linux-kernel - Re: [PATCH v2 4/7] drm/panfrost: Add support for a second regulator for the GPU

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Date:   Thu, 9 Jan 2020 06:52:32 +0800
From:   Nicolas Boichat <drinkcat@...omium.org>
To:     Mark Brown <broonie@...nel.org>
Cc:     Rob Herring <robh+dt@...nel.org>, David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Mark Rutland <mark.rutland@....com>,
        Matthias Brugger <matthias.bgg@...il.com>,
        Tomeu Vizoso <tomeu.vizoso@...labora.com>,
        Steven Price <steven.price@....com>,
        Alyssa Rosenzweig <alyssa.rosenzweig@...labora.com>,
        Liam Girdwood <lgirdwood@...il.com>,
        dri-devel@...ts.freedesktop.org,
        Devicetree List <devicetree@...r.kernel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        linux-arm Mailing List <linux-arm-kernel@...ts.infradead.org>,
        "moderated list:ARM/Mediatek SoC support" 
        <linux-mediatek@...ts.infradead.org>,
        Hsin-Yi Wang <hsinyi@...omium.org>
Subject: Re: [PATCH v2 4/7] drm/panfrost: Add support for a second regulator
 for the GPU

On Wed, Jan 8, 2020 at 9:23 PM Mark Brown <broonie@...nel.org> wrote:
>
> On Wed, Jan 08, 2020 at 01:23:34PM +0800, Nicolas Boichat wrote:
>
> > Some GPUs, namely, the bifrost/g72 part on MT8183, have a second
> > regulator for their SRAM, let's add support for that.
>
> > +     pfdev->regulator_sram = devm_regulator_get_optional(pfdev->dev, "sram");
> > +     if (IS_ERR(pfdev->regulator_sram)) {
>
> This supply is required for the devices that need it so I'd therefore
> expect the driver to request the supply non-optionally based on the
> compatible string rather than just hoping that a missing regulator isn't
> important.

That'd be a bit awkward to match, though... Currently all bifrost
share the same compatible "arm,mali-bifrost", and it'd seem
weird/wrong to match "mediatek,mt8183-mali" in this driver? I have no
idea if any other Mali implementation will require a second regulator,
but with the MT8183 we do need it, see below.

> Though I do have to wonder given the lack of any active
> management of the supply if this is *really* part of the GPU or if it's
> more of a SoC thing, it's not clear what exactly adding this code is
> achieving.

Well if devfreq was working (see patch 7
https://patchwork.kernel.org/patch/11322851/ for a partial
implementation), it would adjust both mali and sram regulators, see
the OPP table in patch 2
(https://patchwork.kernel.org/patch/11322825/): SRAM voltage needs to
be increased for frequencies >=698Mhz.

Now if you have some better idea how to implement this, I'm all ears!

Thanks.