diff mbox series

[v8,2/4] ttynull: Add an option to allow ttynull to be used as a console device

Message ID 20250311033133.1859963-3-adamsimonelli@gmail.com
State New
Headers show
Series Optionally allow ttynull to be selected as a default console | expand

Commit Message

Adam Simonelli March 11, 2025, 3:31 a.m. UTC
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 | 13 +++++++++++++
 2 files changed, 27 insertions(+), 1 deletion(-)

Comments

Petr Mladek March 12, 2025, 3:37 p.m. UTC | #1
On Mon 2025-03-10 23:31:31, 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.

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@pathway.suse.cz
for more details.

Best Regards,
Petr
diff mbox series

Patch

diff --git a/drivers/tty/Kconfig b/drivers/tty/Kconfig
index 63a494d36a1f..edf557c73586 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
+	  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 VT_CONSOLE, but without the dependency on 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..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
+
 static void __exit ttynull_exit(void)
 {
 	unregister_console(&ttynull_console);