How to obtain a drm lease from X for overlay planes as well as a primary plane?

[Simon Ser]

On Wednesday, August 11th, 2021 at 13:40, John Cox <jc at kynesim.co.uk> wrote:

> Raspberry Pi displaying video with subtitles or other controls.  I was
> thinking of the fullscreen case but if zero copy video can be made to
> work to the main desktop then that would even better.
>
> If displaying 4k video the Pi does not have enough bandwidth left for a
> single frame copy, convert or merge so I need hardware scaling,
> composition & display taking the raw video frame (its in a dmabuf).  The
> raw video is in a somewhat unique format, I'd expect the other layers to
> be ARGB.  The Pi h/w can do this and I believe I can make it work via
> DRM if I own the screen so that was where I started.
>
> >Why not use an xdg_toplevel and wl_subsurface?
>
> Probably because I am woefully underinformed about how I should be doing
> stuff properly.  Please feel free to point me in the correct direction -
> any example that takes NV12 video (it isn't NV12 but if NV12 works then
> SAND can probably be made to too) would be a great start.  Also Wayland
> hasn't yet come to the Pi though it will shortly be using mutter.

By SAND do you mean one of these vc4-specific buffer tilings [1]? e.g.
BROADCOM_SAND64, SAND128 or SAND256?

[1]: https://drmdb.emersion.fr/formats?driver=vc4

The fullscreen case may work already on all major Wayland compositors,
assuming the video size matches exactly the current mode. You'll need to use
the linux-dmabuf Wayland extension to pass NV12 buffers to the compositor.

If you want to add scaling into the mix, you'll need to use the viewporter
extension as well. Most compositors aren't yet rigged up for direct scan-out,
they'll fall back to composition. Weston is your best bet if you want to try
this, it supports direct scan-out to multiple KMS planes with scaling and
cropping. There is some active work in wlroots to support this.  I'm not aware
of any effort in this direction for mutter or kwin at the time of writing.

If you want to also use KMS planes with other layers (RGBA or something else),
then you'll need to setup wl_subsurfaces with the rest of the content. As said
above, Weston will do its best to offload the composition work to KMS planes.
You'll need to make sure each buffer you submit can be scanned out by the
display engine -- there's not yet a generic way of doing it, but the upcoming
linux-dmabuf hints protocol will fix that.

If you want to get started, maybe have a look at clients/simple-dmabuf-gbm in
Weston.

Hope this helps!