linux-kernel - Re: R: [RFC PATCH v3 0/2] Add Krait Cache Scaling support

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Message-ID: <6dc62d231c776b2cdfdc36cfe36e4140@codeaurora.org>
Date:   Thu, 03 Sep 2020 12:43:27 +0530
From:   Sibi Sankar <sibis@...eaurora.org>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     ansuelsmth@...il.com, vincent.guittot@...aro.org,
        saravanak@...gle.com, 'Sudeep Holla' <sudeep.holla@....com>,
        "'Rafael J. Wysocki'" <rjw@...ysocki.net>,
        'Rob Herring' <robh+dt@...nel.org>, linux-pm@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: R: [RFC PATCH v3 0/2] Add Krait Cache Scaling support

On 2020-09-03 12:23, Viresh Kumar wrote:
> On 31-08-20, 09:41, ansuelsmth@...il.com wrote:
>> On 31-08-20, Sibi wrote:
>> > On 2020-08-24 16:10, Viresh Kumar wrote:
>> > > +Vincent/Saravana/Sibi
>> > >
>> > > On 21-08-20, 16:00, Ansuel Smith wrote:
>> > >> This adds Krait Cache scaling support using the cpufreq notifier.
>> > >> I have some doubt about where this should be actually placed (clk or
>> > >> cpufreq)?
>> > >> Also the original idea was to create a dedicated cpufreq driver (like
>> > >> it's done in
>> > >> the codeaurora qcom repo) by copying the cpufreq-dt driver and adding
>> > >> the cache
>> > >> scaling logic but i still don't know what is better. Have a very
>> > >> similar driver or
>> > >> add a dedicated driver only for the cache using the cpufreq notifier
>> > >> and do the
>> > >> scale on every freq transition.
>> > >> Thanks to everyone who will review or answer these questions.
>> > >
>> > > Saravana was doing something with devfreq to solve such issues if I
>> > > wasn't mistaken.
>> > >
>> > > Sibi ?
>> >
>> > IIRC the final plan was to create a devfreq device
>> > and devfreq-cpufreq based governor to scale them, this
>> > way one can switch to a different governor if required.
>> 
>> So in this case I should convert this patch to a devfreq driver-
> 
> I think this should happen nevertheless. You are doing DVFS for a
> device which isn't a CPU and devfreq looks to be the right place of
> doing so.
> 
>> Isn't overkill to use a governor for such a task?
>> (3 range based on the cpufreq?)
> 
> I am not sure about the governor part here, maybe it won't be required
> ?

Yeah I don't see it being needed in ^^
case as well. I just mentioned them as
an advantage in case you wanted to switch
to a different governor in the future.

https://lore.kernel.org/lkml/d0bc8877-6d41-f54e-1c4c-2fadbb9dcd0b@samsung.com/

A devfreq governor tracking cpufreq was
generally accepted but using a cpufreq
notifier to achieve that was discouraged.

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