core: dim lockscreen (fixes #974)

core/src/apps/homescreen/lockscreen.py

@@ -1,4 +1,4 @@
-from trezor import res, ui
+from trezor import loop, res, ui
 
 from . import HomescreenBase
 
@@ -33,6 +33,26 @@ class Lockscreen(HomescreenBase):
         )
         ui.display.icon(45, 202, res.load(ui.ICON_CLICK), ui.TITLE_GREY, ui.BG)
 
+    def handle_rendering(self) -> loop.Task:  # type: ignore
+        """Task that is rendering the layout in a busy loop.
+
+        Copy-pasted from ui.Layout.handle_rendering with modification to set the
+        backlight to a lower level while lockscreen is on, and a longer sleep because
+        we never do any redrawing."""
+        # Before the first render, we dim the display.
+        ui.backlight_fade(ui.BACKLIGHT_DIM)
+        # Clear the screen of any leftovers, make sure everything is marked for
+        # repaint (we can be running the same layout instance multiple times)
+        # and paint it.
+        ui.display.clear()
+        self.on_render()
+        ui.refresh()
+        ui.backlight_fade(ui.BACKLIGHT_LOW)
+        # long sleep
+        sleep = loop.sleep(1000 * 1000 * 1000)
+        while True:
+            yield sleep
+
     def on_render(self) -> None:
         self.render_homescreen()
         self.render_lock()

core/src/trezor/ui/style.py

@@ -7,6 +7,7 @@ RADIUS = const(2)
 
 # backlight brightness
 BACKLIGHT_NORMAL = const(150)
+BACKLIGHT_LOW = const(70)
 BACKLIGHT_DIM = const(5)
 BACKLIGHT_NONE = const(2)
 BACKLIGHT_MAX = const(255)