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

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 the 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
[pmladek@suse.com: Slightly update wording.]
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20220216203745.980-1-rdunlap@infradead.org

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 49f495c9a7f80881b9d29b0aba67035f06535c95..46bfadf02bc78cbd9e9490850bc9bc40afae1c51 100644 (file)
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/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 the only console= parameter 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