doc: board: virt: m68k: Detail how to use debug uart

Add a paragraph to explain how to get the address of the goldfish
tty and use it as the debug uart. I think the address is actually
fixed right now but it might change in the future.

Signed-off-by: Daniel Palmer <daniel@0x0f.com>

diff --git a/doc/board/emulation/qemu-m68k.rst b/doc/board/emulation/qemu-m68k.rst
index 6c4de54cf6a8f74344214f7059c9e26f8307557d..2f2e6bd6444a09114f5efe47ec66fdc98f9ed0a4 100644 (file)
--- a/doc/board/emulation/qemu-m68k.rst
+++ b/doc/board/emulation/qemu-m68k.rst
@@ -31,6 +31,16 @@ The minimal QEMU command line to get U-Boot up and running is:
 Note that the `-nographic` option is used to redirect the console to stdio,
 which connects to the emulated Goldfish TTY device.
 
+Debugging U-Boot
+----------------
+
+If you need early debugging output enable `CONFIG_DEBUG_UART_GOLDFISH`.
+The base address for the UART can be found by activating the QEMU monitor,
+running `info qtree`, and then looking for the goldfish tty device and
+taking the mmio address.
+
+Baud rate doesn't matter.
+
 Hardware Support
 ----------------
 The following QEMU virt peripherals are supported in U-Boot: