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

[Greg KH <greg@kroah.com>]

On Tue, Nov 09, 2021 at 04:53:26PM +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>
> 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
> ---
> 
> This is backport of the commit 3cffa06aeef7ece30f6b5ac0e
> ("printk/console: Allow to disable console output by using
> console="" or console=null") for stable release:
> 
>     + 4.4, 4.9, 4.14, 4.19, 5.4
> 
> Please, use the original upstream commit for stable release:
> 
>    + 5.10
> 
> It should fix the problem reported at
> https://www.spinics.net/lists/stable/msg509616.html

Thanks, now all queued up!

greg k-h