tty: add 'active' sysfs attribute to tty0 and console device

tty: add 'active' sysfs attribute to tty0 and console device

Userspace can query the actual virtual console, and the configured
console devices behind /dev/tt0 and /dev/console.

The last entry in the list of devices is the active device, analog
to the console= kernel command line option.

The attribute supports poll(), which is raised when the virtual
console is changed or /dev/console is reconfigured.

Signed-off-by: Kay Sievers <kay.sievers@vrfy.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

index 0000000..b138b66

kernel/printk.c

@@ -1332,6 +1332,7 @@ void register_console(struct console *newcon)
 		spin_unlock_irqrestore(&logbuf_lock, flags);
 	}
 	release_console_sem();
+	console_sysfs_notify();
 
 	/*
 	 * By unregistering the bootconsoles after we enable the real console
@@ -1390,6 +1391,7 @@ int unregister_console(struct console *console)
 		console_drivers->flags |= CON_CONSDEV;
 
 	release_console_sem();
+	console_sysfs_notify();
 	return res;
 }
 EXPORT_SYMBOL(unregister_console);