Revert "init/console: Use ttynull as a fallback when there is no console"

This reverts commit 757055ae8dedf5333af17b3b5b4b70ba9bc9da4e.

The commit caused that ttynull was used as the default console
on several systems[1][2][3]. As a result, the console was
blank even when a better alternative existed.

It happened when there was no console configured
on the command line and ttynull_init() was the first initcall
calling register_console().

Or it happened when /dev/ did not exist when console_on_rootfs()
was called. It was not able to open /dev/console even though
a console driver was registered. It tried to add ttynull console
but it obviously did not help. But ttynull became the preferred
console and was used by /dev/console when it was available later.

The commit tried to fix a historical problem that have been there
for ages. The primary motivation was the commit 3cffa06aeef7ece30f6
("printk/console: Allow to disable console output by using console=""
or console=null"). It provided a clean solution for a workaround
that was widely used and worked only by chance.

This revert causes that the console="" or console=null command line
options will again work only by chance. These options will cause that
a particular console will be preferred and the default (tty) ones
will not get enabled. There will be no console registered at
all. As a result there won't be stdin, stdout, and stderr for
the init process. But it worked exactly this way even before.

The proper solution has to fulfill many conditions:

+ Register ttynull only when explicitly required or as
the ultimate fallback.

+ ttynull should get associated with /dev/console but it must
not become preferred console when used as a fallback.
Especially, it must still be possible to replace it
by a better console later.

Such a change requires clean up of the register_console() code.
Otherwise, it would be even harder to follow. Especially, the use
of has_preferred_console and CON_CONSDEV flag is tricky. The clean
up is risky. The ordering of consoles is not well defined. And
any changes tend to break existing user settings.

Do the revert at the least risky solution for now.

[1] https://lore.kernel.org/linux-kselftest/[e-mail]/
[2] https://lore.kernel.org/lkml/[e-mail]/
[3] https://patchwork.ozlabs.org/project/linux-um/patch/[e-mail]/

Reported-by: Andy Shevchenko <[e-mail]>
Reported-by: Vineet Gupta <[e-mail]>
Reported-by: Thomas Meyer <[e-mail]>
Signed-off-by: Petr Mladek <[e-mail]>
Acked-by: Greg Kroah-Hartman <[e-mail]>
Acked-by: Sergey Senozhatsky <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

drivers/tty/Kconfig

···
 	help
 	  FDC channel number to use for KGDB.
 
+config NULL_TTY
+	tristate "NULL TTY driver"
+	help
+	  Say Y here if you want a NULL TTY which simply discards messages.
+
+	  This is useful to allow userspace applications which expect a console
+	  device to work without modifications even when no console is
+	  available or desired.
+
+	  In order to use this driver, you should redirect the console to this
+	  TTY, or boot the kernel with console=ttynull.
+
+	  If unsure, say N.
+
 config TRACE_ROUTER
 	tristate "Trace data router for MIPI P1149.7 cJTAG standard"
 	depends on TRACE_SINK

drivers/tty/Makefile

···
 obj-$(CONFIG_TTY)		+= tty_io.o n_tty.o tty_ioctl.o tty_ldisc.o \
 				   tty_buffer.o tty_port.o tty_mutex.o \
 				   tty_ldsem.o tty_baudrate.o tty_jobctrl.o \
-				   n_null.o ttynull.o
+				   n_null.o
 obj-$(CONFIG_LEGACY_PTYS)	+= pty.o
 obj-$(CONFIG_UNIX98_PTYS)	+= pty.o
 obj-$(CONFIG_AUDIT)		+= tty_audit.o
···
 obj-$(CONFIG_MOXA_INTELLIO)	+= moxa.o
 obj-$(CONFIG_MOXA_SMARTIO)	+= mxser.o
 obj-$(CONFIG_NOZOMI)		+= nozomi.o
+obj-$(CONFIG_NULL_TTY)	        += ttynull.o
 obj-$(CONFIG_ROCKETPORT)	+= rocket.o
 obj-$(CONFIG_SYNCLINK_GT)	+= synclink_gt.o
 obj-$(CONFIG_PPC_EPAPR_HV_BYTECHAN) += ehv_bytechan.o

drivers/tty/ttynull.c

···
 /*
  * Copyright (C) 2019 Axis Communications AB
  *
- * The console is useful for userspace applications which expect a console
- * device to work without modifications even when no console is available
- * or desired.
- *
- * In order to use this driver, you should redirect the console to this
- * TTY, or boot the kernel with console=ttynull.
- *
  * Based on ttyprintk.c:
  *  Copyright (C) 2010 Samo Pogacnik
  */
···
 	.name = "ttynull",
 	.device = ttynull_device,
 };
-
-void __init register_ttynull_console(void)
-{
-	if (!ttynull_driver)
-		return;
-
-	if (add_preferred_console(ttynull_console.name, 0, NULL))
-		return;
-
-	register_console(&ttynull_console);
-}
 
 static int __init ttynull_init(void)
 {

include/linux/console.h

···
 extern int braille_unregister_console(struct console *);
 #ifdef CONFIG_TTY
 extern void console_sysfs_notify(void);
-extern void register_ttynull_console(void);
 #else
 static inline void console_sysfs_notify(void)
-{ }
-static inline void register_ttynull_console(void)
 { }
 #endif
 extern bool console_suspend_enabled;

init/main.c

···
 	struct file *file = filp_open("/dev/console", O_RDWR, 0);
 
 	if (IS_ERR(file)) {
-		pr_err("Warning: unable to open an initial console. Fallback to ttynull.\n");
-		register_ttynull_console();
-
-		file = filp_open("/dev/console", O_RDWR, 0);
-		if (IS_ERR(file)) {
-			pr_err("Warning: Failed to add ttynull console. No stdin, stdout, and stderr for the init process!\n");
-			return;
-		}
+		pr_err("Warning: unable to open an initial console.\n");
+		return;
 	}
 	init_dup(file);
 	init_dup(file);