USB: cdc-wdm: support back-to-back USB_CDC_NOTIFY_RESPONSE_AVAILABLE notifications

Some MBIM devices send back-to-back USB_CDC_NOTIFY_RESPONSE_AVAILABLE notifications
when sending a message over multiple fragments or when there are unsolicited
messages available.

Count up the number of USB_CDC_NOTIFY_RESPONSE_AVAILABLE notifications received
and decrement the count and submit the urb for the next response each time userspace
completes a read the response.
Signed-off-by: Greg Suarez <gsuarez@smithmicro.com>
Acked-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/class/cdc-wdm.c

@@ -101,6 +101,7 @@ struct wdm_device {
 	struct work_struct	rxwork;
 	int			werr;
 	int			rerr;
+	int                     resp_count;
 
 	struct list_head	device_list;
 	int			(*manage_power)(struct usb_interface *, int);
@@ -262,9 +263,9 @@ static void wdm_int_callback(struct urb *urb)
 	}
 
 	spin_lock(&desc->iuspin);
-	clear_bit(WDM_READ, &desc->flags);
 	responding = test_and_set_bit(WDM_RESPONDING, &desc->flags);
-	if (!responding && !test_bit(WDM_DISCONNECTING, &desc->flags)
+	if (!desc->resp_count++ && !responding
+		&& !test_bit(WDM_DISCONNECTING, &desc->flags)
 		&& !test_bit(WDM_SUSPENDING, &desc->flags)) {
 		rv = usb_submit_urb(desc->response, GFP_ATOMIC);
 		dev_dbg(&desc->intf->dev, "%s: usb_submit_urb %d",
@@ -521,10 +522,36 @@ static ssize_t wdm_read
 
 	desc->length -= cntr;
 	/* in case we had outstanding data */
-	if (!desc->length)
+	if (!desc->length) {
 		clear_bit(WDM_READ, &desc->flags);
 
-	spin_unlock_irq(&desc->iuspin);
+		if (--desc->resp_count) {
+			set_bit(WDM_RESPONDING, &desc->flags);
+			spin_unlock_irq(&desc->iuspin);
+
+			rv = usb_submit_urb(desc->response, GFP_KERNEL);
+			if (rv) {
+				dev_err(&desc->intf->dev,
+					"%s: usb_submit_urb failed with result %d\n",
+					__func__, rv);
+				spin_lock_irq(&desc->iuspin);
+				clear_bit(WDM_RESPONDING, &desc->flags);
+				spin_unlock_irq(&desc->iuspin);
+
+				if (rv == -ENOMEM) {
+					rv = schedule_work(&desc->rxwork);
+					if (rv)
+						dev_err(&desc->intf->dev, "Cannot schedule work\n");
+				} else {
+					spin_lock_irq(&desc->iuspin);
+					desc->resp_count = 0;
+					spin_unlock_irq(&desc->iuspin);
+				}
+			}
+		} else
+			spin_unlock_irq(&desc->iuspin);
+	} else
+		spin_unlock_irq(&desc->iuspin);
 
 	rv = cntr;
 
@@ -635,6 +662,9 @@ static int wdm_release(struct inode *inode, struct file *file)
 		if (!test_bit(WDM_DISCONNECTING, &desc->flags)) {
 			dev_dbg(&desc->intf->dev, "wdm_release: cleanup");
 			kill_urbs(desc);
+			spin_lock_irq(&desc->iuspin);
+			desc->resp_count = 0;
+			spin_unlock_irq(&desc->iuspin);
 			desc->manage_power(desc->intf, 0);
 		} else {
 			/* must avoid dev_printk here as desc->intf is invalid */