libcamera: ipu3: Report sensor timestamp

Report the sensor's timestamp in the Request metadata by using the
CIO2 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: Niklas Söderlund <niklas.soderlund@ragnatech.se>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
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/ipu3/ipu3.cpp

@@ -1255,6 +1255,15 @@ void IPU3CameraData::cio2BufferReady(FrameBuffer *buffer)
 
 	Request *request = info->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);
+
 	/* If the buffer is cancelled force a complete of the whole request. */
 	if (buffer->metadata().status == FrameMetadata::FrameCancelled) {
 		for (auto it : request->buffers()) {