linux-kernel - Re: [PATCH 2/2] PM / devfreq: Generic cpufreq governor

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Date:   Tue, 5 Jun 2018 16:26:31 -0700
From:   Saravana Kannan <skannan@...eaurora.org>
To:     myungjoo.ham@...sung.com,
        Kyungmin Park <kyungmin.park@...sung.com>,
        Chanwoo Choi <cw00.choi@...sung.com>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>
Cc:     Rajendra Nayak <rjendra@...eaurora.org>,
        Amit Kucheria <amit.kucheria@...aro.org>,
        "linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] PM / devfreq: Generic cpufreq governor



On 05/27/2018 11:00 PM, MyungJoo Ham wrote:
>> Many CPU architectures have caches that can scale independent of the CPUs.
>> Frequency scaling of the caches is necessary to make sure the cache is not
>> a performance bottleneck that leads to poor performance and power. The same
>> idea applies for RAM/DDR.
>>
>> To achieve this, this patch series adds a generic devfreq governor that can
>> listen to the frequency transitions of each CPU frequency domain and then
>> adjusts the frequency of the cache (or any devfreq device) based on the
>> frequency of the CPUs.
> I agree that we have some hardware pieces that want to configure
> frequencies based on the CPUfreq.
>
> Creating a devfreq governor that configures devfreq-freq
> based on incoming events (CPUFreq-transition-event in this case)
> is indeed a good idea.
>
> However, I would like to ask the followings:
> The overall code appears to be overly complex compared what you need.
> - Do you really need to revive "CPUFREQ POLICY" events for this?
> especially when you are going to look at "first CPU" only?
>
>
> Cheers,
> MyungJoo
>
Sorry, didn't notice this email earlier. My message filters seem to be 
messed up.

The POLICY notifiers are necessary for cases when all CPUs in a policy 
are hotplugged off -- we need to ignore their frequencies to avoid 
getting the devfreq device stuck at a high frequency. Looking at "first 
CPU" is just an optimization to ignore multiple transition notifiers for 
the each CPU in a policy -- we'd want to do that even if we don't have 
policy notifiers. Not having policy notifier won't really simplify the 
code by much. We'd be forced to check for policy->related_cpus for every 
transition notifier call if the CPU state hasn't been already initialized.

-Saravana

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