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

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Date:   Wed, 1 Aug 2018 09:30:04 +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 Tue, Jul 31, 2018 at 9:21 PM,  <skannan@...eaurora.org> wrote:
> 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.

What are the other events triggering it, then?

>> 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.

OK, fair enough.