Commit 3cffa06a authored Nov 11, 2020 by Petr Mladek

printk/console: Allow to disable console output by using console="" or console=null

The commit 48021f98 ("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@gmail.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20201111135450.11214-3-pmladek@suse.com

parent 757055ae

kernel/printk/printk.c

+8 −1

Original line number	Diff line number	Diff line
		@@ -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;

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