gst: libcamerasrc: Implement timestamp support

This is an experimental patch adding timestamp support to the libcamerasrc
element. This patch currently assume that the driver timestamp are relative to
the system monotonic clock. Without a reference clock source, the timestamp are
otherwise unusable, and without timestamp only minor use case can be achieved.

Signed-off-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

src/gstreamer/gstlibcamerapad.cpp

@@ -63,9 +63,24 @@ gst_libcamera_pad_get_property(GObject *object, guint prop_id, GValue *value,
 	}
 }
 
+static gboolean
+gst_libcamera_pad_query(GstPad *pad, GstObject *parent, GstQuery *query)
+{
+	auto *self = GST_LIBCAMERA_PAD(pad);
+
+	if (query->type != GST_QUERY_LATENCY)
+		return gst_pad_query_default(pad, parent, query);
+
+	/* TRUE here means live, we assumes that max latency is the same as min
+	 * as we have no idea that duration of frames. */
+	gst_query_set_latency(query, TRUE, self->latency, self->latency);
+	return TRUE;
+}
+
 static void
 gst_libcamera_pad_init(GstLibcameraPad *self)
 {
+	GST_PAD_QUERYFUNC(self) = gst_libcamera_pad_query;
 }
 
 static GType
@@ -173,3 +188,11 @@ gst_libcamera_pad_has_pending(GstPad *pad)
 	GLibLocker lock(GST_OBJECT(self));
 	return self->pending_buffers.length > 0;
 }
+
+void
+gst_libcamera_pad_set_latency(GstPad *pad, GstClockTime latency)
+{
+	auto *self = GST_LIBCAMERA_PAD(pad);
+	GLibLocker lock(GST_OBJECT(self));
+	self->latency = latency;
+}