From a98fb5280c69d90e85bb875177a68880089312b0 Mon Sep 17 00:00:00 2001
From: Phil Elwell <phil@raspberrypi.org>
Date: Wed, 9 Mar 2016 13:28:24 +0000
Subject: [PATCH] serial: Take care starting a hung-up tty's port

tty_port_hangup sets a port's tty field to NULL (holding the port lock),
but uart_tx_stopped, called from __uart_start (with the port lock),
uses the tty field without checking for NULL.

Change uart_tx_stopped to treat a NULL tty field as another stopped
indication.

Signed-off-by: Phil Elwell <phil@raspberrypi.org>
---
 include/linux/serial_core.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/include/linux/serial_core.h
+++ b/include/linux/serial_core.h
@@ -397,7 +397,7 @@ int uart_resume_port(struct uart_driver
 static inline int uart_tx_stopped(struct uart_port *port)
 {
 	struct tty_struct *tty = port->state->port.tty;
-	if (tty->stopped || port->hw_stopped)
+	if (!tty || tty->stopped || port->hw_stopped)
 		return 1;
 	return 0;
 }
