serial: atmel_usart: Support enable an early debug UART

Add support to enable an early debug UART for debugging.

Signed-off-by: Wenyou Yang <wenyou.yang@atmel.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Andreas Bießmann <andreas@biessmann.org>

drivers/serial/Kconfig

@@ -98,6 +98,13 @@ config DEBUG_UART_AR933X
 	  driver will be available until the real driver model serial is
 	  running.
 
+config DEBUG_UART_ATMEL
+	bool "Atmel USART"
+	help
+	  Select this to enable a debug UART using the atmel usart driver. You
+	  will need to provide parameters to make this work. The driver will
+	  be available until the real driver-model serial is running.
+
 config DEBUG_UART_NS16550
 	bool "ns16550"
 	help

drivers/serial/atmel_usart.c

@@ -11,6 +11,7 @@
 #include <errno.h>
 #include <watchdog.h>
 #include <serial.h>
+#include <debug_uart.h>
 #include <linux/compiler.h>
 
 #include <asm/io.h>
@@ -226,3 +227,24 @@ U_BOOT_DRIVER(serial_atmel) = {
 	.priv_auto_alloc_size	= sizeof(struct atmel_serial_priv),
 };
 #endif
+
+#ifdef CONFIG_DEBUG_UART_ATMEL
+static inline void _debug_uart_init(void)
+{
+	atmel_usart3_t *usart = (atmel_usart3_t *)CONFIG_DEBUG_UART_BASE;
+
+	atmel_serial_setbrg_internal(usart, 0, CONFIG_BAUDRATE);
+}
+
+static inline void _debug_uart_putc(int ch)
+{
+	atmel_usart3_t *usart = (atmel_usart3_t *)CONFIG_DEBUG_UART_BASE;
+
+	while (!(readl(&usart->csr) & USART3_BIT(TXRDY)))
+		;
+
+	writel(ch, &usart->thr);
+}
+
+DEBUG_UART_FUNCS
+#endif