diff --git a/drivers/serial/uart_handlers.c b/drivers/serial/uart_handlers.c
index 3212841977..44620d4c45 100644
--- a/drivers/serial/uart_handlers.c
+++ b/drivers/serial/uart_handlers.c
@@ -49,7 +49,7 @@ static inline void z_vrfy_uart_poll_out(struct device *dev,
  */
 
 static inline int z_vrfy_uart_tx(struct device *dev, const u8_t *buf,
-				 size_t len, u32_t timeout)
+				 size_t len, s32_t timeout)
 {
 	Z_OOPS(Z_SYSCALL_DRIVER_UART(dev, tx));
 	Z_OOPS(Z_SYSCALL_MEMORY_READ(buf, len));
@@ -61,7 +61,7 @@ UART_SIMPLE(tx_abort);
 #include <syscalls/uart_tx_abort_mrsh.c>
 
 static inline int z_vrfy_uart_rx_enable(struct device *dev, u8_t *buf,
-					size_t len, u32_t timeout)
+					size_t len, s32_t timeout)
 {
 	Z_OOPS(Z_SYSCALL_DRIVER_UART(dev, rx_enable));
 	Z_OOPS(Z_SYSCALL_MEMORY_WRITE(buf, len));
diff --git a/include/drivers/uart.h b/include/drivers/uart.h
index efb6b3240b..4de313e670 100644
--- a/include/drivers/uart.h
+++ b/include/drivers/uart.h
@@ -350,11 +350,11 @@ struct uart_driver_api {
 			    void *user_data);
 
 	int (*tx)(struct device *dev, const u8_t *buf, size_t len,
-		  u32_t timeout);
+		  s32_t timeout);
 	int (*tx_abort)(struct device *dev);
 
 	int (*rx_enable)(struct device *dev, u8_t *buf, size_t len,
-			 u32_t timeout);
+			 s32_t timeout);
 	int (*rx_buf_rsp)(struct device *dev, u8_t *buf, size_t len);
 	int (*rx_disable)(struct device *dev);
 
@@ -460,16 +460,17 @@ static inline int uart_callback_set(struct device *dev,
  * @param dev     UART device structure.
  * @param buf     Pointer to transmit buffer.
  * @param len     Length of transmit buffer.
- * @param timeout Timeout in milliseconds. Valid only if flow control is enabled
+ * @param timeout Timeout in milliseconds. Valid only if flow control is
+ *		  enabled. K_FOREVER disables timeout.
  *
  * @retval -EBUSY There is already an ongoing transfer.
  * @retval 0	  If successful, negative errno code otherwise.
  */
 __syscall int uart_tx(struct device *dev, const u8_t *buf, size_t len,
-		      u32_t timeout);
+		      s32_t timeout);
 
 static inline int z_impl_uart_tx(struct device *dev, const u8_t *buf,
-				 size_t len, u32_t timeout)
+				 size_t len, s32_t timeout)
 
 {
 	const struct uart_driver_api *api =
@@ -508,17 +509,17 @@ static inline int z_impl_uart_tx_abort(struct device *dev)
  * @param dev     UART device structure.
  * @param buf     Pointer to receive buffer.
  * @param len     Buffer length.
- * @param timeout Timeout in milliseconds.
+ * @param timeout Timeout in milliseconds. K_FOREVER disables timeout.
  *
  * @retval -EBUSY RX already in progress.
  * @retval 0	  If successful, negative errno code otherwise.
  *
  */
 __syscall int uart_rx_enable(struct device *dev, u8_t *buf, size_t len,
-			     u32_t timeout);
+			     s32_t timeout);
 
 static inline int z_impl_uart_rx_enable(struct device *dev, u8_t *buf,
-					size_t len, u32_t timeout)
+					size_t len, s32_t timeout)
 {
 	const struct uart_driver_api *api =
 				(const struct uart_driver_api *)dev->driver_api;