printk: Track nbcon consoles

Add a global flag @have_nbcon_console to identify if any nbcon
consoles are registered. This will be used in follow-up commits
to preserve legacy behavior when no nbcon consoles are registered.
Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20240820063001.36405-29-john.ogness@linutronix.deSigned-off-by: Petr Mladek <pmladek@suse.com>

kernel/printk/printk.c

@@ -470,6 +470,11 @@ static DEFINE_MUTEX(syslog_lock);
  */
 static bool have_legacy_console;
 
+/*
+ * Specifies if an nbcon console is registered.
+ */
+static bool have_nbcon_console;
+
 /*
  * Specifies if a boot console is registered. If boot consoles are present,
  * nbcon consoles cannot print simultaneously and must be synchronized by
@@ -3638,6 +3643,7 @@ void register_console(struct console *newcon)
 	init_seq = get_init_console_seq(newcon, bootcon_registered);
 
 	if (newcon->flags & CON_NBCON) {
+		have_nbcon_console = true;
 		nbcon_seq_force(newcon, init_seq);
 	} else {
 		have_legacy_console = true;
@@ -3718,6 +3724,7 @@ static int unregister_console_locked(struct console *console)
 {
 	bool use_device_lock = (console->flags & CON_NBCON) && console->write_atomic;
 	bool found_legacy_con = false;
+	bool found_nbcon_con = false;
 	bool found_boot_con = false;
 	unsigned long flags;
 	struct console *c;
@@ -3785,13 +3792,18 @@ static int unregister_console_locked(struct console *console)
 	for_each_console(c) {
 		if (c->flags & CON_BOOT)
 			found_boot_con = true;
-		if (!(c->flags & CON_NBCON))
+
+		if (c->flags & CON_NBCON)
+			found_nbcon_con = true;
+		else
 			found_legacy_con = true;
 	}
 	if (!found_boot_con)
 		have_boot_console = found_boot_con;
 	if (!found_legacy_con)
 		have_legacy_console = found_legacy_con;
+	if (!found_nbcon_con)
+		have_nbcon_console = found_nbcon_con;
 
 	return res;
 }