Date:   Fri, 18 Feb 2022 12:12:52 +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>,
        Daniel Vetter <daniel.vetter@...ll.ch>
Subject: Re: [Intel-gfx] [PATCH v8 1/3] gpu: drm: separate panel orientation property creating and value setting

On Friday, February 18th, 2022 at 12:54, Hans de Goede <hdegoede@...hat.com> wrote:

> On 2/18/22 12:39, Simon Ser wrote:
> > 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.
>
> If before the initial probe of the connector there is a /dev/dri/card0
> which plymouth can access, then plymouth may at this point decide
> to disable any seemingly unused crtcs, which will make the screen go black...
>
> I'm not sure if plymouth will actually do this, but AFAICT this would
> not be invalid behavior for a userspace kms consumer to do and I
> believe it is likely that mutter will disable unused crtcs.
>
> IMHO it is just a bad idea to register /dev/dri/card0 with userspace
> before the initial connector probe is done. Nothing good can come
> of that.
>
> If all the exposed connectors initially are going to show up as
> disconnected anyways what is the value in registering /dev/dri/card0
> with userspace early ?

OK. I'm still unsure how I feel about this, but I think I agree with
you. That said, the amdgpu architecture is quite involved with multiple
abstraction levels, so I don't think I'm equipped to write a patch to
fix this...

cc Daniel Vetter: can you confirm probing all connectors is a good thing
to do on driver module load?

> >> 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.
>
> The DMI based quirking does, yes. But e.g. the quirk code directly
> reading this from the Intel VBT does not rely on the mode.
>
> But if you are planning on using a DMI based quirk for the steamdeck
> then yes that needs the mode.
>
> Thee mode check is there for 2 reasons:
>
> 1. To avoid also applying the quirk to external displays, but
> I think that that is also solved in most drivers by only checking for
> a quirk at all on the eDP connector
>
> 2. Some laptop models ship with different panels in different badges
> some of these are portrait (so need a panel-orient) setting and others
> are landscape.

That makes sense. So yeah the EDID mode based matching logic needs to
stay to accomodate for these cases.

> > 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.
>
> We could extend the DMI quirk mechanism to allow quirks which don't
> do the mode check, for use on devices where we can guarantee neither
> 1 nor 2 happens, then amdgpu could call the quirk code early simply
> passing 0x0 as resolution.

Yeah. But per the above amdgpu should maybe probe connectors on module
load. If/when amdgpu is fixed to do this, then we don't need to disable
the mode matching logic in panel-orientation quirks anymore.

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