driver: usbvideodriver: add support for overlaying FPS

We encourage addition of multiple quality settings for USB Video
devices, but there is no easy way to get feedback how many frames per
second are actually processed on the receiver side, which could
influence which quality settings would be chosen for Labgrid.

Let's add a fps parameter to USBVideoDriver::stream() that enables an
overlay with FPS statistics if enabled.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>

Author: a3f

CHANGES.rst

@@ -11,6 +11,9 @@ New Features in 25.1
 
 - Guermok HDMI to USB 3.0 capture dongle supported
 
+- The `USBVideoDriver`'s ``stream`` method can now be called with ``fps=True``
+  to embed an overlay with FPS statistics into the video stream.
+
 Breaking changes in 25.1
 ~~~~~~~~~~~~~~~~~~~~~~~~
 - The deprecated pytest plugin option ``--env-config`` has been removed. Use

labgrid/driver/usbvideodriver.py

@@ -139,13 +139,15 @@ def get_pipeline(self, path, caps, controls=None):
         return pipeline
 
     @Driver.check_active
-    def stream(self, caps_hint=None, controls=None):
+    def stream(self, caps_hint=None, controls=None, fps=False):
         caps = self.select_caps(caps_hint)
         pipeline = self.get_pipeline(self.video.path, caps, controls)
 
         tx_cmd = self.video.command_prefix + ["gst-launch-1.0", "-q"]
         tx_cmd += pipeline.split()
         rx_cmd = ["gst-launch-1.0", "playbin3", "buffer-duration=0", "uri=fd://0"]
+        if fps:
+            rx_cmd += ['video-sink=fpsdisplaysink']
 
         tx = subprocess.Popen(
             tx_cmd,