[PATCH] serial: goldfish: Add DEBUG_UART support

Daniel Palmer
2026-03-09T02:33:22Z

Add debug support for the goldfish tty so it can be used for
early debugging. This will be really useful when adding support
for relocation etc.

Signed-off-by: Daniel Palmer <daniel@0x0f.com>
---
doc/board/emulation/qemu-m68k.rst | 10 ++++++++++
drivers/serial/Kconfig | 8 ++++++++
drivers/serial/serial_goldfish.c | 18 ++++++++++++++++++
3 files changed, 36 insertions(+)

diff --git a/doc/board/emulation/qemu-m68k.rst b/doc/board/emulation/qemu-m68k.rst
index 6c4de54cf6a8..2f2e6bd6444a 100644
--- 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:
diff --git a/drivers/serial/Kconfig b/drivers/serial/Kconfig
index b84cb9ec781d..f6a696e6c94c 100644
--- a/drivers/serial/Kconfig
+++ b/drivers/serial/Kconfig
@@ -510,6 +510,14 @@ config DEBUG_UART_XTENSA_SEMIHOSTING
start up driver model. The driver will be available until the real
driver model serial is running.

+config DEBUG_UART_GOLDFISH
+ bool "Goldfish TTY"
+ help
+ Select this to enable the debug UART using the Goldfish TTY driver.
+ This provides basic serial output from the console without needing to
+ start up driver model. The driver will be available until the real
+ driver model serial is running.
+
endchoice

config DEBUG_UART_BASE
diff --git a/drivers/serial/serial_goldfish.c b/drivers/serial/serial_goldfish.c
index 4ac2cfb62315..58c494918786 100644
--- a/drivers/serial/serial_goldfish.c
+++ b/drivers/serial/serial_goldfish.c
@@ -115,3 +115,21 @@ U_BOOT_DRIVER(serial_goldfish) = {
.priv_auto = sizeof(struct goldfish_tty_priv),
.flags = DM_FLAG_PRE_RELOC,
};
+
+#ifdef CONFIG_DEBUG_UART_GOLDFISH
+
+#include <debug_uart.h>
+
+static inline void _debug_uart_init(void)
+{
+}
+
+static inline void _debug_uart_putc(int ch)
+{
+ void *base = (void *)CONFIG_DEBUG_UART_BASE;
+
+ __raw_writel(ch, base + GOLDFISH_TTY_PUT_CHAR);
+}
+
+DEBUG_UART_FUNCS
+#endif
--
2.51.0