USB: ch341: fix ignored TIOCMIWAIT mask

Make sure the TIOCMIWAIT mask is always honoured.

The CH341 interrupt status has a multiple-status changed flag which
indicates that multiple status changes has occurred since last interrupt
event. Unfortunately, if the final status is the same, there appears to
be no way to determine which signal(s) has changed (an even number of
times).

This means that the multiple-status flag should not be used in
TIOCMIWAIT as it leads to the signal mask argument being ignored (e.g.
TIOCMIWAIT could return if DSR changes twice, even though the user only
cares about carrier changes).
Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/serial/ch341.c

@@ -83,7 +83,6 @@ struct ch341_private {
 	unsigned baud_rate; /* set baud rate */
 	u8 line_control; /* set line control value RTS/DTR */
 	u8 line_status; /* active status of modem control inputs */
-	u8 multi_status_change; /* status changed multiple since last call */
 };
 
 static int ch341_control_out(struct usb_device *dev, u8 request,
@@ -174,7 +173,6 @@ static int ch341_get_status(struct usb_device *dev, struct ch341_private *priv)
 		r = 0;
 		spin_lock_irqsave(&priv->lock, flags);
 		priv->line_status = (~(*buffer)) & CH341_BITS_MODEM_STAT;
-		priv->multi_status_change = 0;
 		spin_unlock_irqrestore(&priv->lock, flags);
 	} else
 		r = -EPROTO;
@@ -457,10 +455,11 @@ static void ch341_update_line_status(struct usb_serial_port *port,
 	spin_lock_irqsave(&priv->lock, flags);
 	delta = status ^ priv->line_status;
 	priv->line_status = status;
-	if (data[1] & CH341_MULT_STAT)
-		priv->multi_status_change = 1;
 	spin_unlock_irqrestore(&priv->lock, flags);
 
+	if (data[1] & CH341_MULT_STAT)
+		dev_dbg(&port->dev, "%s - multiple status change\n", __func__);
+
 	if (delta & CH341_BIT_DCD) {
 		tty = tty_port_tty_get(&port->port);
 		if (tty) {
@@ -516,14 +515,12 @@ static int ch341_tiocmiwait(struct tty_struct *tty, unsigned long arg)
 	u8 prevstatus;
 	u8 status;
 	u8 changed;
-	u8 multi_change = 0;
 
 	spin_lock_irqsave(&priv->lock, flags);
 	prevstatus = priv->line_status;
-	priv->multi_status_change = 0;
 	spin_unlock_irqrestore(&priv->lock, flags);
 
-	while (!multi_change) {
+	for (;;) {
 		interruptible_sleep_on(&port->port.delta_msr_wait);
 		/* see if a signal did it */
 		if (signal_pending(current))
@@ -534,7 +531,6 @@ static int ch341_tiocmiwait(struct tty_struct *tty, unsigned long arg)
 
 		spin_lock_irqsave(&priv->lock, flags);
 		status = priv->line_status;
-		multi_change = priv->multi_status_change;
 		spin_unlock_irqrestore(&priv->lock, flags);
 
 		changed = prevstatus ^ status;