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Subject: Re: [PATCH] devfreq: Register devfreq as a cooling device
To:     Daniel Lezcano <daniel.lezcano@linaro.org>
Cc:     cwchoi00@gmail.com, kyungmin.park@samsung.com,
        myungjoo.ham@samsung.com, linux-pm@vger.kernel.org,
        linux-kernel@vger.kernel.org, Chanwoo Choi <cw00.choi@samsung.com>,
        Qiang Yu <yuq825@gmail.com>, David Airlie <airlied@linux.ie>,
        Daniel Vetter <daniel@ffwll.ch>,
        Rob Clark <robdclark@gmail.com>, Sean Paul <sean@poorly.run>,
        Rob Herring <robh@kernel.org>,
        Tomeu Vizoso <tomeu.vizoso@collabora.com>,
        Steven Price <steven.price@arm.com>,
        Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>,
        "open list:DRM DRIVERS FOR LIMA" <dri-devel@lists.freedesktop.org>,
        "moderated list:DRM DRIVERS FOR LIMA" <lima@lists.freedesktop.org>,
        "open list:DRM DRIVER FOR MSM ADRENO GPU" 
        <linux-arm-msm@vger.kernel.org>,
        "open list:DRM DRIVER FOR MSM ADRENO GPU" 
        <freedreno@lists.freedesktop.org>
References: <20210304125034.28404-1-daniel.lezcano@linaro.org>
From:   Lukasz Luba <lukasz.luba@arm.com>
Message-ID: <5f06e0c5-b2d9-5e11-01b6-fdd0dac635a7@arm.com>
Date:   Thu, 4 Mar 2021 13:47:37 +0000
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Hi Daniel,

On 3/4/21 12:50 PM, Daniel Lezcano wrote:
> Currently the default behavior is to manually having the devfreq
> backend to register themselves as a devfreq cooling device.
> 
> There are no so many and actually it makes more sense to register the
> devfreq device when adding it.
> 
> Consequently, every devfreq becomes a cooling device like cpufreq is.
> 
> Having a devfreq being registered as a cooling device can not mitigate
> a thermal zone if it is not bound to this one. Thus, the current
> configurations are not impacted by this change.

There are also different type of devices, which register into devfreq
framework like NoC buses, UFS/eMMC, jpeg and video accelerators, ISP,
etc.
In some platforms there are plenty of those devices and they all would
occupy memory due to private freq_table in devfreq_cooling, function:
devfreq_cooling_gen_tables().

IIRC in OdroidXU4 there are ~20 devfreq devs for NoC buses.

It's true that they will not affect thermal zones, but unnecessarily,
they all will show up in the /sys/class/thermal/ as
thermal-devfreq-X.

IMO the devfreq shouldn't be tight with devfreq cooling thermal.

CpuFreq is different because it handles only CPUs. Here we have
many different devices.

Regards,
Lukasz