linux-kernel - Re: [PATCH 8/8] thermal/drivers/cpu_cooling: Add the combo cpu cooling device

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Date:   Fri, 2 Feb 2018 15:30:10 +0100
From:   Daniel Lezcano <daniel.lezcano@...aro.org>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     edubezval@...il.com, kevin.wangtao@...aro.org, leo.yan@...aro.org,
        vincent.guittot@...aro.org, amit.kachhap@...il.com,
        linux-kernel@...r.kernel.org, Zhang Rui <rui.zhang@...el.com>,
        Javi Merino <javi.merino@...nel.org>,
        "open list:THERMAL" <linux-pm@...r.kernel.org>,
        daniel.thompson@...aro.org
Subject: Re: [PATCH 8/8] thermal/drivers/cpu_cooling: Add the combo cpu
 cooling device

On 02/02/2018 11:42, Viresh Kumar wrote:
> Hi Daniel,

Hi Viresh,


> I have gone through the other review comments, specially from Daniel T.. While I
> share some of his concerns, I have few more of mine.
> 
> On 23-01-18, 16:34, Daniel Lezcano wrote:
>> +late_initcall(cpu_cooling_init);
> 
> For example, this thing isn't going to fly nicely as you have assumed cpufreq
> and cpuidle drivers are going to be part of the kernel itself. What if they are
> modules and are inserted after late-init ? There are more cases like this where
> the cpufreq driver may unregister the cpufreq cooling device on the fly and then
> add it back. And so this stuff is a bit tricky.

The cpuidle driver can not be compiled as a module.

Agree the cpufreq driver can be unloaded and this part needs some
adjustments.

> Here is how I see the whole thing now:
> 
> - Yes we need individual support for both cpufreq and cpuidle cooling devices,
>   and no one disagrees on that I believe.
> 
> - There is nothing in the thermal framework that disallows both cpufreq and
>   cpuidle cooling devices to co-exist. Both would be part of the same thermal
>   zone and so will get throttled with the same thermal sensor event. And so we
>   will end up trying to cool down the SoC using both cpufreq and cpuidle
>   technique.

No. It does not work because we will need different state for each
cooling device and we need some logic behind.

> - Now I am just wondering if we really need the "combo" functionality or not.
>   Can we fine tune the DT cpu-cooling properties (existing ones) for a platform,
>   so that it automatically acts as a combo cooling device? I am not 100% sure
>   its gonna fly, but just wanted to make sure its not possible to work around
>   with and then only try the combo device thing.
> 
> For example, suppose that with just cpufreq-cooling device we need to take the
> CPU down to 1 GHz from 2 GHz if we cross temperature 'X'. What if we can change
> this policy from DT and say the cpufreq-cooling device goes to 1.5 GHz and
> cpuidle-cooling device takes us to idle for 'y' us, and the effect of
> combination of these two is >= the effect of the 1 GHz for just the
> cpufreq-cooling device.
> 
> Is there any possibility of this to work ?

It does not make sense. The combo does that automatically by computing
the power equivalence more precisely.


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