linux-kernel - [PATCH v6 2/3] ttynull: Add an option to allow ttynull to be used as a console device

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Message-ID: <20250304035447.3138221-3-adamsimonelli@gmail.com>
Date: Mon,  3 Mar 2025 22:54:46 -0500
From: adamsimonelli@...il.com
To: linux-serial@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Jiri Slaby <jirislaby@...nel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Andy Shevchenko <andy.shevchenko@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	John Ogness <john.ogness@...utronix.de>,
	Sergey Senozhatsky <senozhatsky@...omium.org>,
	Petr Mladek <pmladek@...e.com>
Cc: Adam Simonelli <adamsimonelli@...il.com>
Subject: [PATCH v6 2/3] ttynull: Add an option to allow ttynull to be used as a console device

From: Adam Simonelli <adamsimonelli@...il.com>

The new config option, CONFIG_NULL_TTY_DEFAULT_ CONSOLE will allow
ttynull to be initialized by console_initcall() and selected as a
possible console device.

Signed-off-by: Adam Simonelli <adamsimonelli@...il.com>
---
 drivers/tty/Kconfig   | 15 ++++++++++++++-
 drivers/tty/ttynull.c | 11 +++++++++++
 2 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/drivers/tty/Kconfig b/drivers/tty/Kconfig
index 63a494d36a1f..868aec2ccbb3 100644
--- a/drivers/tty/Kconfig
+++ b/drivers/tty/Kconfig
@@ -383,7 +383,20 @@ config NULL_TTY
 	  available or desired.
 
 	  In order to use this driver, you should redirect the console to this
-	  TTY, or boot the kernel with console=ttynull.
+	  TTY, boot the kernel with console=ttynull, or enable
+	  CONFIG_NULL_TTY_DEFAULT_CONSOLE.
+
+	  If unsure, say N.
+
+config NULL_TTY_DEFAULT_CONSOLE
+        bool "Support for console on ttynull"
+        depends on NULL_TTY=y && !VT_CONSOLE
+	help
+	  Say Y here if you want the NULL TTY to be used as a /dev/console
+	  device.
+
+	  This is similar to CONFIG_VT_CONSOLE, but without the dependency on
+	  CONFIG_VT. It uses the ttynull driver as the system console.
 
 	  If unsure, say N.
 
diff --git a/drivers/tty/ttynull.c b/drivers/tty/ttynull.c
index d8b51edde628..cb1e363b726d 100644
--- a/drivers/tty/ttynull.c
+++ b/drivers/tty/ttynull.c
@@ -96,6 +96,17 @@ static int __init ttynull_init(void)
 	return 0;
 }
 
+#ifdef CONFIG_NULL_TTY_DEFAULT_CONSOLE
+static int __init ttynull_register(void)
+{
+	if (!console_set_on_cmdline)
+		add_preferred_console("ttynull", 0, NULL);;
+
+	return 0;
+}
+console_initcall(ttynull_register);
+#endif
+
 static void __exit ttynull_exit(void)
 {
 	unregister_console(&ttynull_console);
-- 
2.45.2