USB: add flow control to usb-serial generic driver.

I added two fields to struct usb_serial_port to keep track of the
throttle state. Other usb-serial drivers typically use private data for
such things, but the generic driver can not really do that because some
of its code is also used by other drivers (which may have their own
private data needs).

As it is, I am not sure that this patch is useful in all scenarios.
It is certainly helpful for low-bandwidth devices that can hold their
data in response to throttling. But for devices that pump data in
real-time as fast as possible (webcam, A/D converter, etc), throttling
may actually cause more data loss.

From: Joris van Rantwijk <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/usb/serial/generic.c

···
 	.num_bulk_out =		NUM_DONT_CARE,
 	.num_ports =		1,
 	.shutdown =		usb_serial_generic_shutdown,
+	.throttle =		usb_serial_generic_throttle,
+	.unthrottle =		usb_serial_generic_unthrottle,
 };
 
 static int generic_probe(struct usb_interface *interface,
···
 {
 	struct usb_serial *serial = port->serial;
 	int result = 0;
+	unsigned long flags;
 
 	dbg("%s - port %d", __FUNCTION__, port->number);
 
···
 	if (port->tty)
 		port->tty->low_latency = 1;
 
-	/* if we have a bulk interrupt, start reading from it */
+	/* clear the throttle flags */
+	spin_lock_irqsave(&port->lock, flags);
+	port->throttled = 0;
+	port->throttle_req = 0;
+	spin_unlock_irqrestore(&port->lock, flags);
+
+	/* if we have a bulk endpoint, start reading from it */
 	if (serial->num_bulk_in) {
 		/* Start reading from the device */
 		usb_fill_bulk_urb (port->read_urb, serial->dev,
···
 	return (chars);
 }
 
-void usb_serial_generic_read_bulk_callback (struct urb *urb)
+/* Push data to tty layer and resubmit the bulk read URB */
+static void flush_and_resubmit_read_urb (struct usb_serial_port *port)
 {
-	struct usb_serial_port *port = (struct usb_serial_port *)urb->context;
 	struct usb_serial *serial = port->serial;
-	struct tty_struct *tty;
-	unsigned char *data = urb->transfer_buffer;
+	struct urb *urb = port->read_urb;
+	struct tty_struct *tty = port->tty;
 	int result;
 
-	dbg("%s - port %d", __FUNCTION__, port->number);
-
-	if (urb->status) {
-		dbg("%s - nonzero read bulk status received: %d", __FUNCTION__, urb->status);
-		return;
-	}
-
-	usb_serial_debug_data(debug, &port->dev, __FUNCTION__, urb->actual_length, data);
-
-	tty = port->tty;
+	/* Push data to tty */
 	if (tty && urb->actual_length) {
 		tty_buffer_request_room(tty, urb->actual_length);
-		tty_insert_flip_string(tty, data, urb->actual_length);
-	  	tty_flip_buffer_push(tty);
+		tty_insert_flip_string(tty, urb->transfer_buffer, urb->actual_length);
+	  	tty_flip_buffer_push(tty); /* is this allowed from an URB callback ? */
 	}
 
-	/* Continue trying to always read  */
+	/* Continue reading from device */
 	usb_fill_bulk_urb (port->read_urb, serial->dev,
 			   usb_rcvbulkpipe (serial->dev,
 				   	    port->bulk_in_endpointAddress),
···
 	result = usb_submit_urb(port->read_urb, GFP_ATOMIC);
 	if (result)
 		dev_err(&port->dev, "%s - failed resubmitting read urb, error %d\n", __FUNCTION__, result);
+}
+
+void usb_serial_generic_read_bulk_callback (struct urb *urb)
+{
+	struct usb_serial_port *port = (struct usb_serial_port *)urb->context;
+	unsigned char *data = urb->transfer_buffer;
+	int is_throttled;
+	unsigned long flags;
+
+	dbg("%s - port %d", __FUNCTION__, port->number);
+
+	if (urb->status) {
+		dbg("%s - nonzero read bulk status received: %d", __FUNCTION__, urb->status);
+		return;
+	}
+
+	usb_serial_debug_data(debug, &port->dev, __FUNCTION__, urb->actual_length, data);
+
+	/* Throttle the device if requested by tty */
+	if (urb->actual_length) {
+		spin_lock_irqsave(&port->lock, flags);
+		is_throttled = port->throttled = port->throttle_req;
+		spin_unlock_irqrestore(&port->lock, flags);
+		if (is_throttled) {
+			/* Let the received data linger in the read URB;
+			 * usb_serial_generic_unthrottle() will pick it
+			 * up later. */
+			dbg("%s - throttling device", __FUNCTION__);
+			return;
+		}
+	}
+
+	/* Handle data and continue reading from device */
+	flush_and_resubmit_read_urb(port);
 }
 EXPORT_SYMBOL_GPL(usb_serial_generic_read_bulk_callback);
 
···
 	usb_serial_port_softint(port);
 }
 EXPORT_SYMBOL_GPL(usb_serial_generic_write_bulk_callback);
+
+void usb_serial_generic_throttle (struct usb_serial_port *port)
+{
+	unsigned long flags;
+
+	dbg("%s - port %d", __FUNCTION__, port->number);
+
+	/* Set the throttle request flag. It will be picked up
+	 * by usb_serial_generic_read_bulk_callback(). */
+	spin_lock_irqsave(&port->lock, flags);
+	port->throttle_req = 1;
+	spin_unlock_irqrestore(&port->lock, flags);
+}
+
+void usb_serial_generic_unthrottle (struct usb_serial_port *port)
+{
+	int was_throttled;
+	unsigned long flags;
+
+	dbg("%s - port %d", __FUNCTION__, port->number);
+
+	/* Clear the throttle flags */
+	spin_lock_irqsave(&port->lock, flags);
+	was_throttled = port->throttled;
+	port->throttled = port->throttle_req = 0;
+	spin_unlock_irqrestore(&port->lock, flags);
+
+	if (was_throttled) {
+		/* Handle pending data and resume reading from device */
+		flush_and_resubmit_read_urb(port);
+	}
+}
 
 void usb_serial_generic_shutdown (struct usb_serial *serial)
 {

include/linux/usb/serial.h

···
  * @write_wait: a wait_queue_head_t used by the port.
  * @work: work queue entry for the line discipline waking up.
  * @open_count: number of times this port has been opened.
+ * @throttled: nonzero if the read urb is inactive to throttle the device
+ * @throttle_req: nonzero if the tty wants to throttle us
  *
  * This structure is used by the usb-serial core and drivers for the specific
  * ports of a device.
···
 	wait_queue_head_t	write_wait;
 	struct work_struct	work;
 	int			open_count;
+	char			throttled;
+	char			throttle_req;
 	struct device		dev;
 };
 #define to_usb_serial_port(d) container_of(d, struct usb_serial_port, dev)
···
 extern int usb_serial_generic_chars_in_buffer (struct usb_serial_port *port);
 extern void usb_serial_generic_read_bulk_callback (struct urb *urb);
 extern void usb_serial_generic_write_bulk_callback (struct urb *urb);
+extern void usb_serial_generic_throttle (struct usb_serial_port *port);
+extern void usb_serial_generic_unthrottle (struct usb_serial_port *port);
 extern void usb_serial_generic_shutdown (struct usb_serial *serial);
 extern int usb_serial_generic_register (int debug);
 extern void usb_serial_generic_deregister (void);