v4l2object: query minimum required buffers for output

Some v4l2 devices could require a minimum buffers different from default
values. Rather than blindly propose a pool with min-buffers set to the
default value, it ask the device using control ioctl.

https://bugzilla.gnome.org/show_bug.cgi?id=733750

diff --git a/sys/v4l2/gstv4l2object.c b/sys/v4l2/gstv4l2object.c
index 583c3d4..f8a6c4d 100644 (file)
--- a/sys/v4l2/gstv4l2object.c
+++ b/sys/v4l2/gstv4l2object.c
@@ -3412,6 +3412,7 @@ gst_v4l2_object_propose_allocation (GstV4l2Object * obj, GstQuery * query)
   guint size, min, max;
   GstCaps *caps;
   gboolean need_pool;
+  struct v4l2_control ctl = { 0, };
 
   /* Set defaults allocation parameters */
   size = obj->info.size;
@@ -3444,6 +3445,15 @@ gst_v4l2_object_propose_allocation (GstV4l2Object * obj, GstQuery * query)
     gst_structure_free (config);
   }
 
+  /* Some devices may expose a minimum */
+  ctl.id = V4L2_CID_MIN_BUFFERS_FOR_OUTPUT;
+  if (v4l2_ioctl (obj->video_fd, VIDIOC_G_CTRL, &ctl) >= 0) {
+    GST_DEBUG_OBJECT (obj->element, "driver require a miminum of %d buffers",
+        ctl.value);
+
+    min = MAX (ctl.value, GST_V4L2_MIN_BUFFERS);
+  }
+
   gst_query_add_allocation_pool (query, pool, size, min, max);
 
   /* we also support various metadata */