videobuf: Avoid deadlock with QBUF and bring up to spec for empty queue

Add a waitqueue to wait on when there are no buffers in the buffer queue.
DQBUF waits on this queue without holding vb_lock to allow a QBUF to happen.
Once a buffer has been queued we recheck that the queue is still streaming and
wait on the new buffer's waitqueue while holding the vb_lock.  The driver
should come along in a timely manner and put the buffer into its next state
finishing the DQBUF.

By implementing this waitqueue it also brings the videobuf DQBUF up to spec and
it now blocks on O_NONBLOCK even when no buffers have been queued via QBUF:

"By default VIDIOC_DQBUF blocks when no buffer is in the outgoing queue."
 - V4L2 spec

CC: Trent Piepho <xyzzy@speakeasy.org>
CC: Carl Karsten <carl@personnelware.com>
CC: Jonathan Corbet <corbet@lwn.net>

Signed-off-by: Brandon Philips <bphilips@suse.de>

---
 linux/drivers/media/video/videobuf-core.c |   96 +++++++++++++++++++++++-------
 linux/include/media/videobuf-core.h       |    2
 2 files changed, 78 insertions(+), 20 deletions(-)

1 files changed, 2 insertions, 0 deletions

diff --git a/linux/include/media/videobuf-core.h b/linux/include/media/videobuf-core.h
index 7c80e8adf..18b0ba615 100644
--- a/linux/include/media/videobuf-core.h
+++ b/linux/include/media/videobuf-core.h
@@ -159,6 +159,8 @@ struct videobuf_queue {
 	spinlock_t                 *irqlock;
 	struct device		   *dev;
 
+	wait_queue_head_t	   wait; /* wait if queue is empty */
+
 	enum v4l2_buf_type         type;
 	unsigned int               inputs; /* for V4L2_BUF_FLAG_INPUT */
 	unsigned int               msize;