[PATCH] serial: goldfish: Add DEBUG_UART support

[PATCH] serial: goldfish: Add DEBUG_UART support

[Daniel Palmer]

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

Re: [PATCH] serial: goldfish: Add DEBUG_UART support

[Kuan-Wei Chiu]

On Mon, Mar 09, 2026 at 11:32:34AM +0900, Daniel Palmer wrote:
> 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(+)

I think we should also call debug_uart_init() in qemu-m68k.c. This way,
the <debug_uart> string will be properly printed when the
CONFIG_DEBUG_UART_ANNOUNCE option is enabled.

> 
> 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;

It might be better to use CONFIG_VAL(DEBUG_UART_BASE) here to keep it
robust for potential future SPL/TPL builds.

Also, since this is a mmio register, I think casting to void __iomem *
would be more appropriate.

Regards,
Kuan-Wei

> +
> +	__raw_writel(ch, base + GOLDFISH_TTY_PUT_CHAR);
> +}
> +
> +DEBUG_UART_FUNCS
> +#endif
> -- 
> 2.51.0
> 

Re: [PATCH] serial: goldfish: Add DEBUG_UART support

[Daniel Palmer]

Hi Kuan-Wei,

On Mon, 9 Mar 2026 at 18:10, Kuan-Wei Chiu <visitorckw@gmail.com> wrote:

> I think we should also call debug_uart_init() in qemu-m68k.c. This way,
> the <debug_uart> string will be properly printed when the
> CONFIG_DEBUG_UART_ANNOUNCE option is enabled.

I looked up the other callers to debug_uart_init() and it looks like a
lot of them are in the start.S for the arch.
Maybe it should be added to arch/m68k/cpu/m680x0/start.S?

> > +     void *base = (void *)CONFIG_DEBUG_UART_BASE;
>
> It might be better to use CONFIG_VAL(DEBUG_UART_BASE) here to keep it
> robust for potential future SPL/TPL builds.
>
> Also, since this is a mmio register, I think casting to void __iomem *
> would be more appropriate.

Noted. I'll fix that up.

Cheers,

Daniel