linux-kernel - Re: [PATCH 1/2] docs: Clarify abstract scale usage for power values in Energy Model

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Date:   Wed, 30 Sep 2020 09:25:21 +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,
        Jonathan Corbet <corbet@....net>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Dietmar.Eggemann@....com, Quentin Perret <qperret@...gle.com>,
        Matthias Kaehlcke <mka@...omium.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Rajendra Nayak <rnayak@...eaurora.org>
Subject: Re: [PATCH 1/2] docs: Clarify abstract scale usage for power values
 in Energy Model

Hi Douglas,

On 9/30/20 12:53 AM, Doug Anderson wrote:
> Hi,
> 
> On Tue, Sep 29, 2020 at 5:16 AM Lukasz Luba <lukasz.luba@....com> wrote:
>>
>> The Energy Model (EM) can store power values in milli-Watts or in abstract
>> scale. This might cause issues in the subsystems which use the EM for
>> estimating the device power, such as:
>> - mixing of different scales in a subsystem which uses multiple
>>    (cooling) devices (e.g. thermal Intelligent Power Allocation (IPA))
>> - assuming that energy [milli-Joules] can be derived from the EM power
>>    values which might not be possible since the power scale doesn't have to
>>    be in milli-Watts
>>
>> To avoid misconfiguration add the needed documentation to the EM and
>> related subsystems: EAS and IPA.
>>
>> Signed-off-by: Lukasz Luba <lukasz.luba@....com>
>> ---
>>   .../driver-api/thermal/power_allocator.rst          |  8 ++++++++
>>   Documentation/power/energy-model.rst                | 13 +++++++++++++
>>   Documentation/scheduler/sched-energy.rst            |  5 +++++
>>   3 files changed, 26 insertions(+)
> 
> I haven't read through these files in massive detail, but the quick
> skim makes me believe that your additions seem sane.  In general, I'm
> happy with documenting reality, thus:
> 
> Reviewed-by: Douglas Anderson <dianders@...omium.org>

Thank you for the review.

> 
> I will note: you haven't actually updated the device tree bindings.
> Thus, presumably, anyone who is specifying these numbers in the device
> tree is still supposed to specify them in a way that mW can be
> recovered, right?  Said another way: nothing about your patches makes
> it OK to specify numbers in device trees using an "abstract scale",
> right?

For completeness, we are talking here about the binding from:
Documentation/devicetree/bindings/arm/cpus.yaml
which is 'dynamic-power-coefficient'. Yes, it stays untouched, also the
unit (uW/MHz/V^2) which then allows to have mW in the power
values in the EM.

Regards,
Lukasz