serial-core: restore termios settings when resume console ports

The commit 4547be78 rewrites suspend and resume functions. According
to this rewrite, when a serial port is a printk console device and
can suspend(without set no_console_suspend flag), it will definitely
call set_termios function during its resume, but parameter termios
isn't initialized, this will pass an unpredictable config to the
serial port. If this serial port is not a userspace opened tty device
, a suspend and resume action will make this serial port unusable.
I.E. ttyS0 is a printk console device, ttyS1 or keyboard+display is
userspace tty device, a suspend/resume action will make ttyS0
unusable.

If a serial port is both a printk console device and an opened tty
device, this issue can be overcome because it will call set_termios
again with the correct parameter in the uart_change_speed function.

Refer to the deleted content of commit 4547be78, revert parts relate
to restore settings into parameter termios. It is safe because if
a serial port is a printk console only device, the only meaningful
field in termios is c_cflag and its old config is saved in
uport->cons->cflag, if this port is also an opened tty device,
it will clear uport->cons->cflag in the uart_open and the old config
is saved in tty->termios.
Signed-off-by: Jason Wang <jason77.wang@gmail.com>
Acked-by: Stanislav Brabec <sbrabec@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/serial/serial_core.c

@@ -2066,6 +2066,18 @@ int uart_resume_port(struct uart_driver *drv, struct uart_port *uport)
 	 * Re-enable the console device after suspending.
 	 */
 	if (console_suspend_enabled && uart_console(uport)) {
+		/*
+		 * First try to use the console cflag setting.
+		 */
+		memset(&termios, 0, sizeof(struct ktermios));
+		termios.c_cflag = uport->cons->cflag;
+
+		/*
+		 * If that's unset, use the tty termios setting.
+		 */
+		if (port->tty && port->tty->termios && termios.c_cflag == 0)
+			termios = *(port->tty->termios);
+
 		uart_change_pm(state, 0);
 		uport->ops->set_termios(uport, &termios, NULL);
 		console_start(uport->cons);