[Bug 779466] v4l2: improve v4l2dec auto-negotiation

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=779466

--- Comment #13 from Michael Tretter <m.tretter at pengutronix.de> ---
(In reply to Nicolas Dufresne (stormer) from comment #11)
> See:
> GST_V4L2_ALLOCATOR_CAN_REQUEST()
> GST_V4L2_ALLOCATOR_CAN_ALLOCATE()
> 
> Obviously you need to create the allocator a bit earlier, but you'll need it
> anyway.

If I understand correctly, the allocator should already know, if the driver
supports dmabuf, but currently the mode of the v4l2object (capture) is not
updated. Therefore, it doesn't use GST_V4L2_IO_DMABUF, but always falls back to
GST_V4L2_IO_MMAP.

The better solution would be to update the req_mode after probing the allocator
and switch to GST_V4L2_IO_DMABUF if supported. At least it should solve my
issue to automatically select GST_V4L2_IO_DMABUF if the device supports it and
I do not specify it as property.

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