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: <4305907.CQOukoFCf9@nerdopolis2>
Date: Sun, 23 Feb 2025 00:08:49 -0500
From: Adam Simonelli <adamsimonelli@...il.com>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
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 Wednesday, February 19, 2025 9:00:07 AM EST Greg Kroah-Hartman wrote:
> 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?
> 
> >  };
> >  
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
>