel Lezcano wrote:
> >>>> Can you elaborate a bit ? I'm not sure to get the point.
> >>>
> >>> Sure. With your current code on Hikey960 (big/LITTLE), you end up
> >>> creating two cooling devices, one for the big cluster and one for
> >>> small cluster. Which is the right thing to do, as we also have two
> >>> cpufreq cooling devices.
> >>>
> >>> But with the change Sudeep is referring to, the helper you used to get
> >>> cluster id will return 0 (SoC id) for all the 8 CPUs. So your code
> >>> will end up creating a single cpuidle cooling device for all the CPUs.
> >>> Which would be wrong.
> >>
> >> Is the semantic of topology_physical_package_id changing ?
> > 
> > That's what I understood from his email.
> > 
> >> I don't
> >> understand the change Sudeep is referring to.
> 
> Actually there is no impact with the change Sudeep is referring to. It
> is for ACPI, we are DT based. Confirmed with Jeremy.
> 
> So AFAICT, it is not a problem.

It is a problem - DT or ACPI alike. Sudeep was referring to the notion
of "cluster" that has no architectural meaning whatsoever and using
topology_physical_package_id() to detect a "cluster" was/is/will always
be the wrong thing to do. The notion of cluster must not appear in the
kernel at all, it has no architectural meaning. I understand you need
to group CPUs but that has to be done in a different way, through
cooling devices, thermal domains or power domains DT/ACPI bindings but
not by using topology masks.

You should be able to figure out this week at OSPM the reasoning behind
what I am saying above.

Cheers,
Lorenzo