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Subject: Re: [PATCH 1/4] PM / EM: and devices to Energy Model
To:     Quentin Perret <qperret@google.com>
Cc:     Dietmar Eggemann <dietmar.eggemann@arm.com>,
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From:   Lukasz Luba <lukasz.luba@arm.com>
Message-ID: <5621c137-7057-1d5c-646a-95707105df25@arm.com>
Date:   Tue, 21 Jan 2020 10:49:19 +0000
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On 1/21/20 10:08 AM, Quentin Perret wrote:
> On Monday 20 Jan 2020 at 16:20:49 (+0000), Lukasz Luba wrote:
>> On 1/20/20 3:28 PM, Quentin Perret wrote:
>>> 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.
> 
> Right so I suppose the easiest solution would be to do the opposite of
> my first suggestion. That is, make em_get_pd() call em_cpu_get() if the
> device is a CPU device, or proceed to the PD list iteration for other
> devices. And em_cpu_get() can remain as you originally suggested (that
> is, iterate over the PDs and test the mask).

Exactly, something like:
---------------------------->8-------------------------
288 struct em_perf_domain *em_get_pd(struct device *dev)
289 {
290         struct em_device *em_dev;
291
292         if (IS_ERR_OR_NULL(dev))
293                 return NULL;
294
295         if (_is_cpu_device(dev))
296                 return em_cpu_get(dev->id);
....
------------------------8<-----------------------------

> 
> That should ensure em_get_pd() always works, em_cpu_get() is still there
> handy for the scheduler and such, and the two EM lookup functions (for
> CPUs or for devices) are kept cleanly separated.
> 
> Thoughts ?

Agree. Then we can have these two functions and em_get_pd() will also
work fine.

> 
> Thanks,
> Quentin
> 

Regards,
Lukasz