libcamera: raspberry: Report sensor timestamp

Report the sensor's timestamp in the Request metadata by using the
Unicam::Image buffer timestamp as an initial approximation.

The buffer's timestamp is recorded at DMA-transfer time, and it does not
theoretically matches the 'start of exposure' definition, but when used
to compare two consecutive frames it gives an acceptable estimation of
the sensor frame period duration.

Reviewed-by: Hirokazu Honda <hiroh@chromium.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Naushir Patuck <naush@raspberrypi.com>
Signed-off-by: Jacopo Mondi <jacopo@jmondi.org>

src/libcamera/pipeline/raspberrypi/raspberrypi.cpp

@@ -1409,6 +1409,18 @@ void RPiCameraData::unicamBufferDequeue(FrameBuffer *buffer)
 			<< ", timestamp: " << buffer->metadata().timestamp;
 
 	if (stream == &unicam_[Unicam::Image]) {
+		/*
+		 * Record the sensor timestamp in the Request.
+		 *
+		 * \todo Do not assume the request in the front of the queue
+		 * is the correct one
+		 */
+		Request *request = requestQueue_.front();
+		ASSERT(request);
+
+		request->metadata().set(controls::SensorTimestamp,
+					buffer->metadata().timestamp);
+
 		/*
 		 * Lookup the sensor controls used for this frame sequence from
 		 * DelayedControl and queue them along with the frame buffer.