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

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Message-Id: <20220216203745.980-1-rdunlap@infradead.org>
Date:   Wed, 16 Feb 2022 12:37:45 -0800
From:   Randy Dunlap <rdunlap@...radead.org>
To:     linux-kernel@...r.kernel.org
Cc:     Randy Dunlap <rdunlap@...radead.org>,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        John Ogness <john.ogness@...utronix.de>,
        Jonathan Corbet <corbet@....net>, linux-doc@...r.kernel.org
Subject: [PATCH v2] 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 commit a91bd6223ecd ("Revert
"init/console: Use ttynull as a fallback when there is no console").]

Signed-off-by: Randy Dunlap <rdunlap@...radead.org>
Cc: Petr Mladek <pmladek@...e.com>
Cc: Sergey Senozhatsky <senozhatsky@...omium.org>
Cc: Steven Rostedt <rostedt@...dmis.org>
Cc: John Ogness <john.ogness@...utronix.de>
Cc: Jonathan Corbet <corbet@....net>
Cc: linux-doc@...r.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