Weston: Single CRTC cloning on multiple connectors. How?

[Pekka Paalanen]

On Mon, 29 Mar 2021 15:00:08 +0000
"Sebert, Holger.ext" <Holger.Sebert.ext at karlstorz.com> wrote:

> Hi Pekka,
> 
> thanks for pointing me to "drm_info".
> 
> I built and installed it and indeed, all Encoder entries look
> like this:
> 
> ...
> ├───Encoders
> │   ├───Encoder 0
> │   │   ├───Object ID: 94
> │   │   ├───Type: TMDS
> │   │   ├───CRTCS: {0, 1, 2}
> │   │   └───Clones: {0}
> ...
> 
> where the index ranges from 0 to 9.
> 
> Does that mean that the kernel doesn't support cloning displays?

If the Clones line only ever lists a single index (the connector's own
index), then indeed, the driver says that this does not support
shared-CRTC cloning.

> The system is x86_64 with Intel UHD Graphics 630. I cannot image
> that such consumer grade hardware does not support such a standard
> use-case as display cloning.

I would think the opposite. On consumer PC hardware of today's era,
shared-CRTC cloning is very rare. You may have more luck with some
embedded boards.

As Simon said, shared-CRTC mode is not the only way to clone outputs. I
would think that cloning with independent CRTCs is much more common,
and also does not require explicit hardware support.

However, Weston does not support independent-CRTC clone mode for the
silly reason that its internal damage region tracking breaks with
overlapping outputs. That would lead to partially outdated content on
random outputs.

> 
> Is there maybe some Kernel option that I need to enable?
> 
> For testing, I installed Ubuntu 2020.04 and there display cloning
> worked perfectly (but on Xorg, not Wayland).

Xorg is not as limited as Weston with its cloning options. It can do
independent-CRTC cloning just fine.

I bet GNOME Wayland can do cloning too. It's really just Weston
that doesn't implement "the generic" way of cloning.


Thanks,
pq
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