serial: Support an early UART for debugging

This came up in a discussion on the mailing list here:

https://patchwork.ozlabs.org/patch/384613/

My concerns at the time were:
- it doesn't need to be written in assembler
- it doesn't need to be ARM-specific

This patch provides a possible alternative. It works by allowing any serial
driver to export one init function and provide a putc() function. These
can be used to output debug data before the real serial driver is available.

This implementation does not depend on driver model, and it is possible for
it to operate without a stack on some architectures (e.g. PowerPC, ARM). It
provides the same features as the ARM-specific debug.S but with more UART
and architecture support.

Signed-off-by: Simon Glass <sjg@chromium.org>

1 files changed, 46 insertions, 0 deletions

diff --git a/drivers/serial/Kconfig b/drivers/serial/Kconfig
index c94353ba6a..6313258eac 100644
--- a/drivers/serial/Kconfig
+++ b/drivers/serial/Kconfig
@@ -7,6 +7,52 @@ config DM_SERIAL
 	  implements serial_putc() etc. The uclass interface is
 	  defined in include/serial.h.
 
+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