linux-kernel - Re: [PATCH v5 1/5] PM / EM: add devices to Energy Model

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Message-ID: <cb7f670a-a04f-ba6f-1486-0421f3cce2e9@arm.com>
Date:   Mon, 6 Apr 2020 14:29:56 +0100
From:   Lukasz Luba <lukasz.luba@....com>
To:     Daniel Lezcano <daniel.lezcano@...aro.org>,
        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
Cc:     Morten.Rasmussen@....com, Dietmar.Eggemann@....com,
        javi.merino@....com, cw00.choi@...sung.com,
        b.zolnierkie@...sung.com, rjw@...ysocki.net, sudeep.holla@....com,
        viresh.kumar@...aro.org, nm@...com, sboyd@...nel.org,
        rui.zhang@...el.com, amit.kucheria@...durent.com, mingo@...hat.com,
        peterz@...radead.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, rostedt@...dmis.org,
        qperret@...gle.com, bsegall@...gle.com, mgorman@...e.de,
        shawnguo@...nel.org, s.hauer@...gutronix.de, festevam@...il.com,
        kernel@...gutronix.de, khilman@...nel.org, agross@...nel.org,
        bjorn.andersson@...aro.org, robh@...nel.org,
        matthias.bgg@...il.com, steven.price@....com,
        tomeu.vizoso@...labora.com, alyssa.rosenzweig@...labora.com,
        airlied@...ux.ie, daniel@...ll.ch, liviu.dudau@....com,
        lorenzo.pieralisi@....com, patrick.bellasi@...bug.net,
        orjan.eide@....com, rdunlap@...radead.org, mka@...omium.org
Subject: Re: [PATCH v5 1/5] PM / EM: add devices to Energy Model

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@...gle.com>
>> Signed-off-by: Lukasz Luba <lukasz.luba@....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.
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.

Regards,
Lukasz

[1] 
https://elixir.bootlin.com/linux/latest/source/drivers/cpufreq/cpufreq.c#L1328
[2] 
https://elixir.bootlin.com/linux/latest/source/drivers/cpufreq/cpufreq.c#L1350
[3] 
https://elixir.bootlin.com/linux/latest/source/drivers/cpufreq/cpufreq.c#L1374