linux-kernel - Re: [PATCH 1/4] PM / EM: and devices to Energy Model

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Message-ID: <8332c4ac-2a7d-1e2d-76e9-7c979a666257@arm.com>
Date:   Mon, 20 Jan 2020 19:27:26 +0100
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Quentin Perret <qperret@...gle.com>,
        Lukasz Luba <lukasz.luba@....com>
Cc:     linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        dri-devel@...ts.freedesktop.org, linux-omap@...r.kernel.org,
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Subject: Re: [PATCH 1/4] PM / EM: and devices to Energy Model

On 20/01/2020 16:09, Quentin Perret wrote:
> Hey Lukasz,
> 
> On Monday 20 Jan 2020 at 14:52:07 (+0000), Lukasz Luba wrote:
>> On 1/17/20 10:54 AM, Quentin Perret wrote:
>>> Suggested alternative: have two registration functions like so:
>>>
>>> 	int em_register_dev_pd(struct device *dev, unsigned int nr_states,
>>> 			       struct em_data_callback *cb);
>>> 	int em_register_cpu_pd(cpumask_t *span, unsigned int nr_states,
>>> 			       struct em_data_callback *cb);
>>
>> Interesting, in the internal review Dietmar asked me to remove these two
>> functions. I had the same idea, which would simplify a bit the
>> registration and it does not need to check the dev->bus if it is CPU.
>>
>> Unfortunately, we would need also two function in drivers/opp/of.c:
>> dev_pm_opp_of_register_cpu_em(policy->cpus);
>> and
>> dev_pm_opp_of_register_dev_em(dev);
>>
>> Thus, I have created only one registration function, which you can see
>> in this patch set.
> 
> Right, I can see how having a unified API would be appealing, but the
> OPP dependency is a nono, so we'll need to work around one way or
> another.
> 
> FWIW, I don't think having separate APIs for CPUs and other devices is
> that bad given that we already have entirely different frameworks to
> drive their respective frequencies. And the _cpu variants are basically
> just wrappers around the _dev ones, so not too bad either IMO :).

It's true that we need the policy->cpus cpumask only for cpu devices and
we have it available when we call em_register_perf_domain()
[scmi-cpufreq.c driver] or the OPP wrapper dev_pm_opp_of_register_em()
[e.g. cpufreq-dt.c driver].

And we shouldn't make EM code dependent on OPP.

But can't we add 'struct cpumask *mask' as an additional argument to
both which can be set to NULL for (devfreq) devices?

We can check in em_register_perf_domain() that we got a valid cpumask
for a cpu device and ignore it for (devfreq) devices.