[media] media: v4l2-mem2mem: return for polling if a buffer is available

The v4l2_m2m_poll() does not need to wait if there is already a buffer in
done_list of source and destination queues, but current v4l2_m2m_poll() always
waits. So done_list of each queue is checked before calling poll_wait().
Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>
Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Kamil Debski <k.debski@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

drivers/media/v4l2-core/v4l2-mem2mem.c

@@ -486,7 +486,9 @@ unsigned int v4l2_m2m_poll(struct file *file, struct v4l2_m2m_ctx *m2m_ctx,
 	if (m2m_ctx->m2m_dev->m2m_ops->unlock)
 		m2m_ctx->m2m_dev->m2m_ops->unlock(m2m_ctx->priv);
 
+	if (list_empty(&src_q->done_list))
 		poll_wait(file, &src_q->done_wq, wait);
+	if (list_empty(&dst_q->done_list))
 		poll_wait(file, &dst_q->done_wq, wait);
 
 	if (m2m_ctx->m2m_dev->m2m_ops->lock)