USB: fix EHCI periodic transfers

As noted by Stefan Neis <Stefan.Neis@kobil.com>, we had a recent
regression with EHCI periodic transfers, in some (seemingly not
all that common) cases.

The root cause was that the schedule activation was only loosely
coupled to the addition or removal of transfers, so two different
execution contexts could both think they had to deactivate (or
conversely activate) the schedule.  So this fix tightens that
coupling, managing it more like a refcount.
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/usb/host/ehci-sched.c

@@ -437,6 +437,9 @@ static int enable_periodic (struct ehci_hcd *ehci)
 	u32	cmd;
 	int	status;
 
+	if (ehci->periodic_sched++)
+		return 0;
+
 	/* did clearing PSE did take effect yet?
 	 * takes effect only at frame boundaries...
 	 */
@@ -461,6 +464,9 @@ static int disable_periodic (struct ehci_hcd *ehci)
 	u32	cmd;
 	int	status;
 
+	if (--ehci->periodic_sched)
+		return 0;
+
 	/* did setting PSE not take effect yet?
 	 * takes effect only at frame boundaries...
 	 */
@@ -544,13 +550,10 @@ static int qh_link_periodic (struct ehci_hcd *ehci, struct ehci_qh *qh)
 		: (qh->usecs * 8);
 
 	/* maybe enable periodic schedule processing */
-	if (!ehci->periodic_sched++)
-		return enable_periodic (ehci);
-
-	return 0;
+	return enable_periodic(ehci);
 }
 
-static void qh_unlink_periodic (struct ehci_hcd *ehci, struct ehci_qh *qh)
+static int qh_unlink_periodic(struct ehci_hcd *ehci, struct ehci_qh *qh)
 {
 	unsigned	i;
 	unsigned	period;
@@ -586,9 +589,7 @@ static void qh_unlink_periodic (struct ehci_hcd *ehci, struct ehci_qh *qh)
 	qh_put (qh);
 
 	/* maybe turn off periodic schedule */
-	ehci->periodic_sched--;
-	if (!ehci->periodic_sched)
-		(void) disable_periodic (ehci);
+	return disable_periodic(ehci);
 }
 
 static void intr_deschedule (struct ehci_hcd *ehci, struct ehci_qh *qh)
@@ -1562,9 +1563,7 @@ itd_link_urb (
 	urb->hcpriv = NULL;
 
 	timer_action (ehci, TIMER_IO_WATCHDOG);
-	if (unlikely (!ehci->periodic_sched++))
-		return enable_periodic (ehci);
-	return 0;
+	return enable_periodic(ehci);
 }
 
 #define	ISO_ERRS (EHCI_ISOC_BUF_ERR | EHCI_ISOC_BABBLE | EHCI_ISOC_XACTERR)
@@ -1642,7 +1641,7 @@ itd_complete (
 	ehci_urb_done(ehci, urb, 0);
 	retval = true;
 	urb = NULL;
-	ehci->periodic_sched--;
+	(void) disable_periodic(ehci);
 	ehci_to_hcd(ehci)->self.bandwidth_isoc_reqs--;
 
 	if (unlikely (list_empty (&stream->td_list))) {
@@ -1951,9 +1950,7 @@ sitd_link_urb (
 	urb->hcpriv = NULL;
 
 	timer_action (ehci, TIMER_IO_WATCHDOG);
-	if (!ehci->periodic_sched++)
-		return enable_periodic (ehci);
-	return 0;
+	return enable_periodic(ehci);
 }
 
 /*-------------------------------------------------------------------------*/
@@ -2019,7 +2016,7 @@ sitd_complete (
 	ehci_urb_done(ehci, urb, 0);
 	retval = true;
 	urb = NULL;
-	ehci->periodic_sched--;
+	(void) disable_periodic(ehci);
 	ehci_to_hcd(ehci)->self.bandwidth_isoc_reqs--;
 
 	if (list_empty (&stream->td_list)) {
@@ -2243,8 +2240,7 @@ scan_periodic (struct ehci_hcd *ehci)
 			if (unlikely (modified)) {
 				if (likely(ehci->periodic_sched > 0))
 					goto restart;
-				/* maybe we can short-circuit this scan! */
-				disable_periodic(ehci);
+				/* short-circuit this scan */
 				now_uframe = clock;
 				break;
 			}