If the v4l2videoenc receives an QUERY_ALLOCATION, it must not propose a
currently used pool, because it cannot be sure that the allocation query came
from exactly the same upstream element. The QUERY_ALLOCATION will not contain
the internal OUTPUT pool.
The upstream element (the basesrc) detects that the newly proposed pool differs
from the old pool. It deactivates the old pool and switches to the new pool.
If there was a format change, a new OUTPUT buffer pool will be allocated in
gst_v4l2_object_set_format_full() and the CAPTURE task will be stopped to switch
the format. If there hasn't been a format change,
gst_v4l2_object_set_format_full() will not be called. The old pool will be kept
and reused.
Without a format change, the processing task continues running.
This leads to the situation that the processing task is running, but the OUTPUT
buffer pool (the old pool) is deactivated. Therefore, the encoder is not able to
get buffers from the OUTPUT pool and encoding cannot continue.
This situation can be triggered by sending a RECONFIGURE event without a format
change.
Resolve this situation by ensuring that the OUTPUT buffer pool is always
activated when frames arrive at the encoder.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/4235>
v4l2 plugins
============
The idea is a bit the same as the idea for the v4l1 plugins. We want
one generic v4l2element, and a few child objects (probably only two:
v4l2src and v4l2sink):
/-------- v4l2src
v4l2element ---=
\-------- v4l2sink
Both v4l2src and v4l2sink have a uncompressed and a compressed
recording-/playback-mode. Since this is all part of v4l2, the 'client'
of these elements, i.e. an application using v4l2src/v4l2sink, will
hardly notice this. All capsnego stuff is done inside, and the plugin
knows which formats are compressed and which are not.
Please note that the v4l1 and the v4l2 plugins are *not* compatible
concerning properties. Naming has been kept the same where possible,
but in some cases, properties had to be removed or added to make
full use of v4l2.
V4L2 API: http://linux.bytesex.org/v4l2/.
http://v4l2spec.bytesex.org/
/usr/include/linux/videodev2.h or
Kernel patches available from
http://dl.bytesex.org/patches/.
Articles:
http://lwn.net/Articles/203924/
0563a25494