refactor(core): the backlight driver functions 'API has been reviewed. The data types have been adapted to fit the actual data passed/used/returned.

diff --git a/core/embed/io/backlight/inc/io/backlight.h b/core/embed/io/backlight/inc/io/backlight.h
index 42c7580e..be4fb2a1 100644
--- a/core/embed/io/backlight/inc/io/backlight.h
+++ b/core/embed/io/backlight/inc/io/backlight.h
@@ -19,6 +19,8 @@
 
 #pragma once
 
+#include <trezor_types.h>
+
 #define BACKLIGHT_MAX_LEVEL 255
 #define BACKLIGHT_MIN_LEVEL 0
 
@@ -34,7 +36,9 @@ typedef enum {
 // If the action is set to `BACKLIGHT_RESET`, the backlight level
 // is set to zero level. If the action is set to `BACKLIGHT_RETAIN`,
 // the backlight level is not changed (if possible).
-void backlight_init(backlight_action_t action);
+//
+// Returns `true` if the initialization was successful.
+bool backlight_init(backlight_action_t action);
 
 // Deinitialize the backlight driver
 //
@@ -44,20 +48,20 @@ void backlight_init(backlight_action_t action);
 // is kept on.
 void backlight_deinit(backlight_action_t action);
 
-// Request the backlight level in range 0-255 and returns the actual level set.
-// The requested level may be limited if its above the max_level limit.
+// Requests the backlight level in range 0-255 and returns whether
+// the function executed successfully or not. The requested level
+// may be limited if it's above the max_level limit.
 //
-// If the level is outside the range, the function has no effect
-// and just returns the actual level set. If the backlight driver
-// is not initialized, the function returns 0.
-int backlight_set(int val);
+// If the backlight driver is not initialized, the function
+// returns "False".
+bool backlight_set(uint8_t val);
 
 // Gets the backlight level in range 0-255
 //
 // Returns 0 if the backlight driver is not initialized.
-int backlight_get(void);
+uint8_t backlight_get(void);
 
 // Set maximal backlight limit, limits the requested level to max_level value.
 //
-// Returns 0 if the backlight driver is not initialized.
-int backlight_set_max_level(int max_level);
+// Returns "False" if the backlight driver is not initialized.
+bool backlight_set_max_level(uint8_t max_level);
diff --git a/core/embed/io/backlight/stm32/backlight_pin.c b/core/embed/io/backlight/stm32/backlight_pin.c
index 1006c75f..b7bf73d1 100644
--- a/core/embed/io/backlight/stm32/backlight_pin.c
+++ b/core/embed/io/backlight/stm32/backlight_pin.c
@@ -26,7 +26,7 @@ typedef struct {
   // Set if driver is initialized
   bool initialized;
   // Current backlight level in range 0-255
-  int current_level;
+  uint8_t current_level;
 
 } backlight_driver_t;
 
@@ -52,11 +52,11 @@ static void backlight_off(void) {
   HAL_GPIO_Init(BACKLIGHT_PIN_PORT, &GPIO_InitStructure);
 }
 
-void backlight_init(backlight_action_t action) {
+bool backlight_init(backlight_action_t action) {
   backlight_driver_t *drv = &g_backlight_driver;
 
   if (drv->initialized) {
-    return;
+    return true;
   }
 
   BACKLIGHT_PIN_CLK_ENABLE();
@@ -66,6 +66,8 @@ void backlight_init(backlight_action_t action) {
   };
 
   drv->initialized = true;
+
+  return true;
 }
 
 void backlight_deinit(backlight_action_t action) {
@@ -79,10 +81,11 @@ void backlight_deinit(backlight_action_t action) {
   }
 }
 
-int backlight_set(int val) {
+bool backlight_set(uint8_t val) {
   backlight_driver_t *drv = &g_backlight_driver;
+
   if (!drv->initialized) {
-    return 0;
+    return false;
   }
 
   if (val > 0) {
@@ -90,11 +93,13 @@ int backlight_set(int val) {
   } else {
     backlight_off();
   }
+
   drv->current_level = val;
-  return val;
+
+  return true;
 }
 
-int backlight_get(void) {
+uint8_t backlight_get(void) {
   backlight_driver_t *drv = &g_backlight_driver;
   if (!drv->initialized) {
     return 0;
diff --git a/core/embed/io/backlight/stm32/tps61043.c b/core/embed/io/backlight/stm32/tps61043.c
index 1748acf4..148c30c2 100644
--- a/core/embed/io/backlight/stm32/tps61043.c
+++ b/core/embed/io/backlight/stm32/tps61043.c
@@ -36,7 +36,7 @@ typedef struct {
   // Set if driver is initialized
   bool initialized;
   // Current backlight level in range 0-255
-  int current_level;
+  uint8_t current_level;
 
 } backlight_driver_t;
 
@@ -45,11 +45,11 @@ static backlight_driver_t g_backlight_driver = {
     .initialized = false,
 };
 
-void backlight_init(backlight_action_t action) {
+bool backlight_init(backlight_action_t action) {
   backlight_driver_t *drv = &g_backlight_driver;
 
   if (drv->initialized) {
-    return;
+    return true;
   }
 
   memset(drv, 0, sizeof(backlight_driver_t));
@@ -188,6 +188,8 @@ void backlight_init(backlight_action_t action) {
   drv->initialized = true;
 
   backlight_set(initial_level);
+
+  return true;
 }
 
 void backlight_deinit(backlight_action_t action) {
@@ -265,30 +267,28 @@ static void backlight_wakeup_pulse(void) {
   HAL_GPIO_Init(TPS61043_PORT, &GPIO_InitStructure);
 }
 
-int backlight_set(int level) {
+bool backlight_set(uint8_t level) {
   backlight_driver_t *drv = &g_backlight_driver;
 
   if (!drv->initialized) {
-    return 0;
+    return false;
   }
 
-  if (level >= 0 && level <= 255) {
-    // TPS61043 goes to shutdown when duty cycle is 0 (after 32ms),
-    // so we need to set GPIO to high for at least 500us
-    // to wake it up.
-    if (TPS61043_TIM->TPS61043_TIM_CCR == 0 && level != 0) {
-      backlight_wakeup_pulse();
-    }
+  // TPS61043 goes to shutdown when duty cycle is 0 (after 32ms),
+  // so we need to set GPIO to high for at least 500us
+  // to wake it up.
+  if (TPS61043_TIM->TPS61043_TIM_CCR == 0 && level != 0) {
+    backlight_wakeup_pulse();
+  }
 
-    TPS61043_TIM->CCR1 = (LED_PWM_TIM_PERIOD * level) / 255;
+  TPS61043_TIM->CCR1 = (LED_PWM_TIM_PERIOD * level) / 255;
 
-    drv->current_level = level;
-  }
+  drv->current_level = level;
 
-  return drv->current_level;
+  return true;
 }
 
-int backlight_get(void) {
+uint8_t backlight_get(void) {
   backlight_driver_t *drv = &g_backlight_driver;
 
   if (!drv->initialized) {
diff --git a/core/embed/io/backlight/stm32u5/tps61062.c b/core/embed/io/backlight/stm32u5/tps61062.c
index 0fd3ae26..99d9df25 100644
--- a/core/embed/io/backlight/stm32u5/tps61062.c
+++ b/core/embed/io/backlight/stm32u5/tps61062.c
@@ -49,14 +49,14 @@ typedef struct {
   bool initialized;
 
   // Level requested (0-255)
-  int requested_level;
-  int current_level;
+  uint8_t requested_level;
+  uint8_t current_level;
 
   // Current step in range 0-32
   int current_step;
 
   // Max backlight level
-  int max_level;
+  uint8_t max_level;
 
   DMA_HandleTypeDef dma;
   TIM_HandleTypeDef tim;
@@ -73,11 +73,11 @@ static void backlight_control_up(uint32_t *data, int steps);
 static void backlight_control_down(uint32_t *data, int steps);
 static void backlight_shutdown();
 
-void backlight_init(backlight_action_t action) {
+bool backlight_init(backlight_action_t action) {
   backlight_driver_t *drv = &g_backlight_driver;
 
   if (drv->initialized) {
-    return;
+    return true;
   }
 
   memset(drv, 0, sizeof(backlight_driver_t));
@@ -161,6 +161,8 @@ void backlight_init(backlight_action_t action) {
   drv->requested_level = BACKLIGHT_MIN_LEVEL;
 
   drv->initialized = true;
+
+  return true;
 }
 
 void backlight_deinit(backlight_action_t action) {
@@ -189,29 +191,24 @@ void backlight_deinit(backlight_action_t action) {
   drv->initialized = false;
 }
 
-int backlight_set(int val) {
+bool backlight_set(uint8_t val) {
   backlight_driver_t *drv = &g_backlight_driver;
 
   if (!drv->initialized) {
-    return 0;
-  }
-
-  // Requested level out of range
-  if (val < BACKLIGHT_MIN_LEVEL || val > BACKLIGHT_MAX_LEVEL) {
-    return drv->current_level;
+    return false;
   }
 
   // Capture requested level.
   drv->requested_level = val;
 
-  int requested_level_limited = drv->requested_level;
+  uint8_t requested_level_limited = drv->requested_level;
   if (drv->requested_level > drv->max_level) {
     requested_level_limited = drv->max_level;
   }
 
   // No action required
   if (requested_level_limited == drv->current_level) {
-    return drv->current_level;
+    return true;
   }
 
   // New backlight level
@@ -222,7 +219,7 @@ int backlight_set(int val) {
   if (set_step == 0) {
     backlight_shutdown();
     drv->current_step = 0;
-    return drv->current_level;
+    return true;
   }
 
   if (HAL_DMA_GetState(&drv->dma) == HAL_DMA_STATE_BUSY) {
@@ -261,10 +258,10 @@ int backlight_set(int val) {
 
   drv->current_step = set_step;
 
-  return drv->current_level;
+  return true;
 }
 
-int backlight_get(void) {
+uint8_t backlight_get(void) {
   backlight_driver_t *drv = &g_backlight_driver;
 
   if (!drv->initialized) {
@@ -275,25 +272,16 @@ int backlight_get(void) {
 }
 
 // Set maximal backlight level
-int backlight_set_max_level(int max_level) {
+bool backlight_set_max_level(uint8_t max_level) {
   backlight_driver_t *drv = &g_backlight_driver;
 
   if (!drv->initialized) {
-    return 0;
-  }
-
-  if (max_level < BACKLIGHT_MIN_LEVEL) {
-    max_level = BACKLIGHT_MIN_LEVEL;
-  }
-
-  if (max_level > BACKLIGHT_MAX_LEVEL) {
-    max_level = BACKLIGHT_MAX_LEVEL;
+    return false;
   }
 
   drv->max_level = max_level;
-  backlight_set(drv->requested_level);
 
-  return drv->current_level;
+  return backlight_set(drv->requested_level);
 }
 
 static void backlight_control_up(uint32_t *data, int steps) {