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

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Message-ID: <4647568.Icojqenx9y@nerdopolis2>
Date: Mon, 03 Mar 2025 22:52:29 -0500
From: Adam Simonelli <adamsimonelli@...il.com>
To: Petr Mladek <pmladek@...e.com>
Cc: linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
 Jiri Slaby <jirislaby@...nel.org>,
 Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 Andy Shevchenko <andy.shevchenko@...il.com>,
 Steven Rostedt <rostedt@...dmis.org>,
 John Ogness <john.ogness@...utronix.de>,
 Sergey Senozhatsky <senozhatsky@...omium.org>
Subject:
 Re: [PATCH v5 1/2] ttynull: Add an option to allow ttynull to be used as a
 console device

On Monday, March 3, 2025 8:20:11 AM EST Petr Mladek wrote:
> 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@...il.com wrote:
> > > > From: Adam Simonelli <adamsimonelli@...il.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.
> 
OK, will do. This makes sense.
> > > 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.
> 
Understood, this makes sense.
> > > >  };
> > > >  
> > > 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
> 
Thanks for that, that works.