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Subject: Re: [PATCH v2 3/3] dt-bindings: thermal: update sustainable-power
 with abstract scale
To:     Doug Anderson <dianders@chromium.org>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Linux PM <linux-pm@vger.kernel.org>, linux-doc@vger.kernel.org,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@vger.kernel.org>, Rob Herring <robh+dt@kernel.org>,
        amitk@kernel.org, Jonathan Corbet <corbet@lwn.net>,
        Daniel Lezcano <daniel.lezcano@linaro.org>,
        Dietmar.Eggemann@arm.com, Quentin Perret <qperret@google.com>,
        Matthias Kaehlcke <mka@chromium.org>,
        Rajendra Nayak <rnayak@codeaurora.org>,
        "Rafael J. Wysocki" <rjw@rjwysocki.net>
References: <20201002114426.31277-1-lukasz.luba@arm.com>
 <20201002114426.31277-4-lukasz.luba@arm.com>
 <CAD=FV=UbNP5-G1z95F37Fmv8=n0JPSSwnPQO_K==WpAc4vAHWQ@mail.gmail.com>
 <e9b6fc5a-45d3-168d-db38-6c068da26f6b@arm.com>
 <CAD=FV=Xkg1zpsMW5rERbibnjrgY6opZi8Z9DUFkWebb7NHtU5w@mail.gmail.com>
From:   Lukasz Luba <lukasz.luba@arm.com>
Message-ID: <e80dc3ac-c115-887f-6c72-e0f3d8cd9c76@arm.com>
Date:   Wed, 7 Oct 2020 10:03:32 +0100
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Hi Doug,

On 10/2/20 4:47 PM, Doug Anderson wrote:
> Hi,
> 
> On Fri, Oct 2, 2020 at 8:13 AM Lukasz Luba <lukasz.luba@arm.com> wrote:
>>
>> Hi Doug,
>>
>> On 10/2/20 3:31 PM, Doug Anderson wrote:
>>> Hi,
>>>
>>> On Fri, Oct 2, 2020 at 4:45 AM Lukasz Luba <lukasz.luba@arm.com> wrote:
>>>>
>>>> Update the documentation for the binding 'sustainable-power' and allow
>>>> to provide values in an abstract scale. It is required when the cooling
>>>> devices use an abstract scale for their power values.
>>>>
>>>> Signed-off-by: Lukasz Luba <lukasz.luba@arm.com>
>>>> ---
>>>>    .../devicetree/bindings/thermal/thermal-zones.yaml  | 13 +++++++++----
>>>>    1 file changed, 9 insertions(+), 4 deletions(-)
>>>>
>>>> diff --git a/Documentation/devicetree/bindings/thermal/thermal-zones.yaml b/Documentation/devicetree/bindings/thermal/thermal-zones.yaml
>>>> index 3ec9cc87ec50..4d8f2e37d1e6 100644
>>>> --- a/Documentation/devicetree/bindings/thermal/thermal-zones.yaml
>>>> +++ b/Documentation/devicetree/bindings/thermal/thermal-zones.yaml
>>>> @@ -99,10 +99,15 @@ patternProperties:
>>>>          sustainable-power:
>>>>            $ref: /schemas/types.yaml#/definitions/uint32
>>>>            description:
>>>> -          An estimate of the sustainable power (in mW) that this thermal zone
>>>> -          can dissipate at the desired control temperature. For reference, the
>>>> -          sustainable power of a 4-inch phone is typically 2000mW, while on a
>>>> -          10-inch tablet is around 4500mW.
>>>> +          An estimate of the sustainable power (in mW or in an abstract scale)
>>>> +         that this thermal zone can dissipate at the desired control
>>>> +         temperature. For reference, the sustainable power of a 4-inch phone
>>>> +         is typically 2000mW, while on a 10-inch tablet is around 4500mW.
>>>> +
>>>> +         It is possible to express the sustainable power in an abstract
>>>> +         scale. This is the case when the related cooling devices use also
>>>> +         abstract scale to express their power usage. The scale must be
>>>> +         consistent.
>>>
>>> Two thoughts:
>>>
>>> 1. If we're going to allow "sustainable-power" to be in abstract
>>> scale, why not allow "dynamic-power-coefficient" to be in abstract
>>> scale too?  I assume that the whole reason against that originally was
>>> the idea of device tree purity, but if we're allowing the abstract
>>> scale here then there seems no reason not to allow it for
>>> "dynamic-power-coefficient".
>>
>> With this binding it's a bit more tricky.
>> I also have to discuss a few things internally. This requirement of
>> uW/MHz/V^2 makes the code easier also for potential drivers
>> like GPU (which are going to register the devfreq cooling with EM).
>>
>> Let me think about it, but for now I would just update these bits.
>> These are required to proper IPA operation, the dyn.-pow.-coef. is a
>> nice to have and possible next step.
> 
> I guess the problem is that Rajendra is currently planning to remove
> all the "dynamic-power-coefficient" values from device tree right now
> and move them to the source code because the numbers we currently have
> in the device tree _are_ in abstract scale and thus violate the
> bindings.  Moving this to source code won't help us get to more real
> power numbers (since it'll still be abstract scale), it'll just be
> pure churn.  If we're OK with the abstract scale in general then we
> should allow it everywhere and not add churn for no reason.
> 
> 

I just want to notify you that I had internal conversation about this
'dynamic-power-coefficient' binding and abstract scale. We would
change it as well, similarly to 'sustainable-power'. It must pass
internal review and I will send the v3 of this series.

Regards,
Lukasz