printk/console: Allow to disable console output by using console="" or console=null
stable inclusion from stable-5.10.79 commit ab4af56ae2508df3bdabcb192874086f5f07d98c bugzilla: 185793 https://gitee.com/openeuler/kernel/issues/I4K65C Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=ab4af56ae2508df3bdabcb192874086f5f07d98c -------------------------------- commit 3cffa06a upstream. 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:
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