libcamera: rkisp1: Report sensor timestamp

Report the sensor's timestamp in the Request metadata using the
completed buffer timestamp.

The buffer's timestamp is recorded at DMA-transfer time, and it does not
theoretically matches the 'start of exposure' definition. Record this with
a \todo entry.

Reviewed-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>
Reviewed-by: Hirokazu Honda <hiroh@chromium.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Jacopo Mondi <jacopo@jmondi.org>

src/libcamera/pipeline/rkisp1/rkisp1.cpp

@@ -1070,6 +1070,15 @@ void PipelineHandlerRkISP1::bufferReady(FrameBuffer *buffer)
 {
 	Request *request = buffer->request();
 
+	/*
+	 * Record the sensor's timestamp in the request metadata.
+	 *
+	 * \todo The sensor timestamp should be better estimated by connecting
+	 * to the V4L2Device::frameStart signal.
+	 */
+	request->metadata().set(controls::SensorTimestamp,
+				buffer->metadata().timestamp);
+
 	completeBuffer(request, buffer);
 	tryCompleteRequest(request);
 }