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

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Date:   Mon, 20 Jan 2020 16:20:49 +0000
From:   Lukasz Luba <lukasz.luba@....com>
To:     Quentin Perret <qperret@...gle.com>,
        Dietmar Eggemann <dietmar.eggemann@....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,
        linux-mediatek@...ts.infradead.org, linux-arm-msm@...r.kernel.org,
        linux-imx@....com, Morten.Rasmussen@....com, Chris.Redpath@....com,
        ionela.voinescu@....com, javi.merino@....com,
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        tomeu.vizoso@...labora.com, alyssa.rosenzweig@...labora.com,
        airlied@...ux.ie, daniel@...ll.ch, patrick.bellasi@...bug.net
Subject: Re: [PATCH 1/4] PM / EM: and devices to Energy Model



On 1/20/20 3:28 PM, Quentin Perret wrote:
> On Monday 20 Jan 2020 at 15:53:35 (+0100), Dietmar Eggemann wrote:
>> Would be really nice if this wouldn't be required. We should really aim
>> for 1 framework == 1 set of interfaces.
>>
>> What happens if someone calls em_get_pd() on a CPU EM?
>>
>> E.g:
>>
>>   static struct perf_domain *pd_init(int cpu)
>>   {
>> -       struct em_perf_domain *obj = em_cpu_get(cpu);
>> +       struct device *dev = get_cpu_device(cpu);
>> +       struct em_perf_domain *obj = em_pd_get(dev);
>>          struct perf_domain *pd;
>>
>> Two versions of one functionality will confuse API user from the
>> beginning ...
> 
> Agreed, this looks a bit confusing. It should be trivial to make
> em_dev_get() (or whatever we end up calling it) work for CPUs too,
> though. And we could always have a em_cpu_get(int cpu) API that is a
> basically a wrapper around em_dev_get() for convenience.

The problem not only here is that we have a CPU index 'int cpu'
and if we ask for device like:

struct device *dev = get_cpu_device(cpu);

It might be not the same device that was used during the
registration, when we had i.e. 4 CPUs for the same policy:

int cpu_id = cpumask_first(policy->cpus);
struct device *cpu_dev = get_cpu_device(cpu_id);
em_register_perf_domain(cpu_dev, nr_opp, &em_cb);

That's why the em_cpu_get() is different than em_get_pd(), mainly by:
if (cpumask_test_cpu(cpu, em_span_cpus(em_pd)))

It won't be simple wrapper, let me think how it could be handled
differently than it is now.

Regards,
Lukasz

> 
> Thanks,
> Quentin
>