[Bug 711155] wayland: add wl_drm support to wayland sink

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=711155

--- Comment #59 from Nicolas Dufresne (stormer) <nicolas.dufresne at collabora.co.uk> ---
It's not because we thought it was fine before that it's fine now (there has
been more then a year since we started this discussion). If an allocator has no
->alloc it can only be part of the proposed allocation if there is a negotiated
caps feature to back it. GL allocator is only offered in the case where it can
do memory of type SystemMemory (the GL Upload slow path). For EGLImage, it is
backed with caps feature.

Again, cause I'm really repeating myself, if you have an allocator (or a pool
using an internal allocator) producing mappable DMABUF like V4L2 and the
proposed DRM Dumb Buffers allocator, it can be proposed with a
memory:SystemMemory caps feature. Hence, no, videotestsrc ! waylandsink would
not need anything special if waylandsink can allocate memory of type
SystemMemory. That would depend on having Dumb Buffer support first, which we
don't have in upstream GStreamer, maybe your focus should be on offering
mergeable patches (against git master) for this allocator ?

Why we need an ->alloc is simple. If we proposed an allocator to videotestsrc,
it will do:

  * Ok I got an allocator but no pool
  * Ok, let's create a videopool and pass it the downstream allocator
  * Much later ... allocate fails, pipeline fails.

This is not acceptable.

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