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Subject: Re: [PATCH v5 1/5] PM / EM: add devices to Energy Model
To:     Daniel Lezcano <daniel.lezcano@linaro.org>,
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 <cb7f670a-a04f-ba6f-1486-0421f3cce2e9@arm.com>
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 <4cee98ce-62a6-7448-a99c-3a1af6c87cf4@linaro.org>
From:   Lukasz Luba <lukasz.luba@arm.com>
Message-ID: <6c2fcd42-5245-ff45-1852-0f2ec800517f@arm.com>
Date:   Tue, 7 Apr 2020 10:32:55 +0100
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On 4/6/20 10:17 PM, Daniel Lezcano wrote:
> On 06/04/2020 18:07, Lukasz Luba wrote:
>>
>>
>> On 4/6/20 3:58 PM, Daniel Lezcano wrote:
>>>
>>> Hi Lukasz,
>>>
>>>
>>> On 06/04/2020 15:29, Lukasz Luba wrote:
>>>> Hi Daniel,
>>>>
>>>> Thank you for the review.
>>>>
>>>> On 4/3/20 5:05 PM, Daniel Lezcano wrote:
>>>>>
>>>>> Hi Lukasz,
>>>>>
>>>>>
>>>>> On 18/03/2020 12:45, Lukasz Luba wrote:
>>>>>> Add support of other devices into the Energy Model framework not only
>>>>>> the
>>>>>> CPUs. Change the interface to be more unified which can handle other
>>>>>> devices as well.
>>>>>
>>>>> thanks for taking care of that. Overall I like the changes in this
>>>>> patch
>>>>> but it hard to review in details because the patch is too big :/
>>>>>
>>>>> Could you split this patch into smaller ones?
>>>>>
>>>>> eg. (at your convenience)
>>>>>
>>>>>     - One patch renaming s/cap/perf/
>>>>>
>>>>>     - One patch adding a new function:
>>>>>
>>>>>        em_dev_register_perf_domain(struct device *dev,
>>>>>                   unsigned int nr_states,
>>>>>                   struct em_data_callback *cb);
>>>>>
>>>>>       (+ EXPORT_SYMBOL_GPL)
>>>>>
>>>>>        And em_register_perf_domain() using it.
>>>>>
>>>>>     - One converting the em_register_perf_domain() user to
>>>>>       em_dev_register_perf_domain
>>>>>
>>>>>     - One adding the different new 'em' functions
>>>>>
>>>>>     - And finally one removing em_register_perf_domain().
>>>>
>>>> I agree and will do the split. I could also break the dependencies
>>>> for future easier merge.
>>>>
>>>>>
>>>>>
>>>>>> Acked-by: Quentin Perret <qperret@google.com>
>>>>>> Signed-off-by: Lukasz Luba <lukasz.luba@arm.com>
>>>>>> ---
>>>>>
>>>>> [ ... ]
>>>>>
>>>>>>     2. Core APIs
>>>>>> @@ -70,14 +72,16 @@ CONFIG_ENERGY_MODEL must be enabled to use the EM
>>>>>> framework.
>>>>>>     Drivers are expected to register performance domains into the EM
>>>>>> framework by
>>>>>>     calling the following API::
>>>>>>     -  int em_register_perf_domain(cpumask_t *span, unsigned int
>>>>>> nr_states,
>>>>>> -                  struct em_data_callback *cb);
>>>>>> +  int em_register_perf_domain(struct device *dev, unsigned int
>>>>>> nr_states,
>>>>>> +        struct em_data_callback *cb, cpumask_t *cpus);
>>>>>
>>>>> Isn't possible to get rid of this cpumask by using
>>>>> cpufreq_cpu_get() which returns the cpufreq's policy and from their get
>>>>> the related cpus ?
>>>>
>>>> We had similar thoughts with Quentin and I've checked this.
>>>
>>> Yeah, I suspected you already think about that :)
>>>
>>>> Unfortunately, if the policy is a 'new policy' [1] it gets
>>>> allocated and passed into cpufreq driver ->init(policy) [2].
>>>> Then that policy is set into per_cpu pointer for each related_cpu [3]:
>>>>
>>>> for_each_cpu(j, policy->related_cpus)
>>>>       per_cpu(cpufreq_cpu_data, j) = policy;
>>>>
>>>>    Thus, any calls of functions (i.e. cpufreq_cpu_get()) which try to
>>>> take this ptr before [3] won't work.
>>>>
>>>> We are trying to register EM from cpufreq_driver->init(policy) and the
>>>> per_cpu policy is likely to be not populated at that phase.
>>>
>>> What is the problem of registering at the end of the cpufreq_online ?
>>
>> We want to enable driver developers to choose one of two options for the
>> registration of Energy Model:
>> 1. a simple one via dev_pm_opp_of_register_em(), which uses default
>>     callback function calculating power based on: voltage, freq
>>     and DT entry 'dynamic-power-coefficient' for each OPP
>> 2. a more sophisticated, when driver provides callback function, which
>>    will be called from EM for each OPP to ask for related power;
>>    This interface could also be used by devices which relay not only
>>    on one source of 'voltage', i.e. manipulate body bias or have
>>    other controlling voltage for gates in the new 3D transistors. They
>>    might provide custom callback function in their cpufreq driver.
>>    This is used i.e. in cpufreq drivers which use firmware to get power,
>>    like scmi-cpufreq.c;
>>
>> To meet this requirement the registration of EM is moved into cpufreq
>> drivers, not in the framework i.e cpufreq_online(). If we could limit
>> the support for only option 1. then we could move the registration
>> call into cpufreq framework and clean the cpufreq drivers.
> 
> I'm not sure to get your point but I think a series setting the scene by
> moving the dev_pm_opp_of_register_em() to cpufreq_online() and remove
> the cpumask may make sense.

Some of the cpufreq drivers don't use dev_pm_opp_of_register_em() but 
instead em_register_perf_domain() with their em_data_callback [1].
It is because of point 2. described above. The dev_pm_opp_of_register_em
won't work for them, so it's not a good candidate to cover all use cases
in the framework.

> 
> Can you send the split version of patch 1/5 as a series without the
> other changes ? So we can focus on first ?

Sure, I will only split patch 1/5 as you suggested and send v6.
Thank you for your time and help.

Regards,
Lukasz

[1] 
https://elixir.bootlin.com/linux/latest/source/drivers/cpufreq/scmi-cpufreq.c#L203