[Bug 796525] when gst-rtsp-server is used in conjunction with v4l2h264enc on an imx6, nobody allocates downstream buffers

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=796525

--- Comment #13 from Michael Tretter <m.tretter at pengutronix.de> ---
(In reply to Nicolas Dufresne (ndufresne) from comment #11)
> Review of attachment 372813 [details] [review]:
> 
> Renegotiation should work, it's just a bug. All bits are in place, no need
> for hacks.

Yes, there shouldn't be the need for the hack and it wasn't meant for inclusion
into master. The patch only helps to track down the actual bug.

During renegotiation the encoder element calls set_format for capture. Since
v4l2 is currently in streaming mode, set_format fails to change the format and
the entire renegotiation fails.

I think during renegotiation we should (1) stop streaming, (2) set the format
and (3) start streaming with the new format. This is currently not implemented,
but this should fix renegotiation for the common case.

The patch that I submitted assumes that the caps are the same as prior to the
renegotiation event (there is no check if this is actually true). If we detect
this condition, we can skip the set_format and just continue the renegotiation.
However, I wasn't able to detect this case in v4l2videoenc, and I don't know if
this should be handled in the videoencoder.

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