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Date:   Thu, 17 Feb 2022 12:28:31 +0100
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Subject: Re: [PATCH 1/2] thermal: cooling: Check Energy Model type in
 cpufreq_cooling and devfreq_cooling
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To:     Lukasz Luba <lukasz.luba@arm.com>,
        Doug Anderson <dianders@chromium.org>,
        Matthias Kaehlcke <mka@chromium.org>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Linux PM <linux-pm@vger.kernel.org>,
        amit daniel kachhap <amit.kachhap@gmail.com>,
        Viresh Kumar <viresh.kumar@linaro.org>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        Amit Kucheria <amitk@kernel.org>,
        Zhang Rui <rui.zhang@intel.com>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Pierre.Gondois@arm.com, Stephen Boyd <swboyd@chromium.org>,
        Rajendra Nayak <rnayak@codeaurora.org>,
        Bjorn Andersson <bjorn.andersson@linaro.org>
References: <20220207073036.14901-1-lukasz.luba@arm.com>
 <20220207073036.14901-2-lukasz.luba@arm.com> <YgG+TmLrCSXX4Bvt@google.com>
 <4a7d4e94-1461-5bac-5798-29998af9793a@arm.com> <YgKnnFl7Gp8AS30X@google.com>
 <e4532f65-7f8a-7e89-97c1-85cc61462040@arm.com> <YgQ9XLcto9v0fyTf@google.com>
 <d120110a-7d01-0cfd-f7eb-d160e17ec2a8@arm.com>
 <CAD=FV=VntGw1_AzJPpdOk0zSpOVZRH2X1JNg84JX+zCeU1jvXg@mail.gmail.com>
 <7c059f4f-7439-0cad-c398-96dbde4e49c1@linaro.org>
 <5b8ca53e-3595-85fd-5ae9-a5e8285e8513@arm.com>
From:   Daniel Lezcano <daniel.lezcano@linaro.org>
In-Reply-To: <5b8ca53e-3595-85fd-5ae9-a5e8285e8513@arm.com>
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On 17/02/2022 11:47, Lukasz Luba wrote:
> Hi Daniel,
> 
> On 2/17/22 10:10 AM, Daniel Lezcano wrote:
>> On 16/02/2022 18:33, Doug Anderson wrote:
>>> Hi,
>>>
>>> On Wed, Feb 16, 2022 at 7:35 AM Lukasz Luba <lukasz.luba@arm.com> wrote:
>>>>
>>>> Hi Matthias,
>>>>
>>>> On 2/9/22 10:17 PM, Matthias Kaehlcke wrote:
>>>>> On Wed, Feb 09, 2022 at 11:16:36AM +0000, Lukasz Luba wrote:
>>>>>>
>>>>>>
>>>>>> On 2/8/22 5:25 PM, Matthias Kaehlcke wrote:
>>>>>>> On Tue, Feb 08, 2022 at 09:32:28AM +0000, Lukasz Luba wrote:
>>>>>>>>
>>>>>>>>
>>>>
>>>> [snip]
>>>>
>>>>>>>> Could you point me to those devices please?
>>>>>>>
>>>>>>> arch/arm64/boot/dts/qcom/sc7180-trogdor-*
>>>>>>>
>>>>>>> Though as per above they shouldn't be impacted by your change, 
>>>>>>> since the
>>>>>>> CPUs always pretend to use milli-Watts.
>>>>>>>
>>>>>>> [skipped some questions/answers since sc7180 isn't actually 
>>>>>>> impacted by
>>>>>>>     the change]
>>>>>>
>>>>>> Thank you Matthias. I will investigate your setup to get better
>>>>>> understanding.
>>>>>
>>>>> Thanks!
>>>>>
>>>>
>>>> I've checked those DT files and related code.
>>>> As you already said, this patch is safe for them.
>>>> So we can apply it IMO.
>>>>
>>>>
>>>> -------------Off-topic------------------
>>>> Not in $subject comments:
>>>>
>>>> AFAICS based on two files which define thermal zones:
>>>> sc7180-trogdor-homestar.dtsi
>>>> sc7180-trogdor-coachz.dtsi
>>>>
>>>> only the 'big' cores are used as cooling devices in the
>>>> 'skin_temp_thermal' - the CPU6 and CPU7.
>>>>
>>>> I assume you don't want to model at all the power usage
>>>> from the Little cluster (which is quite big: 6 CPUs), do you?
>>>> I can see that the Little CPUs have small dyn-power-coeff
>>>> ~30% of the big and lower max freq, but still might be worth
>>>> to add them to IPA. You might give them more 'weight', to
>>>> make sure they receive more power during power split.
>>>>
>>>> You also don't have GPU cooling device in that thermal zone.
>>>> Based on my experience if your GPU is a power hungry one,
>>>> e.g. 2-4Watts, you might get better results when you model
>>>> this 'hot' device (which impacts your temp sensor reported value).
>>>
>>> I think the two boards you point at (homestar and coachz) are just the
>>> two that override the default defined in the SoC dtsi file. If you
>>> look in sc7180.dtsi you'll see 'gpuss1-thermal' which has a cooling
>>> map. You can also see the cooling maps for the littles.
>>>
>>> I guess we don't have a `dynamic-power-coefficient` for the GPU,
>>> though? Seems like we should, but I haven't dug through all the code
>>> here...
>>
>> The dynamic-power-coefficient is available for OPPs which includes 
>> CPUfreq and devfreq. As the GPU is managed by devfreq, setting the 
>> dynamic-power-coefficient makes the energy model available for it.
>>
>> However, the OPPs must define the frequency and the voltage. That is 
>> the case for most platforms except on QCom platform.
>>
>> That may not be specified as it uses a frequency index and the 
>> hardware does the voltage change in our back. The QCom cpufreq backend 
>> get the voltage table from a register (or whatever) and completes the 
>> voltage values for the OPPs, thus adding the information which is 
>> missing in the device tree. The energy model can then initializes 
>> itself and allows the usage of the Energy Aware Scheduler.
>>
>> However this piece of code is missing for the GPU part.
>>
> 
> Thank you for joining the discussion. I don't know about that Qcom
> GPU voltage information is missing.
> 
> If the voltage is not available (only the frequencies), there is
> another way. There is an 'advanced' EM which uses registration function:
> em_dev_register_perf_domain(). It uses a local driver callback to get
> power for each found frequency. It has benefit because there is no
> restriction to 'fit' into the math formula, instead just avg power
> values can be feed into EM. It's called 'advanced' EM [1].
> 
> Now we hit (again) the DT & EM issue (it's an old one, IIRC Morten
> was proposing from ~2014 this upstream, but EAS wasn't merged back
> then):
> where to store these power-freq values, which are then used by the
> callback. 

Why not make it more generic and replace the frequency by a performance 
index, so it can be used by any kind of perf limiter?

> We have the 'dynamic-power-coefficient' in DT, but
> it has limitations. It would be good to have this simple array
> attached to the GPU/CPU node. IMHO it meet the requirement of DT,
> it describes the HW (it would have HZ and Watts values).
> 
> Doug, Matthias could you have a look at that function and its
> usage, please [1]?
> If you guys would support me in this, I would start, with an RFC
> proposal, a discussion on LKML.
> 
> 
> [1] 
> https://elixir.bootlin.com/linux/v5.17-rc4/source/Documentation/power/energy-model.rst#L87 
> 


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