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Date:   Tue, 15 Feb 2022 13:55:34 +0000
To:     Emil Velikov <emil.l.velikov@gmail.com>
From:   Simon Ser <contact@emersion.fr>
Cc:     Hsin-Yi Wang <hsinyi@chromium.org>,
        ML dri-devel <dri-devel@lists.freedesktop.org>,
        David Airlie <airlied@linux.ie>,
        Daniel Vetter <daniel@ffwll.ch>,
        amd-gfx mailing list <amd-gfx@lists.freedesktop.org>,
        Intel Graphics Development <intel-gfx@lists.freedesktop.org>,
        Chun-Kuang Hu <chunkuang.hu@kernel.org>,
        devicetree <devicetree@vger.kernel.org>,
        "Linux-Kernel@Vger. Kernel. Org" <linux-kernel@vger.kernel.org>,
        Maxime Ripard <mripard@kernel.org>,
        Alex Deucher <alexander.deucher@amd.com>,
        Rob Herring <robh+dt@kernel.org>,
        linux-mediatek@lists.infradead.org,
        Thomas Zimmermann <tzimmermann@suse.de>,
        Harry Wentland <harry.wentland@amd.com>,
        Matthias Brugger <matthias.bgg@gmail.com>,
        LAKML <linux-arm-kernel@lists.infradead.org>
Reply-To: Simon Ser <contact@emersion.fr>
Subject: Re: [Intel-gfx] [PATCH v8 1/3] gpu: drm: separate panel orientation property creating and value setting
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On Tuesday, February 15th, 2022 at 13:04, Emil Velikov <emil.l.velikov@gmai=
l.com> wrote:

> Greetings everyone,
>
> Padron for joining in so late o/
>
> On Tue, 8 Feb 2022 at 08:42, Hsin-Yi Wang <hsinyi@chromium.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 followin=
g
> > warning.
> >
> > Add a function to create panel orientation property and set default val=
ue
> > to UNKNOWN, so drivers can call this function to init the property earl=
ier
> > , 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 m=
y
testing.

What we can do is create the prop early during module load, and update it w=
hen
we read the EDID (at the place where we create it right now). User-space wi=
ll
receive a hotplug event after the EDID is read, so will be able to pick up =
the
new value if any.