USB: ftdi_sio: fix status line change handling for TIOCMIWAIT and TIOCGICOUNT

Handling of TIOCMIWAIT was changed by commit 1d749f9afa657f6ee9336b2bc1fcd750a647d157
 USB: ftdi_sio.c: Use ftdi async_icount structure for TIOCMIWAIT, as in other drivers

FTDI_STATUS_B0_MASK does not indicate the changed modem status lines,
it indicates the value of the current modem status lines. An xor is
still required to determine which lines have changed.

The count was only being incremented if the line was high. The only
reason TIOCMIWAIT still worked was because the status packet is
repeated every 1ms, so the count was always changing. The wakeup
itself still ran based on the status lines changing.

This change fixes handling of updates to the modem status lines and
allows multiple processes to use TIOCMIWAIT concurrently.

Tested with two processes waiting on different status lines being
toggled independently.

Signed-off-by: Simon Arlott <simon@fire.lp0.eu>
Cc: Uwe Bonnes <bon@elektron.ikp.physik.tu-darmstadt.de>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/serial/ftdi_sio.c

@@ -75,7 +75,7 @@ struct ftdi_private {
 	unsigned long last_dtr_rts;	/* saved modem control outputs */
 	struct async_icount	icount;
 	wait_queue_head_t delta_msr_wait; /* Used for TIOCMIWAIT */
-	char prev_status, diff_status;        /* Used for TIOCMIWAIT */
+	char prev_status;        /* Used for TIOCMIWAIT */
 	char transmit_empty;	/* If transmitter is empty or not */
 	struct usb_serial_port *port;
 	__u16 interface;	/* FT2232C, FT2232H or FT4232H port interface
@@ -1982,17 +1982,19 @@ static int ftdi_process_packet(struct tty_struct *tty,
 	   N.B. packet may be processed more than once, but differences
 	   are only processed once.  */
 	status = packet[0] & FTDI_STATUS_B0_MASK;
-	if (status & FTDI_RS0_CTS)
-		priv->icount.cts++;
-	if (status & FTDI_RS0_DSR)
-		priv->icount.dsr++;
-	if (status & FTDI_RS0_RI)
-		priv->icount.rng++;
-	if (status & FTDI_RS0_RLSD)
-		priv->icount.dcd++;
 	if (status != priv->prev_status) {
-		priv->diff_status |= status ^ priv->prev_status;
-		wake_up_interruptible(&priv->delta_msr_wait);
+		char diff_status = status ^ priv->prev_status;
+
+		if (diff_status & FTDI_RS0_CTS)
+			priv->icount.cts++;
+		if (diff_status & FTDI_RS0_DSR)
+			priv->icount.dsr++;
+		if (diff_status & FTDI_RS0_RI)
+			priv->icount.rng++;
+		if (diff_status & FTDI_RS0_RLSD)
+			priv->icount.dcd++;
+
+		wake_up_interruptible_all(&priv->delta_msr_wait);
 		priv->prev_status = status;
 	}