diff --git a/src/platforms/f072/platform.h b/src/platforms/f072/platform.h
index b28f7bb..e3a0b69 100644
--- a/src/platforms/f072/platform.h
+++ b/src/platforms/f072/platform.h
@@ -145,7 +145,6 @@
 
 #ifdef ENABLE_DEBUG
 extern bool debug_bmp;
-int usbuart_debug_write(const char *buf, size_t len);
 # define DEBUG printf
 #else
 # define DEBUG(...)
diff --git a/src/platforms/f3/platform.h b/src/platforms/f3/platform.h
index e4310d9..2c46a72 100644
--- a/src/platforms/f3/platform.h
+++ b/src/platforms/f3/platform.h
@@ -138,7 +138,6 @@
 
 #ifdef ENABLE_DEBUG
 extern bool debug_bmp;
-int usbuart_debug_write(const char *buf, size_t len);
 # define DEBUG printf
 #else
 # define DEBUG(...)
diff --git a/src/platforms/native/platform.h b/src/platforms/native/platform.h
index 7ba6cd5..776a8a4 100644
--- a/src/platforms/native/platform.h
+++ b/src/platforms/native/platform.h
@@ -36,7 +36,6 @@
 # define PLATFORM_HAS_DEBUG
 # define USBUART_DEBUG
 extern bool debug_bmp;
-int usbuart_debug_write(const char *buf, size_t len);
 #endif
 
 #define PLATFORM_IDENT          " "
diff --git a/src/platforms/stlink/platform.h b/src/platforms/stlink/platform.h
index 206f96b..50bdf03 100644
--- a/src/platforms/stlink/platform.h
+++ b/src/platforms/stlink/platform.h
@@ -36,7 +36,6 @@
 # define PLATFORM_HAS_DEBUG
 # define USBUART_DEBUG
 extern bool debug_bmp;
-int usbuart_debug_write(const char *buf, size_t len);
 #endif
 
 #define PLATFORM_HAS_USBUART
diff --git a/src/platforms/stm32/usbuart.c b/src/platforms/stm32/usbuart.c
index fc849f8..0c390c5 100644
--- a/src/platforms/stm32/usbuart.c
+++ b/src/platforms/stm32/usbuart.c
@@ -319,7 +319,7 @@ void usbuart_usb_out_cb(usbd_device *dev, uint8_t ep)
 #endif
 
 #ifdef USBUART_DEBUG
-int usbuart_debug_write(const char *buf, size_t len)
+size_t usbuart_debug_write(const char *buf, const size_t len)
 {
 	if (nvic_get_active_irq(USB_IRQ) || nvic_get_active_irq(USBUSART_IRQ) || nvic_get_active_irq(USBUSART_DMA_RX_IRQ))
 		return 0;
diff --git a/src/platforms/swlink/platform.h b/src/platforms/swlink/platform.h
index dab4ab5..635e192 100644
--- a/src/platforms/swlink/platform.h
+++ b/src/platforms/swlink/platform.h
@@ -33,7 +33,6 @@
 # define PLATFORM_HAS_DEBUG
 # define USBUART_DEBUG
 extern bool debug_bmp;
-int usbuart_debug_write(const char *buf, size_t len);
 #endif
 
 #define PLATFORM_HAS_USBUART