USB: OHCI: workaround for hardware bug: retired TDs not added to the Done Queue

This patch (as1636) is a partial workaround for a hardware bug
affecting OHCI controllers by NVIDIA at least, maybe others too.  When
the controller retires a Transfer Descriptor, it is supposed to add
the TD onto the Done Queue.  But sometimes this doesn't happen, with
the result that ohci-hcd never realizes the corresponding transfer has
finished.  Symptoms can vary; a typical result is that USB audio stops
working after a while.

The patch works around the problem by recognizing that TDs are always
processed in order.  Therefore, if a later TD is found on the Done
Queue than all the earlier TDs for the same endpoint must be finished
as well.

Unfortunately this won't solve the problem in cases where the missing
TD is the last one in the endpoint's queue.  A complete fix would
require a signficant amount of change to the driver.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Tested-by: Oliver Neukum <oneukum@suse.de>
CC: <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/host/ohci-q.c

@@ -1128,6 +1128,25 @@ dl_done_list (struct ohci_hcd *ohci)
 
 	while (td) {
 		struct td	*td_next = td->next_dl_td;
+		struct ed	*ed = td->ed;
+
+		/*
+		 * Some OHCI controllers (NVIDIA for sure, maybe others)
+		 * occasionally forget to add TDs to the done queue.  Since
+		 * TDs for a given endpoint are always processed in order,
+		 * if we find a TD on the donelist then all of its
+		 * predecessors must be finished as well.
+		 */
+		for (;;) {
+			struct td	*td2;
+
+			td2 = list_first_entry(&ed->td_list, struct td,
+					td_list);
+			if (td2 == td)
+				break;
+			takeback_td(ohci, td2);
+		}
+
 		takeback_td(ohci, td);
 		td = td_next;
 	}