Re: [PATCH v5 1/2] ttynull: Add an option to allow ttynull to be used as a console device

[Petr Mladek <pmladek@suse.com>]

On Wed 2025-02-26 08:39:23, Adam Simonelli wrote:
> On Tuesday, February 25, 2025 11:19:04 AM EST Petr Mladek wrote:
> > On Mon 2025-02-24 07:39:14, adamsimonelli@gmail.com wrote:
> > > From: Adam Simonelli <adamsimonelli@gmail.com>
> > > 
> > > The new config option, CONFIG_NULL_TTY_CONSOLE will allow ttynull to be
> > > initialized by console_initcall() and selected as a possible console
> > > device.
> > > 
> > > diff --git a/drivers/tty/ttynull.c b/drivers/tty/ttynull.c
> > > index 6b2f7208b564..ec3dd3fd41c0 100644
> > > --- a/drivers/tty/ttynull.c
> > > +++ b/drivers/tty/ttynull.c
> > > @@ -57,6 +57,13 @@ static struct tty_driver *ttynull_device(struct console *c, int *index)
> > >  static struct console ttynull_console = {
> > >  	.name = "ttynull",
> > >  	.device = ttynull_device,
> > > +
> > > +	/*
> > > +	 * Match the index and flags from other boot consoles when CONFIG_NULL_TTY_CONSOLE is
> > > +	 * enabled, otherwise, use the default values for the index and flags.
> > > +	*/
> > > +	.index = IS_ENABLED(CONFIG_NULL_TTY_CONSOLE) ? -1 : 0,
> > 
> > This should not be needed. "con->index" is always initialized to "0"
> > for the default console, see:
> > 
> OK, I had this in an #ifdef before, it was the cleanest way to set it to -1
> that I could think of, other than the ifdef... If I still need this, I will try
> to think of something else to set it to -1 when the option is enabled

Ah, I was not clear enough. It should be perfectly fine to always
statically initialize the value to -1. We should not need any
#ifdef or IS_ENABLED.

I mean to do:

static struct console ttynull_console = {
	.name = "ttynull",
	.device = ttynull_device,
	.index = -1,
};

We might even do this in a separate patch. IMHO, it should have
been done this way since the beginning.

> > static void try_enable_default_console(struct console *newcon)
> > {
> > 	if (newcon->index < 0)
> > 		newcon->index = 0;
> > [...]
> > }
> > 
> > > +	.flags = IS_ENABLED(CONFIG_NULL_TTY_CONSOLE) ? CON_PRINTBUFFER : 0,
> > 
> > This does not make much sense to me.
> > 
> > CON_PRINTBUFFER prevents duplicated output when the same device has
> > already been registered as a boot console. But ttynull does not have
> > a boot console variant. Also it is a "null" device. It never prints
> > anything. The output could never be duplicated by definition.
> > 
> OK, I was duplicating what I saw in other consoles. I can try to remove it

Again, I was not clear enough. My primary concern was that it did not make
much sense to use the IS_ENABLED() check and initialize the value
different way.

Anyway, I would omit the flag. It is a NULL device. It does not matter
whether it prints existing (old) messages during registration or not.

> > >  };
> > >  
> > My proposal is to call:
> > 
> > #ifdef CONFIG_NULL_TTY_DEFAULT_CONSOLE
> > static int __init ttynull_default_console(void)
> > {
> > 	add_preferred_console("ttynull", 0, NULL);
> > 	return 0;
> > }
> > console_initcall(ttynull_register);
> > #endif
> > 
> OK, actually in earlier revisions locally, I did actually have
> 
> 
> 
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index dddb15f48d59..c1554a789de8 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -3712,6 +3712,11 @@ void __init console_init(void)
>  	initcall_t call;
>  	initcall_entry_t *ce;
>  
> +#ifdef CONFIG_NULL_TTY_CONSOLE
> +       if (!strstr(boot_command_line, "console="))
> +               add_preferred_console("ttynull", 0, NULL);

Good point! We should call add_preferred_console() only when
the is no console= command line parameter. Otherwise, it could
not get overridden by the command line.

We could check "console_set_on_cmdline", similar to
xenfb_make_preferred_console().

> +#endif
> +
>  	/* Setup the default TTY line discipline. */
>  	n_tty_init();
> 
> Which worked as far as I could tell, at least on x86. Not sure if that was the
> right place,

I would prefer to keep it in drivers/tty/ttynull.c when possible.
The following might do the trick:

#ifdef CONFIG_NULL_TTY_DEFAULT_CONSOLE
static int __init ttynull_default_console(void)
{
	if (!console_set_on_cmdline)
		add_preferred_console("ttynull", 0, NULL);

	return 0;
}
console_initcall(ttynull_register);
#endif

Best Regards,
Petr