libv4l2 error when dynamically adding to playing pipeline

[Anthony Buckley]

I have a C/GTK program that creates the following pipeline:-

| Camera |    | Video |   | Caps |    | Queue |   | Video    |   | Video |
| v4l2src  |->| Rate   |->| Filter  |->| (blk)     |->| convert |->| sink
   |-> Screen

which gets changed dynamically, adding 2 elements, when a menu option is
selected (there is also a reversing option) into this:-

| Camera |  | Video |  | Caps |    | Queue |   | Video   |   | Cairo    |
| Video   |   | Video |
| v4l2src |->| Rat   |->| Filter |-> | (blk)    |->| convert |->| overlay
|->| convert |->| sink  |-> Screen

It works anywhere from 0 to n times before the stream will freeze sometimes
with the following error (GST_DEBUG=1).

libv4l2: error turning on stream: Invalid argument
0:00:07.395996377 3849 0x8856150 ERROR v4l2
gstv4l2bufferpool.c:566:gst_v4l2_buffer_pool_streamon:<v4l2:pool:src> error
with STREAMON 22 (Invalid argument)

The approach I take when the menu option is selected is:-

probe_id = gst_pad_add_probe (blockpad,
GST_PAD_PROBE_TYPE_BLOCK_DOWNSTREAM,
                                              OnPadProbe, user_data pointer
, NULL);

and the OnPadProbe routine:-
  unlinks the elements in the pipeline
  creates the 'cairooverlay' element and the second 'videoconvert' adapter
  sets up callbacks
  adds the new elements to the pipeline, links them and sets their state to
Playing
  return GST_PAD_PROBE_REMOVE;

The cairooverlay Callbacks for 'caps-changed' and 'draw' are pretty
straightforward.

Is this a valid approach for changing a pipeline on the fly? I've used
debug code and found that the pad is still blocked and that the pipeline is
still in playing state after the video freezes, but I'm unsure what to try
next.

I've found that there is no problem when I switch video sinks from
'xvimagesink' to 'ximagesink', but no idea why.

I've also wondered if there is much penalty in just including the 2
additional elements anyway and connecting / disconnecting the Callbacks as
required.

Any advice or pointers would be gladly received.

Regards.

Tony
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