[U-Boot,17/19] serial: Support an early UART for debugging

diff --git a/drivers/serial/Kconfig b/drivers/serial/Kconfig
index a0b6e02..268bb42 100644
--- a/drivers/serial/Kconfig
+++ b/drivers/serial/Kconfig
@@ -5,6 +5,52 @@  config DM_SERIAL
 	  If you want to use driver model for serial drivers, say Y.
 	  To use legacy serial drivers, say N.
 
+config DEBUG_UART
+	bool "Enable an early debug UART for debugging"
+	help
+	  The debug UART is intended for use very early in U-Boot to debug
+	  problems when an ICE or other debug mechanism is not available.
+
+	  To use it you should:
+	  - Make sure your UART supports this interface
+	  - Enable CONFIG_DEBUG_UART
+	  - Enable the CONFIG for your UART to tell it to provide this interface
+	        (e.g. CONFIG_DEBUG_UART_NS16550)
+	  - Define the required settings as needed (see below)
+	  - Call debug_uart_init() before use
+	  - Call debug_uart_putc() to output a character
+
+	  Depending on your platform it may be possible to use this UART before
+	  a stack is available.
+
+	  If your UART does not support this interface you can probably add
+	  support quite easily. Remember that you cannot use driver model and
+	  it is preferred to use no stack.
+
+	  You must not use this UART once driver model is working and the
+	  serial drivers are up and running (done in serial_init()). Otherwise
+	  the drivers may conflict and you will get strange output.
+
+config DEBUG_UART_BASE
+	hex "Base address of UART"
+	depends on DEBUG_UART
+	help
+	  This is the base address of your UART for memory-mapped UARTs.
+
+	  A default should be provided by your board, but if not you will need
+	  to use the correct value here.
+
+config DEBUG_UART_CLOCK
+	int "UART input clock"
+	depends on DEBUG_UART
+	help
+	  The UART input clock determines the speed of the internal UART
+	  circuitry. The baud rate is derived from this by dividing the input
+	  clock down.
+
+	  A default should be provided by your board, but if not you will need
+	  to use the correct value here.
+
 config UNIPHIER_SERIAL
 	bool "UniPhier on-chip UART support"
 	depends on ARCH_UNIPHIER && DM_SERIAL
diff --git a/include/debug_uart.h b/include/debug_uart.h
new file mode 100644
index 0000000..20d8f34
--- /dev/null
+++ b/include/debug_uart.h
@@ -0,0 +1,139 @@ 
+/*
+ * Early debug UART support
+ *
+ * (C) Copyright 2014 Google, Inc
+ * Writte by Simon Glass <sjg@chromium.org>
+ *
+ * SPDX-License-Identifier:	GPL-2.0+
+ */
+
+#ifndef _DEBUG_UART_H
+#define _DEBUG_UART_H
+
+#include <linux/linkage.h>
+
+/*
+ * The debug UART is intended for use very early in U-Boot to debug problems
+ * when an ICE or other debug mechanism is not available.
+ *
+ * To use it you should:
+ * - Make sure your UART supports this interface
+ * - Enable CONFIG_DEBUG_UART
+ * - Enable the CONFIG for your UART to tell it to provide this interface
+ *       (e.g. CONFIG_DEBUG_UART_NS16550)
+ * - Define the required settings as needed (see below)
+ * - Call debug_uart_init() before use
+ * - Call printch() to output a character
+ *
+ * Depending on your platform it may be possible to use this UART before a
+ * stack is available.
+ *
+ * If your UART does not support this interface you can probably add support
+ * quite easily. Remember that you cannot use driver model and it is preferred
+ * to use no stack.
+ *
+ * You must not use this UART once driver model is working and the serial
+ * drivers are up and running (done in serial_init()). Otherwise the drivers
+ * may conflict and you will get strange output.
+ *
+ *
+ * To enable the debug UART in your serial driver:
+ *
+ * - #include <debug_uart.h>
+ * - Define debug_uart_init(), trying to avoid using the stack
+ * - 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.)
+ */
+
+/**
+ * debug_uart_init() - Set up the debug UART ready for use
+ *
+ * This sets up the UART with the correct baud rate, etc.
+ *
+ * Available CONFIG is:
+ *
+ *    - CONFIG_DEBUG_UART_BASE: Base address of UART
+ *    - CONFIG_BAUDRATE: Requested baud rate
+ *    - CONFIG_DEBUG_UART_CLOCK: Input clock for UART
+ */
+void debug_uart_init(void);
+
+/**
+ * printch() - Output a character to the debug UART
+ *
+ * @ch:		Character to output
+ */
+void printch(int ch);
+
+/**
+ * printascii() - Output an ASCII string to the debug UART
+ *
+ * @str:	String to output
+ */
+void printascii(const char *str);
+
+/**
+ * printhex2() - Output a 2-digit hex value
+ *
+ * @value:	Value to output
+ */
+void printhex2(uint value);
+
+/**
+ * printhex4() - Output a 4-digit hex value
+ *
+ * @value:	Value to output
+ */
+void printhex4(uint value);
+
+/**
+ * printhex8() - Output a 8-digit hex value
+ *
+ * @value:	Value to output
+ */
+void printhex8(uint value);
+
+/*
+ * Now define some functions - this should be inserted into the serial driver
+ */
+#define DEBUG_UART_FUNCS \
+	asmlinkage void printch(int ch) \
+	{ \
+		_debug_uart_putc(ch); \
+	} \
+\
+	asmlinkage void printascii(const char *str) \
+	{ \
+		while (*str) \
+			_debug_uart_putc(*str++); \
+	} \
+\
+	static inline void printhex1(uint digit) \
+	{ \
+		digit &= 0xf; \
+		_debug_uart_putc(digit > 9 ? digit - 10 + 'a' : digit + '0'); \
+	} \
+\
+	static inline void printhex(uint value, int digits) \
+	{ \
+		while (digits-- > 0) \
+			printhex1(value >> (4 * digits)); \
+	} \
+\
+	asmlinkage void printhex2(uint value) \
+	{ \
+		printhex(value, 2); \
+	} \
+\
+	asmlinkage void printhex4(uint value) \
+	{ \
+		printhex(value, 4); \
+	} \
+\
+	asmlinkage void printhex8(uint value) \
+	{ \
+		printhex(value, 8); \
+	}
+
+#endif