linux-kernel - Re: [PATCH 2/2] printk/console: Allow to disable console output by using console="" or console=null

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Message-ID: <20201111225248.GB211321@roeck-us.net>
Date:   Wed, 11 Nov 2020 14:52:48 -0800
From:   Guenter Roeck <linux@...ck-us.net>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        John Ogness <john.ogness@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Shreyas Joshi <shreyas.joshi@...mp.com>,
        shreyasjoshi15@...il.com,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] printk/console: Allow to disable console output by
 using console="" or console=null

On Wed, Nov 11, 2020 at 02:54:50PM +0100, Petr Mladek wrote:
> The commit 48021f98130880dd74 ("printk: handle blank console arguments
> passed in.") prevented crash caused by empty console= parameter value.
> 
> Unfortunately, this value is widely used on Chromebooks to disable
> the console output. The above commit caused performance regression
> because the messages were pushed on slow console even though nobody
> was watching it.
> 
> Use ttynull driver explicitly for console="" and console=null
> parameters. It has been created for exactly this purpose.
> 
> It causes that preferred_console is set. As a result, ttySX and ttyX
> are not used as a fallback. And only ttynull console gets registered by
> default.
> 
> It still allows to register other consoles either by additional console=
> parameters or SPCR. It prevents regression because it worked this way even
> before. Also it is a sane semantic. Preventing output on all consoles
> should be done another way, for example, by introducing mute_console
> parameter.
> 
> Link: https://lore.kernel.org/r/20201006025935.GA597@jagdpanzerIV.localdomain
> Suggested-by: Sergey Senozhatsky <sergey.senozhatsky@...il.com>
> Signed-off-by: Petr Mladek <pmladek@...e.com>

Reviewed-and-tested-by: Guenter Roeck <linux@...ck-us.net>

> ---
>  kernel/printk/printk.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> index fe64a49344bf..ac440b879a2c 100644
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -2189,8 +2189,15 @@ static int __init console_setup(char *str)
>  	char *s, *options, *brl_options = NULL;
>  	int idx;
>  
> -	if (str[0] == 0)
> +	/*
> +	 * console="" or console=null have been suggested as a way to
> +	 * disable console output. Use ttynull that has been created
> +	 * for exacly this purpose.
> +	 */
> +	if (str[0] == 0 || strcmp(str, "null") == 0) {
> +		__add_preferred_console("ttynull", 0, NULL, NULL, true);
>  		return 1;
> +	}
>  
>  	if (_braille_console_setup(&str, &brl_options))
>  		return 1;
> -- 
> 2.26.2
>