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

On 11/9/21 7:53 AM, 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.

We actually had to revert this patch on Chromebooks, so we'll have to revert it again from stable releases after it gets there. The problem is two-fold:

First, it is used in Chromebooks to disable the default console in production images; that default console may be set in a devicetree file, and this patch doesn't really disable it. In other words, Chromebooks use "console=" to implement "mute_console" as suggested below, and this patch does not address that use case.

Second, the patch causes some unexplained problems with dm-verity, which inexplicably fails on some Chromebooks when the patch is in place. We never tracked down the root cause because the patch doesn't work for us anyway.

Guenter

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

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 b55dfb3e801f..6d3e1f4961fb 100644 --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -2032,8 +2032,15 @@ 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);
  

  return 1;
• }

if (_braille_console_setup(&str, &brl_options)) return 1;