memory:DMABuf usage with waylandsink

[Damian Hobson-Garcia]

Hello All,

I am working on a plugin for H/W that can output video buffers as
dmabufs for display on a wayland compositor via waylandsink.
It looks like the colour formats supported by the compositor are
reported by waylandsink in 2 separate caps structures, one for SHM
buffers and one for DMABUFs. The DMABUF structure sets the
GST_CAPS_FEATURE_MEMORY_DMABUF caps feature while the SHM one doesn't.

According to the gstdmabuf.h header (plugins-base), though, this should
only be set for non-mappable DMABUF, but since my DMABUFs *are*
mappable, it seems like I shouldn't be setting this flag.  Without it,
though only the formats supported for SHM buffers can be negotiated.  Of
course, its easy enough to just lie and set the flag, but I'm wondering
if there is a more correct solution.

In general, what should upstream plugins be doing to negotiate the
DMABUF formats with waylandsink?  I've tried to find some examples in
other plugins, but grep hasn't given me much to go on.

Thank you,
Damian