[PATCH v2] Docs: printk: add 'console=null|""' to admin/kernel-parameters

[Randy Dunlap <rdunlap@infradead.org>]

Tell about 'console=null|""' and how to use it.

It can be helpful to set (enable) CONFIG_NULL_TTY so that the ttynull
driver is available. This avoids problems with stdin/stdout/stderr of
the init process. Howevere, CONFIG_NULL_TTY cannot be enabled by default
because it can be used by mistake [see commit a91bd6223ecd ("Revert
"init/console: Use ttynull as a fallback when there is no console").]

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Petr Mladek <pmladek@suse.com>
Cc: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: John Ogness <john.ogness@linutronix.de>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
---
v2: "must be the only console=" instead of the first one. (Petr)
    Add info about CONFIG_NULL_TTY to the commit message. (Petr)

 Documentation/admin-guide/kernel-parameters.txt |    6 ++++++
 1 file changed, 6 insertions(+)

--- linux-next-20220216.orig/Documentation/admin-guide/kernel-parameters.txt
+++ linux-next-20220216/Documentation/admin-guide/kernel-parameters.txt
@@ -724,6 +724,12 @@
 		hvc<n>	Use the hypervisor console device <n>. This is for
 			both Xen and PowerPC hypervisors.
 
+		{ null | "" }
+			Use to disable console output, i.e., to have kernel
+			console messages discarded.
+			This must be only console= string used on the
+			kernel command line.
+
 		If the device connected to the port is not a TTY but a braille
 		device, prepend "brl," before the device type, for instance
 			console=brl,ttyS0