usb: xhci: account for num_trbs_free when invalidating TDs

If a ring has a number of TDs enqueued past the dequeue pointer, and the
URBs corresponding to these TDs are dequeued, then num_trbs_free isn't
updated to show that these TDs have been converted to no-ops and
effectively "freed". This means that num_trbs_free creeps downwards
until the count is exhausted, which then triggers xhci_ring_expansion()
and effectively leaks memory by infinitely growing the transfer ring.

This is commonly encounted through the use of a usb-serial port where
the port is repeatedly opened, read, then closed.

Move the num_trbs_free crediting out of the Set TR Dequeue Pointer
handling and into xhci_invalidate_cancelled_tds().

There is a potential for overestimating the actual space on the ring if
the ring is nearly full and TDs are arbitrarily enqueued by a device
driver while it is dequeueing them, but dequeues are usually batched
during device close/shutdown or endpoint error recovery.

See #5088

Signed-off-by: Jonathan Bell <jonathan@raspberrypi.com>

drivers/usb/host/xhci-ring.c

@@ -1035,11 +1035,13 @@ static int xhci_invalidate_cancelled_tds(struct xhci_virt_ep *ep)
 						 td->urb->stream_id, td->urb,
 						 cached_td->urb->stream_id, cached_td->urb);
 				cached_td = td;
+				ring->num_trbs_free += td->num_trbs;
 				break;
 			}
 		} else {
 			td_to_noop(xhci, ring, td, false);
 			td->cancel_status = TD_CLEARED;
+			ring->num_trbs_free += td->num_trbs;
 		}
 	}
 
@@ -1286,10 +1288,7 @@ static void update_ring_for_set_deq_completion(struct xhci_hcd *xhci,
 		unsigned int ep_index)
 {
 	union xhci_trb *dequeue_temp;
-	int num_trbs_free_temp;
-	bool revert = false;
 
-	num_trbs_free_temp = ep_ring->num_trbs_free;
 	dequeue_temp = ep_ring->dequeue;
 
 	/* If we get two back-to-back stalls, and the first stalled transfer
@@ -1304,8 +1303,6 @@ static void update_ring_for_set_deq_completion(struct xhci_hcd *xhci,
 	}
 
 	while (ep_ring->dequeue != dev->eps[ep_index].queued_deq_ptr) {
-		/* We have more usable TRBs */
-		ep_ring->num_trbs_free++;
 		ep_ring->dequeue++;
 		if (trb_is_link(ep_ring->dequeue)) {
 			if (ep_ring->dequeue ==
@@ -1315,15 +1312,10 @@ static void update_ring_for_set_deq_completion(struct xhci_hcd *xhci,
 			ep_ring->dequeue = ep_ring->deq_seg->trbs;
 		}
 		if (ep_ring->dequeue == dequeue_temp) {
-			revert = true;
+			xhci_dbg(xhci, "Unable to find new dequeue pointer\n");
 			break;
 		}
 	}
-
-	if (revert) {
-		xhci_dbg(xhci, "Unable to find new dequeue pointer\n");
-		ep_ring->num_trbs_free = num_trbs_free_temp;
-	}
 }
 
 /*