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

adamsimonelli@gmail.com posted 2 patches 10 months ago
There is a newer version of this series
[PATCH v2 1/2] ttynull: Add an option to allow ttynull to be used as a console device
Posted by adamsimonelli@gmail.com 10 months ago 
From: Adam Simonelli <adamsimonelli@gmail.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@gmail.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
 };
 
 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);
+
+	return 0;
+}
 
+#ifdef CONFIG_NULL_TTY_CONSOLE
+static int __init ttynull_register(void)
+{
+	register_console(&ttynull_console);
 	return 0;
 }
+console_initcall(ttynull_register);
+#endif
 
 static void __exit ttynull_exit(void)
 {
-- 
2.45.2
Re: [PATCH v2 1/2] ttynull: Add an option to allow ttynull to be used as a console device
Posted by Greg Kroah-Hartman 10 months ago 
On Sun, Feb 16, 2025 at 11:07:47PM -0500, adamsimonelli@gmail.com wrote:
> From: Adam Simonelli <adamsimonelli@gmail.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@gmail.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
Re: [PATCH v2 1/2] ttynull: Add an option to allow ttynull to be used as a console device
Posted by Adam Simonelli 9 months, 4 weeks ago 
On Wednesday, February 19, 2025 9:00:07 AM EST Greg Kroah-Hartman wrote:
> On Sun, Feb 16, 2025 at 11:07:47PM -0500, adamsimonelli@gmail.com wrote:
> > From: Adam Simonelli <adamsimonelli@gmail.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@gmail.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?
> 
> >  };
> >  
Maybe IS_ENABLED() is better?  
> >  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?
I thought I had to add that in the console_initcall function because other
drivers seem to have that.
I want it to still work when it is just CONFIG_NULL_TTY without
CONFIG_NULL_TTY_CONSOLE, so I will guard both with console_is_registered
> 
> > +	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,
> 
OK, I will guard both. 
> greg k-h
>

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