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

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Date:   Sun, 29 Jul 2018 12:52:02 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Saravana Kannan <skannan@...eaurora.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" <devicetree@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] PM / devfreq: Generic cpufreq governor

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.