debug_uart: Support board-specific UART initialisation

Some boards need to set things up before the debug UART can be used. On
these boards a call to debug_uart_init() is insufficient. When this option
is enabled, the function board_debug_uart_init() will be called when
debug_uart_init() is called. You can put any code here that is needed to
set up the UART ready for use, such as set pin multiplexing or enable
clocks.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>

drivers/serial/Kconfig

@@ -109,6 +109,17 @@ config DEBUG_UART_SHIFT
 	  value. Use this value to specify the shift to use, where 0=byte
 	  registers, 2=32-bit word registers, etc.
 
+config DEBUG_UART_BOARD_INIT
+	bool "Enable board-specific debug UART init"
+	depends on DEBUG_UART
+	help
+	  Some boards need to set things up before the debug UART can be used.
+	  On these boards a call to debug_uart_init() is insufficient. When
+	  this option is enabled, the function board_debug_uart_init() will
+	  be called when debug_uart_init() is called. You can put any code
+	  here that is needed to set up the UART ready for use, such as set
+	  pin multiplexing or enable clocks.
+
 config ROCKCHIP_SERIAL
 	bool "Rockchip on-chip UART support"
 	depends on ARCH_ROCKCHIP && DM_SERIAL

include/debug_uart.h

@@ -42,6 +42,11 @@
  * - Define _debug_uart_putc() as static inline (avoiding stack usage)
  * - Immediately afterwards, add DEBUG_UART_FUNCS to define the rest of the
  *     functionality (printch(), etc.)
+ *
+ * If your board needs additional init for the UART to work, enable
+ * CONFIG_DEBUG_UART_BOARD_INIT and write a function called
+ * board_debug_uart_init() to perform that init. When debug_uart_init() is
+ * called, the init will happen automatically.
  */
 
 /**
@@ -57,6 +62,14 @@
  */
 void debug_uart_init(void);
 
+#ifdef CONFIG_DEBUG_UART_BOARD_INIT
+void board_debug_uart_init(void);
+#else
+static inline void board_debug_uart_init(void)
+{
+}
+#endif
+
 /**
  * printch() - Output a character to the debug UART
  *
@@ -136,6 +149,7 @@ void printhex8(uint value);
 \
 	void debug_uart_init(void) \
 	{ \
+		board_debug_uart_init(); \
 		_debug_uart_init(); \
 	} \