linux-kernel - Re: [PATCH v2 3/3] dt-bindings: thermal: update sustainable-power with abstract scale

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Date:   Fri, 2 Oct 2020 16:12:55 +0100
From:   Lukasz Luba <lukasz.luba@....com>
To:     Doug Anderson <dianders@...omium.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>, linux-doc@...r.kernel.org,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>, Rob Herring <robh+dt@...nel.org>,
        amitk@...nel.org, Jonathan Corbet <corbet@....net>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Dietmar.Eggemann@....com, Quentin Perret <qperret@...gle.com>,
        Matthias Kaehlcke <mka@...omium.org>,
        Rajendra Nayak <rnayak@...eaurora.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>
Subject: Re: [PATCH v2 3/3] dt-bindings: thermal: update sustainable-power
 with abstract scale

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@....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@....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.

> 
> 2. Is it worth adding some type of indication of what type of units
> "sustainable-power" is represented in?  Maybe even a made up unit so
> that you could tell the difference between made up units in the same
> system?  I'd envision something like:
> 
> sustainable-power-units = "qualcomm,sc7180-bogoWatts"
> 
> ...and on the dynamic-power-coefficient side, the same:
> 
> dynamic-power-coefficient-units = "qualcomm,sc7180-bogoWatts"
> 
> One could imagine someone even later (after devices are widely
> distributed) figuring out translations between these bogoWatts numbers
> and real Watts if someone could come up with a case where it matters.

To figure this out we don't need a new binding.
I think a simple comment in the DT would be enough for this, even e.g.:

sustainable-power = <100> /* bogoWatts */


Thank you for your comments.
BTW, I haven't put your 'Reviewed-by' because I have added this
sustainable-power new stuff in patch 1/3. I will grateful if you
have a look on that.

Regards,
Lukasz