linux-kernel - Re: [PATCH printk v3 24/40] tty: nfcon: use console_is

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Message-ID: <Y20DoLPsrfgm528O@alley>
Date:   Thu, 10 Nov 2022 14:58:56 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        linux-m68k@...ts.linux-m68k.org
Subject: Re: [PATCH printk v3 24/40] tty: nfcon: use console_is_registered()

On Mon 2022-11-07 15:22:22, John Ogness wrote:
> Currently CON_ENABLED is being (mis)used to identify if the console
> has been registered. This is not reliable because it can be set even
> though registration failed or it can be unset, even though the console
> is registered.
>
> Signed-off-by: John Ogness <john.ogness@...utronix.de>
> ---
>  arch/m68k/emu/nfcon.c | 10 ++++------
>  1 file changed, 4 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/m68k/emu/nfcon.c b/arch/m68k/emu/nfcon.c
> index 557d60867f98..292669fa480f 100644
> --- a/arch/m68k/emu/nfcon.c
> +++ b/arch/m68k/emu/nfcon.c
> @@ -49,14 +49,14 @@ static void nfcon_write(struct console *con, const char *str,
>  static struct tty_driver *nfcon_device(struct console *con, int *index)
>  {
>  	*index = 0;
> -	return (con->flags & CON_ENABLED) ? nfcon_tty_driver : NULL;
> +	return console_is_registered(con) ? nfcon_tty_driver : NULL;
>  }
>  
>  static struct console nf_console = {
>  	.name	= "nfcon",
>  	.write	= nfcon_write,
>  	.device	= nfcon_device,
> -	.flags	= CON_PRINTBUFFER,
> +	.flags	= CON_PRINTBUFFER | CON_ENABLED,

This causes that register_console() will always put the console into
console_list. It causes regression, see below.


>  	.index	= -1,
>  };
>  
> @@ -106,10 +106,8 @@ static int __init nf_debug_setup(char *arg)
>  		return 0;
>  
>  	stderr_id = nf_get_id("NF_STDERR");
> -	if (stderr_id) {
> -		nf_console.flags |= CON_ENABLED;
> +	if (stderr_id)
>  		register_console(&nf_console);

My understanding is that this should enable the console
when debug=nfcon kernel parameter is used.

It is a non-standard way. This is why CON_ENABLED flag
has to be explicitly set.

> -	}
>  
>  	return 0;
>  }
> @@ -151,7 +149,7 @@ static int __init nfcon_init(void)
>  
>  	nfcon_tty_driver = driver;
>  
> -	if (!(nf_console.flags & CON_ENABLED))
> +	if (!console_is_registered(&nf_console))
>  		register_console(&nf_console);

This calls register_console() when it was not already
registered by the debug=nfcon early parameter.

It is the standard way. It should enable the console only
when console=nfcon is defined on the commandline. Or when
there is no preferred console and no console enabled by default yet.

We should not pre-set CON_ENABLED in this case.

Best Regards,
Petr