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Subject: Re: [PATCH v3 6/7] thermal/drivers/cpu_cooling: Introduce the cpu
 idle cooling driver
To:     Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
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>
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From:   Daniel Lezcano <daniel.lezcano@linaro.org>
Message-ID: <633cdc63-ce6d-89af-26dd-bdc3a27556ed@linaro.org>
Date:   Mon, 16 Apr 2018 15:57:03 +0200
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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() ?

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


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