From: Adam Simonelli <adamsimonelli@gmail.com>

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.

Signed-off-by: Adam Simonelli <adamsimonelli@gmail.com>
---
 drivers/tty/Kconfig   | 15 ++++++++++++++-
 drivers/tty/ttynull.c | 11 +++++++++++
 2 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/drivers/tty/Kconfig b/drivers/tty/Kconfig
index 63a494d36a1f..868aec2ccbb3 100644
--- 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
+	  CONFIG_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.
+
+	  This is similar to CONFIG_VT_CONSOLE, but without the dependency on
+	  CONFIG_VT. It uses the ttynull driver as the system console.
 
 	  If unsure, say N.
 
diff --git a/drivers/tty/ttynull.c b/drivers/tty/ttynull.c
index d8b51edde628..cb1e363b726d 100644
--- a/drivers/tty/ttynull.c
+++ b/drivers/tty/ttynull.c
@@ -96,6 +96,17 @@ 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)
+		add_preferred_console("ttynull", 0, NULL);;
+
+	return 0;
+}
+console_initcall(ttynull_register);
+#endif
+
 static void __exit ttynull_exit(void)
 {
 	unregister_console(&ttynull_console);
-- 
2.45.2

On Tue, Mar 4, 2025 at 5:55 AM <adamsimonelli@gmail.com> wrote:
>
> From: Adam Simonelli <adamsimonelli@gmail.com>
>
> 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.

...

>           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
> +         CONFIG_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.
> +
> +         This is similar to CONFIG_VT_CONSOLE, but without the dependency on
> +         CONFIG_VT. It uses the ttynull driver as the system console.

I'm now at the non-monospace font mail client, but it looks like you
have a TAB/space mix in the above for the indentation. Please, double
check that all lines, except the first in the option starts with a
single TAB and help lines (after the 'help' keyword) additionally
indented by 2 (two) spaces.

>           If unsure, say N.

...

> +#ifdef CONFIG_NULL_TTY_DEFAULT_CONSOLE
> +static int __init ttynull_register(void)
> +{
> +       if (!console_set_on_cmdline)

Invert this. It might be that we want more in the future and this
conditional is not-scalable for that. Inverted does the trick. To be
precise, do like the fb driver for xen does:

  /* User defined console is present, avoid registering "ttynull" */
  if (..._set_on_cmdline)
    return 0;

> +               add_preferred_console("ttynull", 0, NULL);;
> +
> +       return 0;
> +}
> +console_initcall(ttynull_register);
> +#endif

-- 
With Best Regards,
Andy Shevchenko