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Date:   Tue, 15 Feb 2022 16:37:19 +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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References: <20220208084234.1684930-1-hsinyi@chromium.org> <CACvgo53u01BK_D0ZssV+gCepjxSz23Nr5Dy1qXeaAoJuu6VCFQ@mail.gmail.com> <KW6DNh6IRRgVJx9DfOFBnEqc4a0x-AnDXEbMxwpfEbk8dOn_KGVzAfo-slJWq-4nWW728Uc-OVpFh2w4fDE4-bxfkDuz1hFILRVvbcuXqaw=@emersion.fr> <CACvgo532-pC+7DLFCo=DWTX-OnJEJvSoTmQnt3_qLhiT4cqEMg@mail.gmail.com>
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On Tuesday, February 15th, 2022 at 15:38, Emil Velikov <emil.l.velikov@gmai=
l.com> wrote:

> On Tue, 15 Feb 2022 at 13:55, Simon Ser <contact@emersion.fr> wrote:
> >
> > On Tuesday, February 15th, 2022 at 13:04, Emil Velikov <emil.l.velikov@=
gmail.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 foll=
owing
> > > > 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 wil=
l
> > > 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 mus=
t
> > > 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 qu=
irks 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-spac=
e 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?

I'm not referring to GETPROPERTY, I'm referring to the driver getting the E=
DID
from the sink (here, the eDP panel). In my experimentations with amdgpu I
noticed that the driver module load finished before the EDID was available =
to
the driver. Maybe other drivers behave differently and probe connectors whe=
n
loaded, not sure.

> 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 :-\

User-space should re-fetch *all* properties. In practice some user-space ma=
y
only be fetching some properties, but that should get fixed in user-space.

Also the kernel can indicate that only a single connector changed via the
"CONNECTOR" uevent prop, or even a single connector property via "PROPERTY"=
.
See [1] for a user-space implementation. But all of this is purely an optio=
nal
optimization. Re-fetching all properties is a bit slower (especially if som=
e
drmModeGetConnector calls force-probe connectors) but works perfectly fine.

It would be nice to document, if you have the time feel free to send a patc=
h
and CC danvet, pq and me.

[1]: https://gitlab.freedesktop.org/wlroots/wlroots/-/blob/252b2348bd62170d=
97c4e81fb2050f757b56d67e/backend/session/session.c#L144