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

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Date:   Tue, 31 Jul 2018 10:00:13 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Saravana Kannan <skannan@...eaurora.org>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        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" <devicetree@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] PM / devfreq: Generic cpufreq governor

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.

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?

> I'm open to suggestions for the devfreq governor name too. "cpufreq" has
> been very confusing so far.
> cpufreq-map maybe?

It's more like "cpufreq drriven" IMO, but I don't really have any good
names for it in mind ATM I'm afraid.