refactor(core): the display driver functions' API has been reviewed. The data types have been adapted to fit the actual data passed/used/returned. Since the backlight driver has undergone the similar change, the display driver code has been adapted to respect the changes.

diff --git a/core/embed/io/display/display_utils.c b/core/embed/io/display/display_utils.c
index c2e7384b1..647f19b1b 100644
--- a/core/embed/io/display/display_utils.c
+++ b/core/embed/io/display/display_utils.c
@@ -22,7 +22,7 @@
 #include <io/display.h>
 #include <sys/systick.h>
 
-void display_fade(int start, int end, int delay) {
+void display_fade(uint8_t start, uint8_t end, int delay) {
 #ifdef USE_BACKLIGHT
   if (display_get_backlight() == end) {
     return;
diff --git a/core/embed/io/display/inc/io/display.h b/core/embed/io/display/inc/io/display.h
index 264d4e649..fb04fba8e 100644
--- a/core/embed/io/display/inc/io/display.h
+++ b/core/embed/io/display/inc/io/display.h
@@ -80,14 +80,14 @@ void display_set_unpriv_access(bool unpriv);
 //
 // The default backligt level is 0. Without settings it
 // to some higher value the displayed pixels are not visible.
-// Beware that his also applies to the emulator.
+// Beware that this also applies to the emulator.
 //
-// Returns the set level (usually the same value or the
-// closest value to the `level` argument)
-int display_set_backlight(int level);
+// Returns the boolean value. "True" - successful function execution.
+// "False" - a problem occurred.
+bool display_set_backlight(uint8_t level);
 
 // Gets current display level ranging from 0 (off)..255 (maximum).
-int display_get_backlight(void);
+uint8_t display_get_backlight(void);
 
 // Sets the display orientation.
 //
diff --git a/core/embed/io/display/inc/io/display_utils.h b/core/embed/io/display/inc/io/display_utils.h
index 810a749ee..ca8c79815 100644
--- a/core/embed/io/display/inc/io/display_utils.h
+++ b/core/embed/io/display/inc/io/display_utils.h
@@ -25,7 +25,7 @@
  * @param end Target backlight level (0-255)
  * @param delay Total duration of the fade effect in milliseconds
  */
-void display_fade(int start, int end, int delay);
+void display_fade(uint8_t start, uint8_t end, int delay);
 
 /**
  * @brief Starts recording the display output to files
diff --git a/core/embed/io/display/ltdc_dsi/display_driver.c b/core/embed/io/display/ltdc_dsi/display_driver.c
index 7da367f5d..65941024b 100644
--- a/core/embed/io/display/ltdc_dsi/display_driver.c
+++ b/core/embed/io/display/ltdc_dsi/display_driver.c
@@ -442,11 +442,11 @@ void display_deinit(display_content_mode_t mode) {
   memset(drv, 0, sizeof(display_driver_t));
 }
 
-int display_set_backlight(int level) {
+bool display_set_backlight(uint8_t level) {
   display_driver_t *drv = &g_display_driver;
 
   if (!drv->initialized) {
-    return 0;
+    return false;
   }
 
 #ifdef USE_BACKLIGHT
@@ -458,11 +458,11 @@ int display_set_backlight(int level) {
 #else
   // Just emulation, not doing anything
   drv->backlight_level = level;
-  return level;
+  return true;
 #endif
 }
 
-int display_get_backlight(void) {
+uint8_t display_get_backlight(void) {
   display_driver_t *drv = &g_display_driver;
 
   if (!drv->initialized) {
diff --git a/core/embed/io/display/ltdc_dsi/display_internal.h b/core/embed/io/display/ltdc_dsi/display_internal.h
index ec92e195f..e5db3f4a6 100644
--- a/core/embed/io/display/ltdc_dsi/display_internal.h
+++ b/core/embed/io/display/ltdc_dsi/display_internal.h
@@ -52,7 +52,7 @@ typedef struct {
   // Current display orientation (0, 90, 180, 270)
   int orientation_angle;
   // Current backlight level ranging from 0 to 255
-  int backlight_level;
+  uint8_t backlight_level;
   // The current frame buffer selector
 
   DSI_HandleTypeDef hlcd_dsi;
diff --git a/core/embed/io/display/st-7789/display_driver.c b/core/embed/io/display/st-7789/display_driver.c
index b6777f5bc..3e1e5179c 100644
--- a/core/embed/io/display/st-7789/display_driver.c
+++ b/core/embed/io/display/st-7789/display_driver.c
@@ -131,11 +131,11 @@ void display_deinit(display_content_mode_t mode) {
   drv->initialized = false;
 }
 
-int display_set_backlight(int level) {
+bool display_set_backlight(uint8_t level) {
   display_driver_t* drv = &g_display_driver;
 
   if (!drv->initialized) {
-    return 0;
+    return false;
   }
 
 #ifndef BOARDLOADER
@@ -148,7 +148,7 @@ int display_set_backlight(int level) {
   return backlight_set(level);
 }
 
-int display_get_backlight(void) { return backlight_get(); }
+uint8_t display_get_backlight(void) { return backlight_get(); }
 
 int display_set_orientation(int angle) {
   display_driver_t* drv = &g_display_driver;
diff --git a/core/embed/io/display/stm32f429i-disc1/display_driver.c b/core/embed/io/display/stm32f429i-disc1/display_driver.c
index d4ac62a33..fa2330788 100644
--- a/core/embed/io/display/stm32f429i-disc1/display_driver.c
+++ b/core/embed/io/display/stm32f429i-disc1/display_driver.c
@@ -42,7 +42,7 @@ typedef struct {
   // Current display orientation (0, 90, 180, 270)
   int orientation_angle;
   // Current backlight level ranging from 0 to 255
-  int backlight_level;
+  uint8_t backlight_level;
 } display_driver_t;
 
 // Display driver instance
@@ -83,19 +83,19 @@ void display_deinit(display_content_mode_t mode) {
   drv->initialized = false;
 }
 
-int display_set_backlight(int level) {
+bool display_set_backlight(uint8_t level) {
   display_driver_t *drv = &g_display_driver;
 
   if (!drv->initialized) {
-    return 0;
+    return false;
   }
 
   // Just emulation, not doing anything
   drv->backlight_level = level;
-  return level;
+  return true;
 }
 
-int display_get_backlight(void) {
+uint8_t display_get_backlight(void) {
   display_driver_t *drv = &g_display_driver;
 
   if (!drv->initialized) {
diff --git a/core/embed/io/display/unix/display_driver.c b/core/embed/io/display/unix/display_driver.c
index 44c13f4d5..677afaad3 100644
--- a/core/embed/io/display/unix/display_driver.c
+++ b/core/embed/io/display/unix/display_driver.c
@@ -69,7 +69,7 @@ typedef struct {
   // Current display orientation (0 or 180)
   int orientation_angle;
   // Current backlight level ranging from 0 to 255
-  int backlight_level;
+  uint8_t backlight_level;
 
   SDL_Window *window;
   SDL_Renderer *renderer;
@@ -229,26 +229,26 @@ void display_deinit(display_content_mode_t mode) {
   drv->initialized = false;
 }
 
-int display_set_backlight(int level) {
+bool display_set_backlight(uint8_t level) {
   display_driver_t *drv = &g_display_driver;
 
   if (!drv->initialized) {
-    return 0;
+    return false;
   }
 
 #if !USE_BACKLIGHT
   level = 255;
 #endif
 
-  if (drv->backlight_level != level && level >= 0 && level <= 255) {
+  if (drv->backlight_level != level) {
     drv->backlight_level = level;
     display_refresh();
   }
 
-  return drv->backlight_level;
+  return true;
 }
 
-int display_get_backlight(void) {
+uint8_t display_get_backlight(void) {
   display_driver_t *drv = &g_display_driver;
 
   if (!drv->initialized) {
diff --git a/core/embed/io/display/vg-2864/display_driver.c b/core/embed/io/display/vg-2864/display_driver.c
index 4f9992fb8..3f4896f36 100644
--- a/core/embed/io/display/vg-2864/display_driver.c
+++ b/core/embed/io/display/vg-2864/display_driver.c
@@ -67,7 +67,7 @@ typedef struct {
   // Current display orientation (0 or 180)
   int orientation_angle;
   // Current backlight level ranging from 0 to 255
-  int backlight_level;
+  uint8_t backlight_level;
 } display_driver_t;
 
 // Display driver instance
@@ -340,18 +340,18 @@ void display_set_unpriv_access(bool unpriv) {
 }
 #endif  // USE_TRUSTZONE
 
-int display_set_backlight(int level) {
+bool display_set_backlight(uint8_t level) {
   display_driver_t *drv = &g_display_driver;
 
   if (!drv->initialized) {
-    return 0;
+    return false;
   }
 
   drv->backlight_level = 255;
-  return drv->backlight_level;
+  return true;
 }
 
-int display_get_backlight(void) {
+uint8_t display_get_backlight(void) {
   display_driver_t *drv = &g_display_driver;
 
   if (!drv->initialized) {
diff --git a/core/embed/sys/syscall/stm32/syscall_dispatch.c b/core/embed/sys/syscall/stm32/syscall_dispatch.c
index 3b845c92b..145f1d577 100644
--- a/core/embed/sys/syscall/stm32/syscall_dispatch.c
+++ b/core/embed/sys/syscall/stm32/syscall_dispatch.c
@@ -213,7 +213,7 @@ __attribute((no_stack_protector)) void syscall_handler(uint32_t *args,
     } break;
 
     case SYSCALL_DISPLAY_SET_BACKLIGHT: {
-      int level = (int)args[0];
+      uint8_t level = (uint8_t)args[0];
       args[0] = display_set_backlight(level);
     } break;
 
diff --git a/core/embed/sys/syscall/stm32/syscall_stubs.c b/core/embed/sys/syscall/stm32/syscall_stubs.c
index fac5e44a3..77d0d313b 100644
--- a/core/embed/sys/syscall/stm32/syscall_stubs.c
+++ b/core/embed/sys/syscall/stm32/syscall_stubs.c
@@ -166,12 +166,12 @@ void notify_send(notification_event_t event) {
 
 #include <io/display.h>
 
-int display_set_backlight(int level) {
-  return (int)syscall_invoke1((uint32_t)level, SYSCALL_DISPLAY_SET_BACKLIGHT);
+bool display_set_backlight(uint8_t level) {
+  return (bool)syscall_invoke1((uint32_t)level, SYSCALL_DISPLAY_SET_BACKLIGHT);
 }
 
-int display_get_backlight(void) {
-  return (int)syscall_invoke0(SYSCALL_DISPLAY_GET_BACKLIGHT);
+uint8_t display_get_backlight(void) {
+  return (uint8_t)syscall_invoke0(SYSCALL_DISPLAY_GET_BACKLIGHT);
 }
 
 int display_set_orientation(int angle) {