[PATCH 1/2] media: v4l2-mem2mem: consider OUTPUT queue first when polling

[Alexandre Courbot]

If poll() is called on a m2m device with the EPOLLOUT event after the
last buffer of the CAPTURE queue is dequeued, any buffer available on
OUTPUT queue will never be signaled because v4l2_m2m_poll_for_data()
starts by checking whether dst_q->last_buffer_dequeued is set and
returns EPOLLIN in this case, without looking at the state of the OUTPUT
queue.

Fix this by checking the state of the OUTPUT queue before considering
that early-return case.

This also has the side-effect of bringing the two blocks of code dealing
with the CAPTURE queue next to one another, and saves us one spin
lock/unlock cycle, for what it's worth.

Signed-off-by: Alexandre Courbot <gnurou@xxxxxxxxx>
---
 drivers/media/v4l2-core/v4l2-mem2mem.c | 23 +++++++++++------------
 1 file changed, 11 insertions(+), 12 deletions(-)

diff --git a/drivers/media/v4l2-core/v4l2-mem2mem.c b/drivers/media/v4l2-core/v4l2-mem2mem.c
index 95a8f2dc5341d..0d0192119af20 100644
--- a/drivers/media/v4l2-core/v4l2-mem2mem.c
+++ b/drivers/media/v4l2-core/v4l2-mem2mem.c
@@ -862,6 +862,15 @@ static __poll_t v4l2_m2m_poll_for_data(struct file *file,
 	     list_empty(&dst_q->queued_list)))
 		return EPOLLERR;
 
+	spin_lock_irqsave(&src_q->done_lock, flags);
+	if (!list_empty(&src_q->done_list))
+		src_vb = list_first_entry(&src_q->done_list, struct vb2_buffer,
+						done_entry);
+	if (src_vb && (src_vb->state == VB2_BUF_STATE_DONE
+			|| src_vb->state == VB2_BUF_STATE_ERROR))
+		rc |= EPOLLOUT | EPOLLWRNORM;
+	spin_unlock_irqrestore(&src_q->done_lock, flags);
+
 	spin_lock_irqsave(&dst_q->done_lock, flags);
 	if (list_empty(&dst_q->done_list)) {
 		/*
@@ -870,21 +879,11 @@ static __poll_t v4l2_m2m_poll_for_data(struct file *file,
 		 */
 		if (dst_q->last_buffer_dequeued) {
 			spin_unlock_irqrestore(&dst_q->done_lock, flags);
-			return EPOLLIN | EPOLLRDNORM;
+			rc |= EPOLLIN | EPOLLRDNORM;
+			return rc;
 		}
 	}
-	spin_unlock_irqrestore(&dst_q->done_lock, flags);
 
-	spin_lock_irqsave(&src_q->done_lock, flags);
-	if (!list_empty(&src_q->done_list))
-		src_vb = list_first_entry(&src_q->done_list, struct vb2_buffer,
-						done_entry);
-	if (src_vb && (src_vb->state == VB2_BUF_STATE_DONE
-			|| src_vb->state == VB2_BUF_STATE_ERROR))
-		rc |= EPOLLOUT | EPOLLWRNORM;
-	spin_unlock_irqrestore(&src_q->done_lock, flags);
-
-	spin_lock_irqsave(&dst_q->done_lock, flags);
 	if (!list_empty(&dst_q->done_list))
 		dst_vb = list_first_entry(&dst_q->done_list, struct vb2_buffer,
 						done_entry);
-- 
2.28.0