USB: serial: ch341: fix modem-status handling

The modem-status register was read as part of device configuration at
port_probe and then again at open (and reset-resume). During open (and
reset-resume) the MSR was read before submitting the interrupt URB,
something which could lead to an MSR-change going unnoticed when it
races with open (reset-resume).

Fix this by dropping the redundant reconfiguration of the port at every
open, and only read the MSR after the interrupt URB has been submitted.

Fixes: 664d5df92e88 ("USB: usb-serial ch341: support for DTR/RTS/CTS")
Signed-off-by: Johan Hovold <johan@kernel.org>

drivers/usb/serial/ch341.c

@@ -248,21 +248,11 @@ static int ch341_configure(struct usb_device *dev, struct ch341_private *priv)
 	if (r < 0)
 		goto out;
 
-	/* expect 0xff 0xee */
-	r = ch341_get_status(dev, priv);
-	if (r < 0)
-		goto out;
-
 	r = ch341_set_baudrate_lcr(dev, priv, priv->lcr);
 	if (r < 0)
 		goto out;
 
 	r = ch341_set_handshake(dev, priv->line_control);
-	if (r < 0)
-		goto out;
-
-	/* expect 0x9f 0xee */
-	r = ch341_get_status(dev, priv);
 
 out:	kfree(buffer);
 	return r;
@@ -334,14 +324,9 @@ static void ch341_close(struct usb_serial_port *port)
 /* open this device, set default parameters */
 static int ch341_open(struct tty_struct *tty, struct usb_serial_port *port)
 {
-	struct usb_serial *serial = port->serial;
 	struct ch341_private *priv = usb_get_serial_port_data(port);
 	int r;
 
-	r = ch341_configure(serial->dev, priv);
-	if (r)
-		return r;
-
 	if (tty)
 		ch341_set_termios(tty, port, NULL);
 
@@ -353,6 +338,12 @@ static int ch341_open(struct tty_struct *tty, struct usb_serial_port *port)
 		return r;
 	}
 
+	r = ch341_get_status(port->serial->dev, priv);
+	if (r < 0) {
+		dev_err(&port->dev, "failed to read modem status: %d\n", r);
+		goto err_kill_interrupt_urb;
+	}
+
 	r = usb_serial_generic_open(tty, port);
 	if (r)
 		goto err_kill_interrupt_urb;
@@ -619,6 +610,12 @@ static int ch341_reset_resume(struct usb_serial *serial)
 				ret);
 			return ret;
 		}
+
+		ret = ch341_get_status(port->serial->dev, priv);
+		if (ret < 0) {
+			dev_err(&port->dev, "failed to read modem status: %d\n",
+				ret);
+		}
 	}
 
 	return usb_serial_generic_resume(serial);