[Bug 740633] v4l2src: RW io-mode is broken

[GStreamer (bugzilla.gnome.org)]

https://bugzilla.gnome.org/show_bug.cgi?id=740633
  GStreamer | gst-plugins-good | 1.4.4

--- Comment #1 from Nicolas Dufresne (stormer) <nicolas.dufresne at collabora.co.uk> 2014-12-07 22:44:50 UTC ---
I can fix the RW mode, but libv4l2 RW emulation can't ever work with poll()
without hacks. There is no solution to that. Basically, libv4l2 won't know it's
doing READ/WRITE before the first call to read() is made. Because it's an
emulation of read/write over MMAP, only on the first read() it will setup the
MMAP queue and streamon. Before that, poll() will fail, but we need to call
poll() before read.

Never the less, I'll add a hack that call v4l2_read(fd, NULL, 0) in bufferpool
start if we have lib4l2 and the device is a streaming device (mmap). That will
cause the queue to be initialized, after what poll will work normally. I'm
including it since it's the only way I have to frequently test that RW isn't
broken again.

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