serial: 8250: Allow using ports higher than SERIAL_8250_RUNTIME_UARTS

We already allocate CONFIG_SERIAL_8250_NR_UARTS, but only allow using
CONFIG_SERIAL_8250_RUNTIME_UARTS uarts unless nr_uarts module params
is set. This causes issues for using distro kernels on SoCs with a
large number of serial ports.

Let's allow up to CONFIG_SERIAL_8250_NR_UARTS instead. To do this, we init
the ports as needed if the initial uarts was too low. This way there's no
need to set the value for CONFIG_SERIAL_8250_RUNTIME_UARTS to some SoC
specific higher value. Typically the default value of 4 can be used as
set for legacy reasons.

Note that limiting the number of intial uarts still works as before
unless a serial console on a higher port is specified. In this case we
will increase the nr_ports up to the console port specified.
Suggested-by: Andrew Davis <afd@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Link: https://lore.kernel.org/r/20230508111903.39339-1-tony@atomide.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/8250/8250_core.c

@@ -488,28 +488,20 @@ static inline void serial8250_apply_quirks(struct uart_8250_port *up)
 	up->port.quirks |= skip_txen_test ? UPQ_NO_TXEN_TEST : 0;
 }
 
-static void __init serial8250_isa_init_ports(void)
+static struct uart_8250_port *serial8250_setup_port(int index)
 {
 	struct uart_8250_port *up;
-	static int first = 1;
-	int i, irqflag = 0;
 
-	if (!first)
-		return;
-	first = 0;
+	if (index >= UART_NR)
+		return NULL;
 
-	if (nr_uarts > UART_NR)
-		nr_uarts = UART_NR;
+	up = &serial8250_ports[index];
+	up->port.line = index;
 
-	for (i = 0; i < nr_uarts; i++) {
-		struct uart_8250_port *up = &serial8250_ports[i];
-		struct uart_port *port = &up->port;
-
-		port->line = i;
 	serial8250_init_port(up);
 	if (!base_ops)
-			base_ops = port->ops;
-		port->ops = &univ8250_port_ops;
+		base_ops = up->port.ops;
+	up->port.ops = &univ8250_port_ops;
 
 	timer_setup(&up->timer, serial8250_timeout, 0);
 
@@ -517,10 +509,33 @@ static void __init serial8250_isa_init_ports(void)
 
 	if (IS_ENABLED(CONFIG_ALPHA_JENSEN) ||
 	    (IS_ENABLED(CONFIG_ALPHA_GENERIC) && alpha_jensen()))
-			port->set_mctrl = alpha_jensen_set_mctrl;
+		up->port.set_mctrl = alpha_jensen_set_mctrl;
 
 	serial8250_set_defaults(up);
-	}
+
+	return up;
+}
+
+static void __init serial8250_isa_init_ports(void)
+{
+	struct uart_8250_port *up;
+	static int first = 1;
+	int i, irqflag = 0;
+
+	if (!first)
+		return;
+	first = 0;
+
+	if (nr_uarts > UART_NR)
+		nr_uarts = UART_NR;
+
+	/*
+	 * Set up initial isa ports based on nr_uart module param, or else
+	 * default to CONFIG_SERIAL_8250_RUNTIME_UARTS. Note that we do not
+	 * need to increase nr_uarts when setting up the initial isa ports.
+	 */
+	for (i = 0; i < nr_uarts; i++)
+		serial8250_setup_port(i);
 
 	/* chain base port ops to support Remote Supervisor Adapter */
 	univ8250_port_ops = *base_ops;
@@ -586,16 +601,29 @@ static void univ8250_console_write(struct console *co, const char *s,
 
 static int univ8250_console_setup(struct console *co, char *options)
 {
+	struct uart_8250_port *up;
 	struct uart_port *port;
-	int retval;
+	int retval, i;
 
 	/*
 	 * Check whether an invalid uart number has been specified, and
 	 * if so, search for the first available port that does have
 	 * console support.
 	 */
-	if (co->index >= nr_uarts)
+	if (co->index >= UART_NR)
 		co->index = 0;
+
+	/*
+	 * If the console is past the initial isa ports, init more ports up to
+	 * co->index as needed and increment nr_uarts accordingly.
+	 */
+	for (i = nr_uarts; i <= co->index; i++) {
+		up = serial8250_setup_port(i);
+		if (!up)
+			return -ENODEV;
+		nr_uarts++;
+	}
+
 	port = &serial8250_ports[co->index].port;
 	/* link port to console */
 	port->cons = co;
@@ -990,7 +1018,18 @@ int serial8250_register_8250_port(const struct uart_8250_port *up)
 	mutex_lock(&serial_mutex);
 
 	uart = serial8250_find_match_or_unused(&up->port);
-	if (uart && uart->port.type != PORT_8250_CIR) {
+	if (!uart) {
+		/*
+		 * If the port is past the initial isa ports, initialize a new
+		 * port and increment nr_uarts accordingly.
+		 */
+		uart = serial8250_setup_port(nr_uarts);
+		if (!uart)
+			goto unlock;
+		nr_uarts++;
+	}
+
+	if (uart->port.type != PORT_8250_CIR) {
 		struct mctrl_gpios *gpios;
 
 		if (uart->port.dev)
@@ -1120,6 +1159,7 @@ int serial8250_register_8250_port(const struct uart_8250_port *up)
 		}
 	}
 
+unlock:
 	mutex_unlock(&serial_mutex);
 
 	return ret;