--- a/vizzini.c	2013-03-26 11:34:39.362546946 -0500
+++ b/vizzini.c	2013-03-27 10:43:03.123460275 -0500
@@ -92,6 +92,9 @@
 #define N_OUT_URB   4
 #define IN_BUFLEN   4096
 
+#define READ_URB_RUNNING    0
+#define READ_URB_STOPPED    1
+
 static int debug;
 
 
@@ -169,6 +172,7 @@
 
         struct urb   *in_urbs[N_IN_URB];
         char         *in_buffer[N_IN_URB];
+        int           in_urb_flags[N_IN_URB];
 
         int           ctrlin;
         int           ctrlout;
@@ -995,6 +999,8 @@
         char                            *transfer_buffer = urb->transfer_buffer;
         int                              length, room, have_extra_byte;
         int                              err;
+        unsigned long                    flags;
+        int j;
 
         if (debug) dev_dbg(&port->dev, "%s: %p\n", __func__, urb);
 
@@ -1085,9 +1091,26 @@
                 portdata->extra_byte = transfer_buffer[urb->actual_length - 1];
         }
 
-        err = usb_submit_urb(urb, GFP_ATOMIC);
-        if (err)
-                dev_err(&port->dev, "resubmit read urb failed. (%d)\n", err);
+        /* throttle device if requested */
+        spin_lock_irqsave(&port->lock, flags);
+        port->throttled = port->throttle_req;
+        if (! port->throttled) {
+                spin_unlock_irqrestore(&port->lock, flags);
+                err = usb_submit_urb(urb, GFP_ATOMIC);
+                if (err)
+                        dev_err(&port->dev, "resubmit read urb failed. (%d)\n", err);
+        } else {
+                spin_unlock_irqrestore(&port->lock, flags);
+                for (j = 0; j < N_IN_URB; j++) {
+                        if (portdata->in_urbs[j] == urb) {
+                                if (debug) dev_dbg(&port->dev, "throttling urb %d\n", j);
+                                spin_lock_irqsave(&port->lock, flags);
+                                portdata->in_urb_flags[j] = READ_URB_STOPPED;
+                                spin_unlock_irqrestore(&port->lock, flags);
+                                break;
+                        }
+                }
+        }
 }
 
 
@@ -1172,6 +1195,7 @@
         int                              i;
         struct urb                      *urb;
         int                              result;
+        unsigned long                    flags;
 
         portdata = usb_get_serial_port_data(port);
 
@@ -1179,10 +1203,19 @@
 
         acm_set_control(port, portdata->ctrlout = ACM_CTRL_DTR | ACM_CTRL_RTS);
 
+        /* unthrottle device in case TTY was closed while throttled */
+        spin_lock_irqsave(&port->lock, flags);
+        port->throttled = 0;
+        port->throttle_req = 0;
+        spin_unlock_irqrestore(&port->lock, flags);
+
         /* Reset low level data toggle and start reading from endpoints */
         for (i = 0; i < N_IN_URB; i++) {
                 if (debug) dev_dbg(&port->dev, "%s urb %d\n", __func__, i);
 
+                spin_lock_irqsave(&port->lock, flags);
+                portdata->in_urb_flags[i] = READ_URB_RUNNING;
+                spin_unlock_irqrestore(&port->lock, flags);
                 urb = portdata->in_urbs[i];
                 if (!urb)
                         continue;
@@ -1207,8 +1240,6 @@
                 }
         }
 
-        tty->low_latency = 1;
-
         /* start up the interrupt endpoint if we have one */
         if (port->interrupt_in_urb) {
                 result = usb_submit_urb(port->interrupt_in_urb, GFP_KERNEL);
@@ -1592,6 +1623,51 @@
         usb_serial_disconnect(interface);
 }
 
+static void vizzini_throttle(struct tty_struct *tty)
+{ 
+        unsigned long flags;
+        struct usb_serial_port *port = tty->driver_data;
+
+        if (debug) dev_dbg(&port->dev, "TTY buffer approaching limit, stopping USB reads\n");
+
+        spin_lock_irqsave(&port->lock, flags);
+        port->throttle_req = 1;
+        spin_unlock_irqrestore(&port->lock, flags);
+}
+
+static void vizzini_unthrottle(struct tty_struct *tty)
+{ 
+        unsigned long flags;
+        unsigned int was_throttled;
+        struct usb_serial_port *port = tty->driver_data;
+        struct vizzini_port_private *portdata = usb_get_serial_port_data(port);
+        int err;
+        int i;
+
+        if (debug) dev_dbg(&port->dev, "Resuming USB reads\n");
+
+        spin_lock_irqsave(&port->lock, flags);
+        was_throttled = port->throttled;
+        port->throttled = 0;
+        port->throttle_req = 0;
+        spin_unlock_irqrestore(&port->lock, flags);
+
+        if (was_throttled) {
+                for (i = 0; i < N_IN_URB; i++) {
+                        spin_lock_irqsave(&port->lock, flags);
+                        if (portdata->in_urb_flags[i] == READ_URB_STOPPED) {
+                                if (debug) dev_dbg(&port->dev, "unthrottling urb %d\n", i);
+                                portdata->in_urb_flags[i] = READ_URB_RUNNING;
+                                spin_unlock_irqrestore(&port->lock, flags);
+                                err = usb_submit_urb(portdata->in_urbs[i], GFP_ATOMIC);
+                                if (err)
+                                        dev_err(&port->dev, "resubmit read urb failed. (%d)\n", err);
+                        } else {
+                                spin_unlock_irqrestore(&port->lock, flags);
+                        }
+                }
+        }
+}
 
 
 static struct usb_serial_driver vizzini_device = {
@@ -1606,6 +1682,8 @@
         .probe              = vizzini_probe,
         .open               = vizzini_open,
         .close              = vizzini_close,
+        .throttle           = vizzini_throttle,
+        .unthrottle         = vizzini_unthrottle,
         .write              = vizzini_write,
         .write_room         = vizzini_write_room,
         .ioctl              = vizzini_ioctl,