x86: coreboot: Document how to enable the debug UART

This is not obvious so add a little note about how it works.

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

diff --git a/doc/board/coreboot/coreboot.rst b/doc/board/coreboot/coreboot.rst
index 4a5f101cad2e6d981d4340d6d9f8138dd61ad098..0fe95af56d2db60434d30589b86e3fd3855461b8 100644 (file)
--- a/doc/board/coreboot/coreboot.rst
+++ b/doc/board/coreboot/coreboot.rst
@@ -71,3 +71,32 @@ Memory map
               (typically redirects to 7ab10030 or similar)
          500  Location of coreboot sysinfo table, used during startup
   ==========  ==================================================================
+
+
+Debug UART
+----------
+
+It is possible to enable the debug UART with coreboot. To do this, use the
+info from the cbsysinfo command to locate the UART base. For example::
+
+   => cbsysinfo
+   ...
+   Serial I/O port: 00000000
+      base        : 00000000
+      pointer     : 767b51bc
+      type        : 2
+      base        : fe03e000
+      baud        : 0d115200
+      regwidth    : 4
+      input_hz    : 0d1843200
+      PCI addr    : 00000010
+   ...
+
+Here you can see that the UART base is fe03e000, regwidth is 4 (1 << 2) and the
+input clock is 1843200. So you can add the following CONFIG options::
+
+   CONFIG_DEBUG_UART=y
+   CONFIG_DEBUG_UART_BASE=fe03e000
+   CONFIG_DEBUG_UART_CLOCK=1843200
+   CONFIG_DEBUG_UART_SHIFT=2
+   CONFIG_DEBUG_UART_ANNOUNCE=y