On Mon 2025-03-10 23:31:31, adamsimonelli@xxxxxxxxx wrote:
> From: Adam Simonelli <adamsimonelli@xxxxxxxxx>
>
> The new config option, CONFIG_NULL_TTY_DEFAULT_CONSOLE will allow
> ttynull to be initialized by console_initcall() and selected as a
> possible console device.
It would be great to mention here the motivation. It is nicely
explained in the 3rd patch.
> --- a/drivers/tty/Kconfig
> +++ b/drivers/tty/Kconfig
> @@ -383,7 +383,20 @@ config NULL_TTY
> available or desired.
>
> In order to use this driver, you should redirect the console to this
> - TTY, or boot the kernel with console=ttynull.
> + TTY, boot the kernel with console=ttynull, or enable
> + NULL_TTY_DEFAULT_CONSOLE.
> +
> + If unsure, say N.
> +
> +config NULL_TTY_DEFAULT_CONSOLE
> + bool "Support for console on ttynull"
> + depends on NULL_TTY=y && !VT_CONSOLE
> + help
> + Say Y here if you want the NULL TTY to be used as a /dev/console
> + device.
I would provide few more details, something like:
<proposal>
Say Y here if you want the NULL TTY to be used as a /dev/console
device by default.
For example, it might be useful to prevent a VT-less kernel from
writing the system log to a random device connected to
the serial port.
Another console driver still might get preferred via the command line,
SPCR, or the device tree.
</proposal>
> + This is similar to VT_CONSOLE, but without the dependency on VT.
> + It uses the ttynull driver as the system console.
Honestly, I do not find this paragraph much useful. I would omit it,
> If unsure, say N.
>
> diff --git a/drivers/tty/ttynull.c b/drivers/tty/ttynull.c
> index d8b51edde628..67aad3e6f538 100644
> --- a/drivers/tty/ttynull.c
> +++ b/drivers/tty/ttynull.c
> @@ -96,6 +96,19 @@ static int __init ttynull_init(void)
> return 0;
> }
>
> +#ifdef CONFIG_NULL_TTY_DEFAULT_CONSOLE
> +static int __init ttynull_register(void)
> +{
> + if (console_set_on_cmdline)
> + return 0;
> +
> + add_preferred_console("ttynull", 0, NULL);
> +
> + return 0;
> +}
> +console_initcall(ttynull_register);
> +#endif
I have realized that this does not work without shuffling the linking
order (3rd patch).
I would prefer to avoid the linking order hack and rather call this
in console_init() directly, see
https://lore.kernel.org/r/Z9GPVVTnngGbmbuv@xxxxxxxxxxxxxxx
for more details.
Best Regards,
Petr
