USB: see if URB comes from a completion handler

Now that URBs can be completed inside tasklets, we need a way of
determining whether a completion handler for a given endpoint is
currently running.  Otherwise it's not possible to maintain the API
guarantee about keeping isochronous streams synchronous when an
underrun occurs.

This patch adds a field and a routine to check whether a completion
handler for a periodic endpoint is running.  At the moment no
analogous routine appears to be necessary for async endpoints, but one
can always be added.
Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
CC: Ming Lei <tom.leiming@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/core/hcd.c

@@ -1703,7 +1703,9 @@ static void usb_giveback_urb_bh(unsigned long param)
 
 		urb = list_entry(local_list.next, struct urb, urb_list);
 		list_del_init(&urb->urb_list);
+		bh->completing_ep = urb->ep;
 		__usb_hcd_giveback_urb(urb);
+		bh->completing_ep = NULL;
 	}
 
 	/* check if there are new URBs to giveback */

include/linux/usb/hcd.h

@@ -73,6 +73,7 @@ struct giveback_urb_bh {
 	spinlock_t lock;
 	struct list_head  head;
 	struct tasklet_struct bh;
+	struct usb_host_endpoint *completing_ep;
 };
 
 struct usb_hcd {
@@ -378,6 +379,12 @@ static inline int hcd_giveback_urb_in_bh(struct usb_hcd *hcd)
 	return hcd->driver->flags & HCD_BH;
 }
 
+static inline bool hcd_periodic_completion_in_progress(struct usb_hcd *hcd,
+		struct usb_host_endpoint *ep)
+{
+	return hcd->high_prio_bh.completing_ep == ep;
+}
+
 extern int usb_hcd_link_urb_to_ep(struct usb_hcd *hcd, struct urb *urb);
 extern int usb_hcd_check_unlink_urb(struct usb_hcd *hcd, struct urb *urb,
 		int status);