[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 at 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
>