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

Guenter Roeck
2020-11-12T01:45:37Z

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@gmail.com>
> Signed-off-by: Petr Mladek <pmladek@suse.com>

Reviewed-and-tested-by: Guenter Roeck <linux@roeck-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
>