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Date:   Mon, 16 Apr 2018 15:22:43 +0100
From:   Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
To:     Daniel Lezcano <daniel.lezcano@linaro.org>
Cc:     Viresh Kumar <viresh.kumar@linaro.org>,
        Sudeep Holla <sudeep.holla@arm.com>, edubezval@gmail.com,
        kevin.wangtao@linaro.org, leo.yan@linaro.org,
        vincent.guittot@linaro.org, linux-kernel@vger.kernel.org,
        javi.merino@kernel.org, rui.zhang@intel.com,
        daniel.thompson@linaro.org, linux-pm@vger.kernel.org,
        Amit Daniel Kachhap <amit.kachhap@gmail.com>
Subject: Re: [PATCH v3 6/7] thermal/drivers/cpu_cooling: Introduce the cpu
 idle cooling driver
Message-ID: <20180416142243.GB32565@red-moon>
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On Mon, Apr 16, 2018 at 03:57:03PM +0200, Daniel Lezcano wrote:
> On 16/04/2018 14:30, Lorenzo Pieralisi wrote:
> > 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.
> 
> I don't get it. What is the cluster concept defined in the ARM
> documentation?
> 
> ARM Cortex-A53 MPCore Processor Technical Reference Manual
> 
> 4.5.2. Multiprocessor Affinity Register
> 
> I see the documentation says:
> 
> A cluster with two cores, three cores, ...
> 
> How the kernel can represent that if you kill the
> topology_physical_package_id() ?

From an Arm ARM perspective (ARM v8 reference manual), the MPIDR_EL1 has
no notion of cluster which means that a cluster is not architecturally
defined on Arm systems.

Currently, as Morten explained today, topology_physical_package_id()
is supposed to represent a "cluster" and that's completely wrong
because a "cluster" cannot be defined from an architectural perspective.

It was a bodge used as a shortcut, wrongly. We should have never used
that API for that purpose and there must be no code in the kernel
relying on:

topology_physical_package_id()

to define a cluster; the information you require to group CPUs must
come from something else, which is firmware bindings(DT or ACPI) as
I mentioned.

Please speak to Sudeep who will fill you on the reasoning above.

Thanks,
Lorenzo