linux-kernel - [PATCH 1/2] ttynull: Add an option to allow ttynull to be used as a console device

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Message-ID: <20241129041549.778959-2-bluescreen_avenger@verizon.net>
Date: Thu, 28 Nov 2024 23:15:48 -0500
From: n3rdopolis <bluescreen_avenger@...izon.net>
To: linux-serial@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Jiri Slaby <jirislaby@...nel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc: n3rdopolis <bluescreen_avenger@...izon.net>
Subject: [PATCH 1/2] ttynull: Add an option to allow ttynull to be used as a console device

Add a config option CONFIG_NULL_TTY_CONSOLE that will have ttynull be
initialized by console_initcall() and selected as a possible console
device.
Signed-off-by: n3rdopolis <bluescreen_avenger@...izon.net>
---
 drivers/tty/Kconfig   | 18 +++++++++++++++++-
 drivers/tty/ttynull.c | 16 +++++++++++++++-
 2 files changed, 32 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/Kconfig b/drivers/tty/Kconfig
index 63a494d36a1f..afe4e7722d4c 100644
--- a/drivers/tty/Kconfig
+++ b/drivers/tty/Kconfig
@@ -383,7 +383,23 @@ 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_CONSOLE.
+
+	  If unsure, say N.
+
+config NULL_TTY_CONSOLE
+        bool "Supports /dev/ttynull as a console automatically"
+        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 useful for userspace applications that expect a working
+	  console device, without changing the kernel boot options, such as a
+	  distribuition or environment that historically had CONFIG_VT_CONSOLE
+	  enabled, and have now disabled it, but still need /dev/console to be
+	  working for userspace applications.
 
 	  If unsure, say N.
 
diff --git a/drivers/tty/ttynull.c b/drivers/tty/ttynull.c
index 6b2f7208b564..8ba629ae426b 100644
--- a/drivers/tty/ttynull.c
+++ b/drivers/tty/ttynull.c
@@ -57,6 +57,10 @@ static struct tty_driver *ttynull_device(struct console *c, int *index)
 static struct console ttynull_console = {
 	.name = "ttynull",
 	.device = ttynull_device,
+#ifdef CONFIG_NULL_TTY_CONSOLE
+	.index = -1,
+	.flags = CON_PRINTBUFFER,
+#endif
 };
 
 static int __init ttynull_init(void)
@@ -90,10 +94,20 @@ static int __init ttynull_init(void)
 	}
 
 	ttynull_driver = driver;
-	register_console(&ttynull_console);
+	if (!console_is_registered(&ttynull_console))
+		register_console(&ttynull_console);
+
+	return 0;
+}
 
+#ifdef CONFIG_NULL_TTY_CONSOLE
+static int __init ttynull_register(void)
+{
+	register_console(&ttynull_console);
 	return 0;
 }
+console_initcall(ttynull_register);
+#endif
 
 static void __exit ttynull_exit(void)
 {
-- 
2.45.2