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

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Date:   Tue, 31 Jul 2018 12:21:49 -0700
From:   skannan@...eaurora.org
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     MyungJoo Ham <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>,
        Linux PM <linux-pm@...r.kernel.org>,
        devicetree@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        rjwysocki@...il.com
Subject: Re: [PATCH] PM / devfreq: Generic cpufreq governor

On 2018-07-31 01:00, Rafael J. Wysocki wrote:
> On Mon, Jul 30, 2018 at 8:58 PM,  <skannan@...eaurora.org> wrote:
>> On 2018-07-29 03:52, Rafael J. Wysocki wrote:
>>> 
>>> On Sat, Jul 28, 2018 at 5:56 AM, Saravana Kannan 
>>> <skannan@...eaurora.org>
>>> 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 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.
>>>> 
>>>> To decide the frequency of the device, the governor does one of the
>>>> following:
>>>> 
>>>> * Uses a CPU frequency to device frequency mapping table
>>>>   - Either one mapping table used for all CPU freq policies 
>>>> (typically
>>>> used
>>>>     for system with homogeneous cores/clusters that have the same 
>>>> OPPs.
>>>>   - One mapping table per CPU freq policy (typically used for ASMP
>>>> systems
>>>>     with heterogeneous CPUs with different OPPs)
>>>> 
>>>> OR
>>>> 
>>>> * Scales the device frequency in proportion to the CPU frequency. 
>>>> So, if
>>>>   the CPUs are running at their max frequency, the device runs at 
>>>> its max
>>>>   frequency.  If the CPUs are running at their min frequency, the 
>>>> device
>>>>   runs at its min frequency. And interpolated for frequencies in 
>>>> between.
>>> 
>>> 
>>> While not having looked at the details of the patch yet, I would
>>> change the name of the feature to "Generic cpufreq transition
>>> governor" to make it somewhat less ambiguous.
>> 
>> 
>> In my opinion it makes it look MORE like this is a cpufreq governor. 
>> How
>> about the following?
>> PM / devfreq: Generic cpufreq to devfreq mapping governor
>> Seem a lot more clear to me.
> 
> Well, it's not just mapping, but also it triggers on cpufreq 
> transitions AFAICS.

Right, but I'm not sure that's the most important aspect of this 
governor.

> Which makes me wonder if the approach here is the right one at all.
> 
> Shouldn't the cpufreq driver be hooked up to the related HW through
> the OPP framework and sharing access with devfreq rather?

Not sure what you mean here. This devfreq governor is orthogonal to what 
the cpufreq driver does with its HW. This is just trying to scale L3 or 
DDR or whatever other device based on current CPU frequency. Not all 
CPUfreq drivers support OPP. And even if they do, I don't see how it's 
relevant here.

Thanks,
Saravana