[PATCH v4 1/2] ttynull: Add an option to allow ttynull to be used as a console device

[adamsimonelli@gmail.com]

From: Adam Simonelli <adamsimonelli@gmail.com>

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

Signed-off-by: Adam Simonelli <adamsimonelli@gmail.com>
---
 drivers/tty/Kconfig   | 15 ++++++++++++++-
 drivers/tty/ttynull.c | 20 +++++++++++++++++++-
 2 files changed, 33 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/Kconfig b/drivers/tty/Kconfig
index 63a494d36a1f..b4afae8b0e74 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_CONSOLE.
+
+	  If unsure, say N.
+
+config NULL_TTY_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 6b2f7208b564..ec3dd3fd41c0 100644
--- a/drivers/tty/ttynull.c
+++ b/drivers/tty/ttynull.c
@@ -57,6 +57,13 @@ static struct tty_driver *ttynull_device(struct console *c, int *index)
 static struct console ttynull_console = {
 	.name = "ttynull",
 	.device = ttynull_device,
+
+	/*
+	 * Match the index and flags from other boot consoles when CONFIG_NULL_TTY_CONSOLE is
+	 * enabled, otherwise, use the default values for the index and flags.
+	*/
+	.index = IS_ENABLED(CONFIG_NULL_TTY_CONSOLE) ? -1 : 0,
+	.flags = IS_ENABLED(CONFIG_NULL_TTY_CONSOLE) ? CON_PRINTBUFFER : 0,
 };
 
 static int __init ttynull_init(void)
@@ -90,11 +97,22 @@ 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)
+{
+	if (!console_is_registered(&ttynull_console))
+		register_console(&ttynull_console);
+	return 0;
+}
+console_initcall(ttynull_register);
+#endif
+
 static void __exit ttynull_exit(void)
 {
 	unregister_console(&ttynull_console);
-- 
2.45.2