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: <YV87l-2XXzmZ2i6GuGd__uf85s2JQkbXt_qTQDYlbBh1kW2COJoNLUDinmLCokKyy-_0ZnIMUcZeW0GdJ8zF690iYi4ThKrjc09omNMe-0g=@emersion.fr>
Date:   Fri, 18 Feb 2022 11:39:55 +0000
From:   Simon Ser <contact@...rsion.fr>
To:     Hans de Goede <hdegoede@...hat.com>
Cc:     Emil Velikov <emil.l.velikov@...il.com>,
        Maxime Ripard <mripard@...nel.org>,
        Chun-Kuang Hu <chunkuang.hu@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        devicetree <devicetree@...r.kernel.org>,
        David Airlie <airlied@...ux.ie>,
        Intel Graphics Development <intel-gfx@...ts.freedesktop.org>,
        "Linux-Kernel@...r. Kernel. Org" <linux-kernel@...r.kernel.org>,
        amd-gfx mailing list <amd-gfx@...ts.freedesktop.org>,
        Matthias Brugger <matthias.bgg@...il.com>,
        Rob Herring <robh+dt@...nel.org>,
        linux-mediatek@...ts.infradead.org,
        ML dri-devel <dri-devel@...ts.freedesktop.org>,
        Hsin-Yi Wang <hsinyi@...omium.org>,
        Alex Deucher <alexander.deucher@....com>,
        Harry Wentland <harry.wentland@....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 Friday, February 18th, 2022 at 11:38, Hans de Goede <hdegoede@...hat.com> wrote:

> What I'm reading in the above is that it is being considered to allow
> changing the panel-orientation value after the connector has been made
> available to userspace; and let userspace know about this through a uevent.
>
> I believe that this is a bad idea, it is important to keep in mind here
> what userspace (e.g. plymouth) uses this prorty for. This property is
> used to rotate the image being rendered / shown on the framebuffer to
> adjust for the panel orientation.
>
> So now lets assume we apply the correct upside-down orientation later
> on a device with an upside-down mounted LCD panel. Then on boot the
> following could happen:
>
> 1. amdgpu exports a connector for the LCD panel to userspace without
> setting panel-orient=upside-down
> 2. plymouth sees this and renders its splash normally, but since the
> panel is upside-down it will now actually show upside-down

At this point amdgpu hasn't probed the connector yet. So the connector
will be marked as disconnected, and plymouth shouldn't render anything.

> 3. amdgpu adjusts the panel-orient prop to upside-down, sends out
> uevents

That's when amdgpu marks the connector as connected. So everything
should be fine I believe, no bad frame.

> 4. Lets assume plymouth handles this well (i) and now adjust its
> rendering and renders the next frame of the bootsplash 180° rotated
> to compensate for the panel being upside down. Then from now on
> the user will see the splash normally
>
> So this means that the user will briefly see the bootsplash rendered
> upside down which IMHO is not acceptable behavior. Also see my footnote
> about how I seriously doubt plymouth will see the panel-orient change
> at all.
>
> I'm also a bit unsure about:
>
> a) How you can register the panel connector with userspace before
> reading the edid, don't you need the edid to give the physical size +
> modeline to userspace, which you cannot just leave out ?

Yup. The KMS EDID property is created before the EDID is read, and is set
to zero (NULL blob). The width/height in mm and other info are also zero.
You can try inspecting the state printed by drm_info on any disconnected
connector to see for yourself.

> I guess the initial modeline is inherited from the video-bios, but
> what about the physical size? Note that you cannot just change the
> physical size later either, that gets used to determine the hidpi
> scaling factor in the bootsplash, and changing that after the initial
> bootsplash dislay will also look ugly
>
> b) Why you need the edid for the panel-orientation property at all,
> typically the edid prom is part of the panel and the panel does not
> know that it is mounted e.g. upside down at all, that is a property
> of the system as a whole not of the panel as a standalone unit so
> in my experience getting panel-orient info is something which comes
> from the firmware /video-bios not from edid ?

This is an internal DRM thing. The orientation quirks logic uses the
mode size advertised by the EDID. I agree that at least in the Steam
Deck case it may not make a lot of sense to use any info from the
EDID, but that's needed for the current status quo.

Also note, DisplayID has a bit to indicate the panel orientation IIRC.
Would be nice to support parsing this at some point.