v4l2bufferpool: Wait before polling if queue is empty

In kernel before 3.17, polling during queue underrun would unblock right
away and trigger POLLERR. As we are not handling POLLERR, we would endup
blocking in DQBUF call, which won't be unblocked correctly when going
to NULL state. A deadlock at start caused by locking error in libv4l2 was
also seen before this patch. Instead, we wait until the queue is no longer
empty before polling.

https://bugzilla.gnome.org/show_bug.cgi?id=731015

diff --git a/sys/v4l2/gstv4l2bufferpool.c b/sys/v4l2/gstv4l2bufferpool.c
index f5c74c8..eb63a57 100644 (file)
--- a/sys/v4l2/gstv4l2bufferpool.c
+++ b/sys/v4l2/gstv4l2bufferpool.c
@@ -846,6 +846,11 @@ gst_v4l2_buffer_pool_flush_start (GstBufferPool * bpool)
 
   gst_poll_set_flushing (pool->poll, TRUE);
 
+  GST_OBJECT_LOCK (pool);
+  pool->empty = FALSE;
+  g_cond_broadcast (&pool->empty_cond);
+  GST_OBJECT_UNLOCK (pool);
+
   if (pool->other_pool)
     gst_buffer_pool_set_flushing (pool->other_pool, TRUE);
 }
@@ -934,6 +939,11 @@ gst_v4l2_buffer_pool_poll (GstV4l2BufferPool * pool)
 {
   gint ret;
 
+  GST_OBJECT_LOCK (pool);
+  while (pool->empty)
+    g_cond_wait (&pool->empty_cond, GST_OBJECT_GET_LOCK (pool));
+  GST_OBJECT_UNLOCK (pool);
+
   if (!pool->can_poll_device)
     goto done;
 
@@ -1001,6 +1011,11 @@ gst_v4l2_buffer_pool_qbuf (GstV4l2BufferPool * pool, GstBuffer * buf)
   if (!gst_v4l2_allocator_qbuf (pool->vallocator, group))
     goto queue_failed;
 
+  GST_OBJECT_LOCK (pool);
+  pool->empty = FALSE;
+  g_cond_signal (&pool->empty_cond);
+  GST_OBJECT_UNLOCK (pool);
+
   return GST_FLOW_OK;
 
 already_queued:
@@ -1047,7 +1062,11 @@ gst_v4l2_buffer_pool_dqbuf (GstV4l2BufferPool * pool, GstBuffer ** buffer)
 
   /* mark the buffer outstanding */
   pool->buffers[group->buffer.index] = NULL;
-  g_atomic_int_add (&pool->num_queued, -1);
+  if (g_atomic_int_dec_and_test (&pool->num_queued)) {
+    GST_OBJECT_LOCK (pool);
+    pool->empty = TRUE;
+    GST_OBJECT_UNLOCK (pool);
+  }
 
   timestamp = GST_TIMEVAL_TO_TIME (group->buffer.timestamp);
 
@@ -1354,6 +1373,8 @@ gst_v4l2_buffer_pool_init (GstV4l2BufferPool * pool)
 {
   pool->poll = gst_poll_new (TRUE);
   pool->can_poll_device = TRUE;
+  g_cond_init (&pool->empty_cond);
+  pool->empty = TRUE;
 }
 
 static void

diff --git a/sys/v4l2/gstv4l2bufferpool.h b/sys/v4l2/gstv4l2bufferpool.h
index 65486bf..e805615 100644 (file)
--- a/sys/v4l2/gstv4l2bufferpool.h
+++ b/sys/v4l2/gstv4l2bufferpool.h
@@ -53,6 +53,9 @@ struct _GstV4l2BufferPool
   GstPoll *poll;             /* a poll for video_fd */
   gboolean can_poll_device;
 
+  gboolean empty;
+  GCond empty_cond;
+
   GstV4l2Allocator *vallocator;
   GstAllocator *allocator;
   GstAllocationParams params;