controls: Add FrameWallClock control

Add a FrameWallClock control that reports the same moment as the
frame's SensorTimestamp, but in wallclock units.

Signed-off-by: David Plowman <david.plowman@raspberrypi.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Naushir Patuck <naush@raspberrypi.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Naushir Patuck <naush@raspberrypi.com>
Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

src/libcamera/control_ids_core.yaml

@@ -1,6 +1,4 @@
-# SPDX-License-Identifier: LGPL-2.1-or-later
-#
-# Copyright (C) 2019, Google Inc.
+
 #
 %YAML 1.1
 ---
@@ -1268,4 +1266,19 @@ controls:
       description: |
         Enable or disable the debug metadata.
 
+  - FrameWallClock:
+      type: int64_t
+      direction: out
+      description: |
+        This timestamp corresponds to the same moment in time as the
+        SensorTimestamp, but is represented as a wall clock time as measured by
+        the CLOCK_REALTIME clock.
+
+        Being a wall clock measurement, it can be used to synchronise timing
+        across different devices.
+
+        \sa SensorTimestamp
+
+        The FrameWallClock control can only be returned in metadata.
+
 ...