linux-kernel - Re: [RFC PATCH v3 03/10] PM: Introduce an Energy Model management framework

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Date:   Fri, 8 Jun 2018 18:39:56 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Quentin Perret <quentin.perret@....com>
Cc:     Juri Lelli <juri.lelli@...hat.com>, peterz@...radead.org,
        rjw@...ysocki.net, gregkh@...uxfoundation.org,
        linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        mingo@...hat.com, morten.rasmussen@....com, chris.redpath@....com,
        patrick.bellasi@....com, valentin.schneider@....com,
        vincent.guittot@...aro.org, thara.gopinath@...aro.org,
        viresh.kumar@...aro.org, tkjos@...gle.com, joelaf@...gle.com,
        smuckle@...gle.com, adharmap@...cinc.com, skannan@...cinc.com,
        pkondeti@...eaurora.org, edubezval@...il.com,
        srinivas.pandruvada@...ux.intel.com, currojerez@...eup.net,
        javi.merino@...nel.org
Subject: Re: [RFC PATCH v3 03/10] PM: Introduce an Energy Model management
 framework

On 06/08/2018 03:11 PM, Quentin Perret wrote:
> On Friday 08 Jun 2018 at 14:39:33 (+0200), Dietmar Eggemann wrote:

[...]

>>>> Even though we would be forced to get cpufreq's related cpumask from
>>>> somewhere.
>>>
>>> That's the easy part. The difficult part is, where do you get power
>>> values from ? You have to let the lower layers register those values
>>> in a centralized location on a voluntary basis. And then it becomes easy
>>> for consumers to access that data, because they know where it is.
>>
>> The code in the arch could use the same struct em_data_callback em_cb = {
>> &dev_pm_opp_of_estimate_power } that the cpufreq driver is currently using?
> 
> How do you know from the arch code if you should get power from DT
> with dev_pm_opp_of_estimate_power or use another callback that reads
> power from firmware (SCMI) ?

Ah, ok, cpufreq dt, scpi and arm_big_little are dt, cpufreq scmi can be 
different ...

> 
> I don't think it is reasonable to assume a single source of information for
> an arch. It is is already an incorrect assumption even if just you look at
> the Arm world.

Ok, I see.

> 
>>> Again, I don't think that's possible. You have to let the lower layers
>>> tell you where the power values come from, at the very least. You could
>>> let the archs do that aggregation I suppose, but I don't really see the
>>> benefit over one centralized framework with a generic interface ...
>>> What's your opinion ?
>>
>> Don't understand the '... let the lower layers tell you where the power
>> values come from ...' part. Where is the difference whether the arch or the
>> cpufreq driver uses em_data_callback?
> 
> Because different CPUFreq drivers can be used for one arch. There are
> different CPUFreq drivers because there are different ways of getting
> information about the platform, even just for the Arm world (DT, SCPI,
> SCMI, ...). It's the same thing for power values, they don't necessarily
> come from DT.

scpi is dt ? At least scpi-cpufreq.c uses this 
dev_pm_opp_of_estimate_power too.

> The point of having a centralized EM framework with a standardized
> callback prototype is flexibility. You can implement a callback that
> estimates power from the DT. You can implement a callback that reads
> power from firmware. But you can also have a completely ad-hoc EM
> provider in a module if you like. All you have to do to provide data to
> the framework is respect the callback API.

IMHO, this idea is good, there should be also user of this outside 
arm/arm64 ...

[...]