linux-kernel - Re: [PATCH v2 1/2] ttynull: Add an option to allow ttynull to be used as a console device

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Message-ID: <2025021920-uproot-antsy-e2c7@gregkh>
Date: Wed, 19 Feb 2025 15:00:07 +0100
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: adamsimonelli@...il.com
Cc: linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
	Jiri Slaby <jirislaby@...nel.org>
Subject: Re: [PATCH v2 1/2] ttynull: Add an option to allow ttynull to be
 used as a console device

On Sun, Feb 16, 2025 at 11:07:47PM -0500, adamsimonelli@...il.com wrote:
> From: Adam Simonelli <adamsimonelli@...il.com>
> 
> Add a config option CONFIG_NULL_TTY_CONSOLE that will have ttynull be
> initialized by console_initcall() and selected as a possible console
> device.
> 
> Signed-off-by: Adam Simonelli <adamsimonelli@...il.com>
> ---
>  drivers/tty/Kconfig   | 18 +++++++++++++++++-
>  drivers/tty/ttynull.c | 16 +++++++++++++++-
>  2 files changed, 32 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/tty/Kconfig b/drivers/tty/Kconfig
> index 63a494d36a1f..afe4e7722d4c 100644
> --- a/drivers/tty/Kconfig
> +++ b/drivers/tty/Kconfig
> @@ -383,7 +383,23 @@ 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_CONSOLE.
> +
> +	  If unsure, say N.
> +
> +config NULL_TTY_CONSOLE
> +        bool "Supports /dev/ttynull as a console automatically"
> +        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 useful for userspace applications that expect a working
> +	  console device, without changing the kernel boot options, such as a
> +	  distribuition or environment that historically had CONFIG_VT_CONSOLE
> +	  enabled, and have now disabled it, but still need /dev/console to be
> +	  working for userspace applications.
>  
>  	  If unsure, say N.
>  
> diff --git a/drivers/tty/ttynull.c b/drivers/tty/ttynull.c
> index 6b2f7208b564..8ba629ae426b 100644
> --- a/drivers/tty/ttynull.c
> +++ b/drivers/tty/ttynull.c
> @@ -57,6 +57,10 @@ static struct tty_driver *ttynull_device(struct console *c, int *index)
>  static struct console ttynull_console = {
>  	.name = "ttynull",
>  	.device = ttynull_device,
> +#ifdef CONFIG_NULL_TTY_CONSOLE
> +	.index = -1,
> +	.flags = CON_PRINTBUFFER,
> +#endif

There's no way to do this without #ifdef in the .c files?

>  };
>  
>  static int __init ttynull_init(void)
> @@ -90,10 +94,20 @@ static int __init ttynull_init(void)
>  	}
>  
>  	ttynull_driver = driver;
> -	register_console(&ttynull_console);
> +	if (!console_is_registered(&ttynull_console))
> +		register_console(&ttynull_console);
> +

Why do you register this twice?

> +	return 0;
> +}
>  
> +#ifdef CONFIG_NULL_TTY_CONSOLE
> +static int __init ttynull_register(void)
> +{
> +	register_console(&ttynull_console);

Here, why is this registered again?

You should only have to do this once, and not need to check before
trying again, right?

thanks,

greg k-h