chore(core/rust): Add backlight fading fn

core/embed/rust/src/ui/display.rs

@@ -1,4 +1,4 @@
-use crate::trezorhal::display;
+use crate::{micropython::time, time::Duration, trezorhal::display};
 
 #[cfg(not(feature = "model_tt"))]
 use crate::ui::model_t1::constant;
@@ -23,8 +23,30 @@ pub fn screen() -> Rect {
     Rect::from_top_left_and_size(Point::zero(), size())
 }
 
-pub fn backlight(val: i32) -> i32 {
-    display::backlight(val)
+pub fn backlight() -> i32 {
+    display::backlight(-1)
+}
+
+pub fn set_backlight(val: i32) {
+    display::backlight(val);
+}
+
+pub fn fade_backlight(target: i32) {
+    const BACKLIGHT_DELAY: Duration = Duration::from_millis(14);
+    const BACKLIGHT_STEP: usize = 15;
+
+    let current = backlight();
+    if current < target {
+        for val in (current..target).step_by(BACKLIGHT_STEP) {
+            set_backlight(val);
+            time::sleep(BACKLIGHT_DELAY);
+        }
+    } else {
+        for val in (target..current).rev().step_by(BACKLIGHT_STEP) {
+            set_backlight(val);
+            time::sleep(BACKLIGHT_DELAY);
+        }
+    }
 }
 
 pub fn rect(r: Rect, fg_color: Color) {

core/embed/rust/src/ui/model_tt/component/swipe.rs

@@ -77,7 +77,7 @@ impl Swipe {
         let start = self.backlight_start as f32;
         let end = self.backlight_end as f32;
         let value = start + ratio * (end - start);
-        display::backlight(value as i32);
+        display::set_backlight(value as i32);
     }
 }