Re: [PATCH v6 3/4] drm/panfrost: devfreq: Disable devfreq when num_supplies > 1

From: Nicolas Boichat <drinkcat@chromium.org>
To: Steven Price <steven.price@arm.com>
Cc: Rob Herring <robh@kernel.org>,
	Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>,
	Hsin-Yi Wang <hsinyi@chromium.org>,
	Kristian Kristensen <hoegsberg@chromium.org>,
	Fei Shao <fshao@chromium.org>,
	Boris Brezillon <boris.brezillon@collabora.com>,
	Daniel Vetter <daniel@ffwll.ch>, David Airlie <airlied@linux.ie>,
	Tomeu Vizoso <tomeu.vizoso@collabora.com>,
	dri-devel <dri-devel@lists.freedesktop.org>,
	lkml <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v6 3/4] drm/panfrost: devfreq: Disable devfreq when num_supplies > 1
Date: Fri, 8 Jan 2021 07:51:29 +0800	[thread overview]
Message-ID: <CANMq1KCbA9FawgtK4+rjBKO=dFomrAt7kpN8K_180mHRAeUkAg@mail.gmail.com> (raw)
In-Reply-To: <a373044b-3e24-4e00-432f-46b0bd91b213@arm.com>

On Thu, Jan 7, 2021 at 11:59 PM Steven Price <steven.price@arm.com> wrote:
>
> On 05/01/2021 00:11, Nicolas Boichat wrote:
> > GPUs with more than a single regulator (e.g. G-57 on MT8183) will
> > require platform-specific handling, disable devfreq for now.
>
> Can you explain what actually goes wrong here? AFAICT the existing code
> does support controlling multiple regulators - but clearly this is the
> first platform that exercises that code with num_supplies>1.

Sure, I should have expanded in the commit message, will do in v9.

We have support for >1 supplies, and we need to enable them to get the
GPU running _at all_ (and the default voltages should be safe by
design).

For devfreq though:
 1. There are constraints on the voltage difference between the core
and SRAM, we have this caterpillar logic downstream [1], so somebody
will need to port it (TBH I don't think it's overly critical at this
point, as Bifrost support is still not very mature from what I can
see, and devfreq is purely a performance thing).
 2. The core [2] does not support multiple regulators, so we'll need
custom code anyway. Even if we didn't have constraints.

[1] https://chromium.googlesource.com/chromiumos/third_party/kernel/+/refs/heads/chromeos-4.19/drivers/gpu/arm/bifrost/platform/mediatek/mali_kbase_runtime_pm.c#367
[2] https://elixir.bootlin.com/linux/latest/source/drivers/opp/core.c#L679

>
> Steve
>
> >
> > Signed-off-by: Nicolas Boichat <drinkcat@chromium.org>
> > ---
> >
> > Changes in v6:
> >   - New change
> >
> >   drivers/gpu/drm/panfrost/panfrost_devfreq.c | 9 +++++++++
> >   1 file changed, 9 insertions(+)
> >
> > diff --git a/drivers/gpu/drm/panfrost/panfrost_devfreq.c b/drivers/gpu/drm/panfrost/panfrost_devfreq.c
> > index f44d28fad085..1f49043aae73 100644
> > --- a/drivers/gpu/drm/panfrost/panfrost_devfreq.c
> > +++ b/drivers/gpu/drm/panfrost/panfrost_devfreq.c
> > @@ -92,6 +92,15 @@ int panfrost_devfreq_init(struct panfrost_device *pfdev)
> >       struct thermal_cooling_device *cooling;
> >       struct panfrost_devfreq *pfdevfreq = &pfdev->pfdevfreq;
> >
> > +     if (pfdev->comp->num_supplies > 1) {
> > +             /*
> > +              * GPUs with more than 1 supply require platform-specific handling:
> > +              * continue without devfreq
> > +              */
> > +             DRM_DEV_ERROR(dev, "More than 1 supply is not supported yet\n");
> > +             return 0;
> > +     }
> > +
> >       opp_table = dev_pm_opp_set_regulators(dev, pfdev->comp->supply_names,
> >                                             pfdev->comp->num_supplies);
> >       if (IS_ERR(opp_table)) {
> >
>