ui: bg brightness write thread (#38030)

* bg brightness thread

* rn

* later

* better

* rn

selfdrive/ui/ui_state.py

@@ -210,6 +210,8 @@ class Device:
     self._last_brightness: int = 0
     self._brightness_filter = FirstOrderFilter(BACKLIGHT_OFFROAD, 10.00, 1 / gui_app.target_fps)
     self._brightness_thread: threading.Thread | None = None
+    self._brightness_event = threading.Event()
+    self._brightness_target: int = 0
 
   @property
   def awake(self) -> bool:
@@ -235,6 +237,8 @@ class Device:
     self._interactive_timeout_callbacks.append(callback)
 
   def update(self):
+    self._start_brightness_thread()  # start thread after manager forks ui
+
     # do initial reset
     if self._interaction_time <= 0:
       self._reset_interactive_timeout()
@@ -242,6 +246,17 @@ class Device:
     self._update_brightness()
     self._update_wakefulness()
 
+  def _start_brightness_thread(self):
+    if self._brightness_thread is None or not self._brightness_thread.is_alive():
+      self._brightness_thread = threading.Thread(target=self._brightness_worker, daemon=True)
+      self._brightness_thread.start()
+
+  def _brightness_worker(self):
+    while True:
+      self._brightness_event.wait()
+      self._brightness_event.clear()
+      HARDWARE.set_screen_brightness(self._brightness_target)
+
   def set_offroad_brightness(self, brightness: int | None):
     if brightness is None:
       brightness = BACKLIGHT_OFFROAD
@@ -266,10 +281,9 @@ class Device:
       brightness = 0
 
     if brightness != self._last_brightness:
-      if self._brightness_thread is None or not self._brightness_thread.is_alive():
-        self._brightness_thread = threading.Thread(target=HARDWARE.set_screen_brightness, args=(brightness,))
-        self._brightness_thread.start()
-        self._last_brightness = brightness
+      self._brightness_target = brightness
+      self._brightness_event.set()
+      self._last_brightness = brightness
 
   def _update_wakefulness(self):
     # Handle interactive timeout