linux-kernel - Re: [Intel-gfx] [PATCH v8 1/3] gpu: drm: separate panel orientation property creating and value setting

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Message-ID: <CACvgo532-pC+7DLFCo=DWTX-OnJEJvSoTmQnt3_qLhiT4cqEMg@mail.gmail.com>
Date:   Tue, 15 Feb 2022 14:38:45 +0000
From:   Emil Velikov <emil.l.velikov@...il.com>
To:     Simon Ser <contact@...rsion.fr>
Cc:     Hsin-Yi Wang <hsinyi@...omium.org>,
        ML dri-devel <dri-devel@...ts.freedesktop.org>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        amd-gfx mailing list <amd-gfx@...ts.freedesktop.org>,
        Intel Graphics Development <intel-gfx@...ts.freedesktop.org>,
        Chun-Kuang Hu <chunkuang.hu@...nel.org>,
        devicetree <devicetree@...r.kernel.org>,
        "Linux-Kernel@...r. Kernel. Org" <linux-kernel@...r.kernel.org>,
        Maxime Ripard <mripard@...nel.org>,
        Alex Deucher <alexander.deucher@....com>,
        Rob Herring <robh+dt@...nel.org>,
        linux-mediatek@...ts.infradead.org,
        Thomas Zimmermann <tzimmermann@...e.de>,
        Harry Wentland <harry.wentland@....com>,
        Matthias Brugger <matthias.bgg@...il.com>,
        LAKML <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [Intel-gfx] [PATCH v8 1/3] gpu: drm: separate panel orientation
 property creating and value setting

On Tue, 15 Feb 2022 at 13:55, Simon Ser <contact@...rsion.fr> wrote:
>
> On Tuesday, February 15th, 2022 at 13:04, Emil Velikov <emil.l.velikov@...il.com> wrote:
>
> > Greetings everyone,
> >
> > Padron for joining in so late o/
> >
> > On Tue, 8 Feb 2022 at 08:42, Hsin-Yi Wang <hsinyi@...omium.org> wrote:
> > >
> > > drm_dev_register() sets connector->registration_state to
> > > DRM_CONNECTOR_REGISTERED and dev->registered to true. If
> > > drm_connector_set_panel_orientation() is first called after
> > > drm_dev_register(), it will fail several checks and results in following
> > > warning.
> > >
> > > Add a function to create panel orientation property and set default value
> > > to UNKNOWN, so drivers can call this function to init the property earlier
> > > , and let the panel set the real value later.
> > >
> >
> > The warning illustrates a genuine race condition, where userspace will
> > read the old/invalid property value/state. So this patch masks away
> > the WARNING without addressing the actual issue.
> > Instead can we fix the respective drivers, so that no properties are
> > created after drm_dev_register()?
> >
> > Longer version:
> > As we look into drm_dev_register() it's in charge of creating the
> > dev/sysfs nodes (et al). Note that connectors cannot disappear at
> > runtime.
> > For panel orientation, we are creating an immutable connector
> > properly, meaning that as soon as drm_dev_register() is called we must
> > ensure that the property is available (if applicable) and set to the
> > correct value.
>
> Unfortunately we can't quite do this. To apply the panel orientation quirks we
> need to grab the EDID of the eDP connector, and this happened too late in my
> testing.
>
> What we can do is create the prop early during module load, and update it when
> we read the EDID (at the place where we create it right now). User-space will
> receive a hotplug event after the EDID is read, so will be able to pick up the
> new value if any.

Didn't quite get that, are you saying that a GETPROPERTY for the EDID,
the ioctl blocks or that we get an empty EDID?

The EDID hotplug even thing is neat - sounds like it also signals on
panel orientation, correct?
On such an event, which properties userspace should be re-fetching -
everything or guess randomly?

Looking through the documentation, I cannot see a clear answer :-\

Thanks
Emil