USB: serial: keyspan_pda: fix write unthrottling

The driver did not update its view of the available device buffer space
until write() was called in task context. This meant that write_room()
would return 0 even after the device had sent a write-unthrottle
notification, something which could lead to blocked writers not being
woken up (e.g. when using OPOST).

Note that we must also request an unthrottle notification is case a
write() request fills the device buffer exactly.

Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Cc: stable <stable@vger.kernel.org>
Acked-by: NSebastian Andrzej Siewior <bigeasy@linutronix.de>
Reviewed-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: NJohan Hovold <johan@kernel.org>

drivers/usb/serial/keyspan_pda.c

@@ -40,6 +40,8 @@
 #define DRIVER_AUTHOR "Brian Warner <warner@lothar.com>"
 #define DRIVER_DESC "USB Keyspan PDA Converter driver"
 
+#define KEYSPAN_TX_THRESHOLD	16
+
 struct keyspan_pda_private {
 	int			tx_room;
 	int			tx_throttled;
@@ -110,7 +112,7 @@ static void keyspan_pda_request_unthrottle(struct work_struct *work)
 				 7, /* request_unthrottle */
 				 USB_TYPE_VENDOR | USB_RECIP_INTERFACE
 				 | USB_DIR_OUT,
-				 16, /* value: threshold */
+				 KEYSPAN_TX_THRESHOLD,
 				 0, /* index */
 				 NULL,
 				 0,
@@ -129,6 +131,8 @@ static void keyspan_pda_rx_interrupt(struct urb *urb)
 	int retval;
 	int status = urb->status;
 	struct keyspan_pda_private *priv;
+	unsigned long flags;
+
 	priv = usb_get_serial_port_data(port);
 
 	switch (status) {
@@ -171,7 +175,10 @@ static void keyspan_pda_rx_interrupt(struct urb *urb)
 		case 1: /* modemline change */
 			break;
 		case 2: /* tx unthrottle interrupt */
+			spin_lock_irqsave(&port->lock, flags);
 			priv->tx_throttled = 0;
+			priv->tx_room = max(priv->tx_room, KEYSPAN_TX_THRESHOLD);
+			spin_unlock_irqrestore(&port->lock, flags);
 			/* queue up a wakeup at scheduler time */
 			usb_serial_port_softint(port);
 			break;
@@ -505,7 +512,8 @@ static int keyspan_pda_write(struct tty_struct *tty,
 			goto exit;
 		}
 	}
-	if (count > priv->tx_room) {
+
+	if (count >= priv->tx_room) {
 		/* we're about to completely fill the Tx buffer, so
 		   we'll be throttled afterwards. */
 		count = priv->tx_room;
@@ -560,14 +568,17 @@ static void keyspan_pda_write_bulk_callback(struct urb *urb)
 static int keyspan_pda_write_room(struct tty_struct *tty)
 {
 	struct usb_serial_port *port = tty->driver_data;
-	struct keyspan_pda_private *priv;
-	priv = usb_get_serial_port_data(port);
-	/* used by n_tty.c for processing of tabs and such. Giving it our
-	   conservative guess is probably good enough, but needs testing by
-	   running a console through the device. */
-	return priv->tx_room;
-}
+	struct keyspan_pda_private *priv = usb_get_serial_port_data(port);
+	unsigned long flags;
+	int room = 0;
+
+	spin_lock_irqsave(&port->lock, flags);
+	if (test_bit(0, &port->write_urbs_free) && !priv->tx_throttled)
+		room = priv->tx_room;
+	spin_unlock_irqrestore(&port->lock, flags);
 
+	return room;
+}
 
 static int keyspan_pda_chars_in_buffer(struct tty_struct *tty)
 {