[PATCH] boot: Pass baud rate to stdout-path

[Mark Kettenis]

> From: John Watts <contact at jookia.org>
> Date: Thu, 11 Apr 2024 15:03:20 +1000
> 
> Linux might use the wrong baud rate such as 9600 by default, make sure
> to specify it when passing the serial port over.
> 
> Signed-off-by: John Watts <contact at jookia.org>
> ---
> On my board at least (a sunxi T113) the serial console will initialize
> as 9600 baud instead of the set baud. Pass the baud with the serial
> device to Linux to solve this issue.

You probably should fix this by making sure the device tree you're
using has the appropriate stdout-path node.  Because I think the
functionality you're trying to use here is deprecated:

* The linux,stdout-path property has been superseded by the
  stdout-path property.

* The description of the OF_STDOUT_VIA_ALIAS option suggests that it
  is seprecated as well:

    This option currently references CONFIG_CONS_INDEX, which is
    incorrect when used with device tree as this option does not
    exist / should not be used.

  It just happens that sunxi is one of the few remaining "modenr"
  platforms that still uses CONFIG_CONS_INDEX.

That said, the diff is interesting.  To me it doesn't really make
sense that you can change the serial port and its parameters in
U-Boot, but that this choice doesn't always make it into the device
tree that is passed to the OS.

A particular case that I'm dealing with is the default speed of
1500000 that the various Rockchip SoCs use.  This doesn't work with
many of the USB-to-serial interfaces on the market and is also rather
susceptible to line noise.  So for OpenBSD packages I've decide to use
115200 instead.  But that means I have to patch all the device trees
in addition to changing the CONFIG_BAUDRATE setting.  If U-Boot would
tweak the stdout-path property based on CONFIG_BAUDRATE that would
make things easier.

Cheers,

Mark

> ---
>  boot/fdt_support.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/boot/fdt_support.c b/boot/fdt_support.c
> index 090d82ee80..83e62f47b5 100644
> --- a/boot/fdt_support.c
> +++ b/boot/fdt_support.c
> @@ -153,9 +153,9 @@ static int fdt_fixup_stdout(void *fdt, int chosenoff)
>  	}
>  
>  	/* fdt_setprop may break "path" so we copy it to tmp buffer */
> -	memcpy(tmp, path, len);
> +	len = sprintf(tmp, "%.*s:%d", len, (char *)path, CONFIG_BAUDRATE);
>  
> -	err = fdt_setprop(fdt, chosenoff, "linux,stdout-path", tmp, len);
> +	err = fdt_setprop(fdt, chosenoff, "linux,stdout-path", tmp, len + 1);
>  	if (err < 0)
>  		printf("WARNING: could not set linux,stdout-path %s.\n",
>  		       fdt_strerror(err));
> 
> ---
> base-commit: 777c28460947371ada40868dc994dfe8537d7115
> change-id: 20240411-stdout-4f91b566a0f2
> 
> Best regards,
> -- 
> John Watts <contact at jookia.org>
> 
>