serial: mxc: Add debug uart support

Add support for the debug UART to assist with early debugging.
Enable it for i.CoreM6 as an example.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

configs/imx6qdl_icore_mmc_defconfig

@@ -45,3 +45,7 @@ CONFIG_PINCTRL_IMX6=y
 CONFIG_MXC_UART=y
 CONFIG_IMX_THERMAL=y
 CONFIG_VIDEO_IPUV3=y
+CONFIG_DEBUG_UART=y
+CONFIG_DEBUG_UART_MXC=y
+CONFIG_DEBUG_UART_BASE=0x021f0000
+CONFIG_DEBUG_UART_CLOCK=24000000

drivers/serial/Kconfig

@@ -248,6 +248,14 @@ config DEBUG_UART_PIC32
 	  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_MXC
+	bool "IMX Serial port"
+	depends on MXC_UART
+	help
+	  Select this to enable a debug UART using the serial_mxc 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_UNIPHIER
 	bool "UniPhier on-chip UART"
 	depends on ARCH_UNIPHIER

drivers/serial/serial_mxc.c

@@ -357,3 +357,29 @@ U_BOOT_DRIVER(serial_mxc) = {
 	.flags = DM_FLAG_PRE_RELOC,
 };
 #endif
+
+#ifdef CONFIG_DEBUG_UART_MXC
+#include <debug_uart.h>
+
+static inline void _debug_uart_init(void)
+{
+	struct mxc_uart *base = (struct mxc_uart *)CONFIG_DEBUG_UART_BASE;
+
+	_mxc_serial_init(base);
+	_mxc_serial_setbrg(base, CONFIG_DEBUG_UART_CLOCK,
+			   CONFIG_BAUDRATE, false);
+}
+
+static inline void _debug_uart_putc(int ch)
+{
+	struct mxc_uart *base = (struct mxc_uart *)CONFIG_DEBUG_UART_BASE;
+
+	while (!(readl(&base->ts) & UTS_TXEMPTY))
+		WATCHDOG_RESET();
+
+	writel(ch, &base->txd);
+}
+
+DEBUG_UART_FUNCS
+
+#endif