linux-kernel - Re: [RFC v2 1/1] drm/lima: Add optional devfreq support

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Message-ID: <fe45f4f8-8c67-ded2-90bf-8d5fd6874876@arm.com>
Date:   Mon, 30 Dec 2019 00:47:00 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Martin Blumenstingl <martin.blumenstingl@...glemail.com>
Cc:     yuq825@...il.com, dri-devel@...ts.freedesktop.org, robh@...nel.org,
        tomeu.vizoso@...labora.com, airlied@...ux.ie,
        linux-kernel@...r.kernel.org, steven.price@....com,
        linux-rockchip@...ts.infradead.org, wens@...e.org,
        alyssa.rosenzweig@...labora.com, daniel@...ll.ch,
        linux-amlogic@...ts.infradead.org
Subject: Re: [RFC v2 1/1] drm/lima: Add optional devfreq support

On 2019-12-29 11:19 pm, Martin Blumenstingl wrote:
> Hi Robin,
> 
> On Sun, Dec 29, 2019 at 11:58 PM Robin Murphy <robin.murphy@....com> wrote:
>>
>> Hi Martin,
>>
>> On 2019-12-27 5:37 pm, Martin Blumenstingl wrote:
>>> Most platforms with a Mali-400 or Mali-450 GPU also have support for
>>> changing the GPU clock frequency. Add devfreq support so the GPU clock
>>> rate is updated based on the actual GPU usage when the
>>> "operating-points-v2" property is present in the board.dts.
>>>
>>> The actual devfreq code is taken from panfrost_devfreq.c and modified so
>>> it matches what the lima hardware needs:
>>> - a call to dev_pm_opp_set_clkname() during initialization because there
>>>     are two clocks on Mali-4x0 IPs. "core" is the one that actually clocks
>>>     the GPU so we need to control it using devfreq.
>>> - locking when reading or writing the devfreq statistics because (unlike
>>>     than panfrost) we have multiple PP and GP IRQs which may finish jobs
>>>     concurrently.
>>
>> I gave this a quick try on my RK3328, and the clock scaling indeed kicks
>> in nicely on the glmark2 scenes that struggle, however something appears
>> to be missing in terms of regulator association, as the appropriate OPP
>> voltages aren't reflected in the GPU supply (fortunately the initial
>> voltage seems close enough to that of the highest OPP not to cause major
>> problems, on my box at least). With panfrost on RK3399 I do see the
>> supply voltage scaling accordingly, but I don't know my way around
>> devfreq well enough to know what matters in the difference :/
> first of all: thank you for trying this out! :-)
> 
> does your kernel include commit 221bc77914cbcc ("drm/panfrost: Use
> generic code for devfreq") for your panfrost test?
> if I understand the devfreq API correct then I suspect with that
> commit panfrost also won't change the voltage anymore.

Oh, you're quite right - I was already considering that change as 
ancient history, but indeed it's only in 5.5-rc, while that board is 
still on 5.4.y release kernels. No wonder I couldn't make sense of how 
the (current) code could possibly be working :)

I'll try the latest -rc kernel tomorrow to confirm (now that PCIe is 
hopefully fixed), but I'm already fairly confident you've called it 
correctly.

Cheers,
Robin.

> this is probably due to a missing call to dev_pm_opp_set_regulators()
> which is supposed to attach the regulator to the devfreq instance.
> I didn't notice this yet because on Amlogic SoCs the voltage is the
> same for all OPPs.
> 
> I'll debug this in the next days and send an updated patch (and drop
> the RFC prefix if there are no more comments).
> 
> 
> Regards
> Martin
>