serial: core: Introduce wrapper to set @uart_port->cons

Introduce uart_port_set_cons() as a wrapper to set @cons of a
uart_port. The wrapper sets @cons under the port lock in order
to prevent @cons from disappearing while another context is
holding the port lock. This is necessary for a follow-up
commit relating to the port lock wrappers, which rely on @cons
not changing between lock and unlock.
Signed-off-by: John Ogness <john.ogness@linutronix.de>
Tested-by: Théo Lebrun <theo.lebrun@bootlin.com> # EyeQ5, AMBA-PL011
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20240820063001.36405-12-john.ogness@linutronix.deSigned-off-by: Petr Mladek <pmladek@suse.com>

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

@@ -423,11 +423,11 @@ static int univ8250_console_setup(struct console *co, char *options)
 
 	port = &serial8250_ports[co->index].port;
 	/* link port to console */
-	port->cons = co;
+	uart_port_set_cons(port, co);
 
 	retval = serial8250_console_setup(port, options, false);
 	if (retval != 0)
-		port->cons = NULL;
+		uart_port_set_cons(port, NULL);
 	return retval;
 }
 
@@ -485,7 +485,7 @@ static int univ8250_console_match(struct console *co, char *name, int idx,
 			continue;
 
 		co->index = i;
-		port->cons = co;
+		uart_port_set_cons(port, co);
 		return serial8250_console_setup(port, options, true);
 	}
 

drivers/tty/serial/amba-pl011.c

@@ -2480,7 +2480,7 @@ static int pl011_console_match(struct console *co, char *name, int idx,
 			continue;
 
 		co->index = i;
-		port->cons = co;
+		uart_port_set_cons(port, co);
 		return pl011_console_setup(co, options);
 	}
 

drivers/tty/serial/serial_core.c

@@ -3176,8 +3176,15 @@ static int serial_core_add_one_port(struct uart_driver *drv, struct uart_port *u
 	state->uart_port = uport;
 	uport->state = state;
 
+	/*
+	 * If this port is in use as a console then the spinlock is already
+	 * initialised.
+	 */
+	if (!uart_console_registered(uport))
+		uart_port_spin_lock_init(uport);
+
 	state->pm_state = UART_PM_STATE_UNDEFINED;
-	uport->cons = drv->cons;
+	uart_port_set_cons(uport, drv->cons);
 	uport->minor = drv->tty_driver->minor_start + uport->line;
 	uport->name = kasprintf(GFP_KERNEL, "%s%d", drv->dev_name,
 				drv->tty_driver->name_base + uport->line);
@@ -3186,13 +3193,6 @@ static int serial_core_add_one_port(struct uart_driver *drv, struct uart_port *u
 		goto out;
 	}
 
-	/*
-	 * If this port is in use as a console then the spinlock is already
-	 * initialised.
-	 */
-	if (!uart_console_registered(uport))
-		uart_port_spin_lock_init(uport);
-
 	if (uport->cons && uport->dev)
 		of_console_check(uport->dev->of_node, uport->cons->name, uport->line);
 

include/linux/serial_core.h

@@ -608,6 +608,23 @@ static inline void __uart_port_unlock_irqrestore(struct uart_port *up, unsigned
 	spin_unlock_irqrestore(&up->lock, flags);
 }
 
+/**
+ * uart_port_set_cons - Safely set the @cons field for a uart
+ * @up:		The uart port to set
+ * @con:	The new console to set to
+ *
+ * This function must be used to set @up->cons. It uses the port lock to
+ * synchronize with the port lock wrappers in order to ensure that the console
+ * cannot change or disappear while another context is holding the port lock.
+ */
+static inline void uart_port_set_cons(struct uart_port *up, struct console *con)
+{
+	unsigned long flags;
+
+	__uart_port_lock_irqsave(up, &flags);
+	up->cons = con;
+	__uart_port_unlock_irqrestore(up, flags);
+}
 /**
  * uart_port_lock - Lock the UART port
  * @up:		Pointer to UART port structure