diff --git a/sys/dev/usb/ohci.c b/sys/dev/usb/ohci.c
index 12ced6efc821..e7af10575805 100644
--- a/sys/dev/usb/ohci.c
+++ b/sys/dev/usb/ohci.c
@@ -1,4 +1,4 @@
-/*	$NetBSD: ohci.c,v 1.45 1999/09/13 19:49:41 augustss Exp $	*/
+/*	$NetBSD: ohci.c,v 1.46 1999/09/13 21:33:25 augustss Exp $	*/
 
 /*
  * Copyright (c) 1998 The NetBSD Foundation, Inc.
@@ -1133,6 +1133,7 @@ ohci_add_ed(sed, head)
 	ohci_soft_ed_t *sed; 
 	ohci_soft_ed_t *head; 
 {
+	SPLUSBCHECK;
 	sed->next = head->next;
 	sed->ed.ed_nexted = head->ed.ed_nexted;
 	head->next = sed;
@@ -1149,6 +1150,8 @@ ohci_rem_ed(sed, head)
 {
 	ohci_soft_ed_t *p; 
 
+	SPLUSBCHECK;
+
 	/* XXX */
 	for (p = head; p && p->next != sed; p = p->next)
 		;
@@ -1177,6 +1180,8 @@ ohci_hash_add_td(sc, std)
 {
 	int h = HASH(std->physaddr);
 
+	SPLUSBCHECK;
+
 	LIST_INSERT_HEAD(&sc->sc_hash_tds[h], std, hnext);
 }
 
@@ -1186,6 +1191,8 @@ ohci_hash_rem_td(sc, std)
 	ohci_softc_t *sc;
 	ohci_soft_td_t *std;
 {
+	SPLUSBCHECK;
+
 	LIST_REMOVE(std, hnext);
 }
 
@@ -1404,6 +1411,8 @@ ohci_abort_req(reqh, status)
 	struct ohci_pipe *opipe = (struct ohci_pipe *)reqh->pipe;
 	ohci_soft_ed_t *sed;
 
+	SPLUSBCHECK;
+
 	DPRINTF(("ohci_abort_req: reqh=%p pipe=%p\n", reqh, opipe));
 
 	reqh->status = status;
@@ -1549,11 +1558,13 @@ ohci_root_ctrl_transfer(reqh)
 {
 	usbd_status r;
 
+	/* Insert last in queue. */
 	r = usb_insert_transfer(reqh);
 	if (r != USBD_NORMAL_COMPLETION)
 		return (r);
-	else
-		return (ohci_root_ctrl_start(reqh));
+
+	/* Pipe isn't running, start first */
+	return (ohci_root_ctrl_start(SIMPLEQ_FIRST(&reqh->pipe->queue)));
 }
 
 usbd_status
@@ -1564,7 +1575,7 @@ ohci_root_ctrl_start(reqh)
 	usb_device_request_t *req;
 	void *buf;
 	int port, i;
-	int len, value, index, l, totlen = 0;
+	int s, len, value, index, l, totlen = 0;
 	usb_port_status_t ps;
 	usb_hub_descriptor_t hubd;
 	usbd_status r;
@@ -1853,7 +1864,9 @@ ohci_root_ctrl_start(reqh)
 	r = USBD_NORMAL_COMPLETION;
  ret:
 	reqh->status = r;
+	s = splusb();
 	usb_transfer_complete(reqh);
+	splx(s);
 	return (USBD_IN_PROGRESS);
 }
 
@@ -1880,11 +1893,13 @@ ohci_root_intr_transfer(reqh)
 {
 	usbd_status r;
 
+	/* Insert last in queue. */
 	r = usb_insert_transfer(reqh);
 	if (r != USBD_NORMAL_COMPLETION)
 		return (r);
-	else
-		return (ohci_root_intr_start(reqh));
+
+	/* Pipe isn't running, start first */
+	return (ohci_root_intr_start(SIMPLEQ_FIRST(&reqh->pipe->queue)));
 }
 
 usbd_status
@@ -1927,11 +1942,13 @@ ohci_device_ctrl_transfer(reqh)
 {
 	usbd_status r;
 
+	/* Insert last in queue. */
 	r = usb_insert_transfer(reqh);
 	if (r != USBD_NORMAL_COMPLETION)
 		return (r);
-	else
-		return (ohci_device_ctrl_start(reqh));
+
+	/* Pipe isn't running, start first */
+	return (ohci_device_ctrl_start(SIMPLEQ_FIRST(&reqh->pipe->queue)));
 }
 
 usbd_status
@@ -2001,11 +2018,13 @@ ohci_device_bulk_transfer(reqh)
 {
 	usbd_status r;
 
+	/* Insert last in queue. */
 	r = usb_insert_transfer(reqh);
 	if (r != USBD_NORMAL_COMPLETION)
 		return (r);
-	else
-		return (ohci_device_bulk_start(reqh));
+
+	/* Pipe isn't running, start first */
+	return (ohci_device_bulk_start(SIMPLEQ_FIRST(&reqh->pipe->queue)));
 }
 
 usbd_status
@@ -2133,11 +2152,13 @@ ohci_device_intr_transfer(reqh)
 {
 	usbd_status r;
 
+	/* Insert last in queue. */
 	r = usb_insert_transfer(reqh);
 	if (r != USBD_NORMAL_COMPLETION)
 		return (r);
-	else
-		return (ohci_device_intr_start(reqh));
+
+	/* Pipe isn't running, start first */
+	return (ohci_device_intr_start(SIMPLEQ_FIRST(&reqh->pipe->queue)));
 }
 
 usbd_status
diff --git a/sys/dev/usb/uhci.c b/sys/dev/usb/uhci.c
index 0780fb9ae7cf..ea89e3dc8e1f 100644
--- a/sys/dev/usb/uhci.c
+++ b/sys/dev/usb/uhci.c
@@ -1,4 +1,4 @@
-/*	$NetBSD: uhci.c,v 1.51 1999/09/13 19:49:41 augustss Exp $	*/
+/*	$NetBSD: uhci.c,v 1.52 1999/09/13 21:33:25 augustss Exp $	*/
 
 /*
  * Copyright (c) 1998 The NetBSD Foundation, Inc.
@@ -685,6 +685,8 @@ uhci_add_ctrl(sc, sqh)
 {
 	uhci_soft_qh_t *eqh;
 
+	SPLUSBCHECK;
+
 	DPRINTFN(10, ("uhci_add_ctrl: sqh=%p\n", sqh));
 	eqh = sc->sc_ctl_end;
 	sqh->hlink       = eqh->hlink;
@@ -702,6 +704,8 @@ uhci_remove_ctrl(sc, sqh)
 {
 	uhci_soft_qh_t *pqh;
 
+	SPLUSBCHECK;
+
 	DPRINTFN(10, ("uhci_remove_ctrl: sqh=%p\n", sqh));
 	for (pqh = sc->sc_ctl_start; pqh->hlink != sqh; pqh=pqh->hlink)
 #if defined(DIAGNOSTIC) || defined(USB_DEBUG)		
@@ -726,6 +730,8 @@ uhci_add_bulk(sc, sqh)
 {
 	uhci_soft_qh_t *eqh;
 
+	SPLUSBCHECK;
+
 	DPRINTFN(10, ("uhci_add_bulk: sqh=%p\n", sqh));
 	eqh = sc->sc_bulk_end;
 	sqh->hlink       = eqh->hlink;
@@ -743,6 +749,8 @@ uhci_remove_bulk(sc, sqh)
 {
 	uhci_soft_qh_t *pqh;
 
+	SPLUSBCHECK;
+
 	DPRINTFN(10, ("uhci_remove_bulk: sqh=%p\n", sqh));
 	for (pqh = sc->sc_bulk_start; pqh->hlink != sqh; pqh = pqh->hlink)
 #if defined(DIAGNOSTIC) || defined(USB_DEBUG)		
@@ -882,6 +890,7 @@ uhci_check_intr(sc, ii)
 	uhci_idone(ii);
 }
 
+/* Called at splusb() */
 void
 uhci_idone(ii)
 	uhci_intr_info_t *ii;
@@ -1316,11 +1325,13 @@ uhci_device_bulk_transfer(reqh)
 {
 	usbd_status r;
 
+	/* Insert last in queue. */
 	r = usb_insert_transfer(reqh);
 	if (r != USBD_NORMAL_COMPLETION)
 		return (r);
-	else
-		return (uhci_device_bulk_start(reqh));
+
+	/* Pipe isn't running, start first */
+	return (uhci_device_bulk_start(SIMPLEQ_FIRST(&reqh->pipe->queue)));
 }
 
 usbd_status
@@ -1475,11 +1486,13 @@ uhci_device_ctrl_transfer(reqh)
 {
 	usbd_status r;
 
+	/* Insert last in queue. */
 	r = usb_insert_transfer(reqh);
 	if (r != USBD_NORMAL_COMPLETION)
 		return (r);
-	else
-		return (uhci_device_ctrl_start(reqh));
+
+	/* Pipe isn't running, start first */
+	return (uhci_device_ctrl_start(SIMPLEQ_FIRST(&reqh->pipe->queue)));
 }
 
 usbd_status
@@ -1509,11 +1522,13 @@ uhci_device_intr_transfer(reqh)
 {
 	usbd_status r;
 
+	/* Insert last in queue. */
 	r = usb_insert_transfer(reqh);
 	if (r != USBD_NORMAL_COMPLETION)
 		return (r);
-	else
-		return (uhci_device_intr_start(reqh));
+
+	/* Pipe isn't running, start first */
+	return (uhci_device_intr_start(SIMPLEQ_FIRST(&reqh->pipe->queue)));
 }
 
 usbd_status
@@ -1796,7 +1811,7 @@ uhci_device_isoc_transfer(reqh)
 
 	/* and put on interrupt list if the pipe wasn't running */
 	if (r == USBD_NORMAL_COMPLETION)
-		uhci_device_isoc_start(reqh);
+		uhci_device_isoc_start(SIMPLEQ_FIRST(&reqh->pipe->queue));
 
 	return (r);
 }
@@ -2428,11 +2443,13 @@ uhci_root_ctrl_transfer(reqh)
 {
 	usbd_status r;
 
+	/* Insert last in queue. */
 	r = usb_insert_transfer(reqh);
 	if (r != USBD_NORMAL_COMPLETION)
 		return (r);
-	else
-		return (uhci_root_ctrl_start(reqh));
+
+	/* Pipe isn't running, start first */
+	return (uhci_root_ctrl_start(SIMPLEQ_FIRST(&reqh->pipe->queue)));
 }
 
 usbd_status
@@ -2443,7 +2460,7 @@ uhci_root_ctrl_start(reqh)
 	usb_device_request_t *req;
 	void *buf;
 	int port, x;
-	int len, value, index, status, change, l, totlen = 0;
+	int s, len, value, index, status, change, l, totlen = 0;
 	usb_port_status_t ps;
 	usbd_status r;
 
@@ -2762,7 +2779,9 @@ uhci_root_ctrl_start(reqh)
  ret:
 	reqh->status = r;
 	reqh->hcpriv = 0;
+	s = splusb();
 	usb_transfer_complete(reqh);
+	splx(s);
 	return (USBD_IN_PROGRESS);
 }
 
@@ -2802,11 +2821,13 @@ uhci_root_intr_transfer(reqh)
 {
 	usbd_status r;
 
+	/* Insert last in queue. */
 	r = usb_insert_transfer(reqh);
 	if (r != USBD_NORMAL_COMPLETION)
 		return (r);
-	else
-		return (uhci_root_intr_start(reqh));
+
+	/* Pipe isn't running, start first */
+	return (uhci_root_intr_start(SIMPLEQ_FIRST(&reqh->pipe->queue)));
 }
 
 /* Start a transfer on the root interrupt pipe */
diff --git a/sys/dev/usb/usb.c b/sys/dev/usb/usb.c
index 896882bd659b..957e5a59bb34 100644
--- a/sys/dev/usb/usb.c
+++ b/sys/dev/usb/usb.c
@@ -1,4 +1,4 @@
-/*	$NetBSD: usb.c,v 1.20 1999/09/13 19:18:17 augustss Exp $	*/
+/*	$NetBSD: usb.c,v 1.21 1999/09/13 21:33:25 augustss Exp $	*/
 
 /*
  * Copyright (c) 1998 The NetBSD Foundation, Inc.
@@ -84,6 +84,7 @@ MALLOC_DEFINE(M_USBHC, "USBHC", "USB host controller");
 int	usbdebug = 0;
 int	uhcidebug;
 int	ohcidebug;
+int	usb_noexplore = 0;
 #else
 #define DPRINTF(x)
 #define DPRINTFN(n,x)
@@ -205,6 +206,10 @@ usb_event_thread(arg)
 	while (!sc->shutdown) {
 		(void)tsleep(&sc->sc_bus->needs_explore, 
 			     PWAIT, "usbevt", hz*30);
+#ifdef USB_DEBUG
+		if (usb_noexplore)
+			continue;
+#endif
 		DPRINTFN(2,("usb_event_thread: woke up\n"));
 		usb_discover(sc);
 	}
diff --git a/sys/dev/usb/usbdi.c b/sys/dev/usb/usbdi.c
index a5255a688cc3..651ce6f19f62 100644
--- a/sys/dev/usb/usbdi.c
+++ b/sys/dev/usb/usbdi.c
@@ -1,4 +1,4 @@
-/*	$NetBSD: usbdi.c,v 1.39 1999/09/13 19:18:17 augustss Exp $	*/
+/*	$NetBSD: usbdi.c,v 1.40 1999/09/13 21:33:25 augustss Exp $	*/
 
 /*
  * Copyright (c) 1998 The NetBSD Foundation, Inc.
@@ -681,6 +681,8 @@ usbd_ar_pipe(pipe)
 {
 	usbd_request_handle reqh;
 
+	SPLUSBCHECK;
+
 	DPRINTFN(2,("usbd_ar_pipe: pipe=%p\n", pipe));
 #ifdef USB_DEBUG
 	if (usbdebug > 5)
@@ -716,6 +718,7 @@ usbd_init()
 	}
 }
 
+/* Called at splusb() */
 void
 usb_transfer_complete(reqh)
 	usbd_request_handle reqh;
@@ -724,8 +727,10 @@ usb_transfer_complete(reqh)
 	usb_dma_t *dmap = &reqh->dmabuf;
 	int polling;
 
-	DPRINTFN(5, ("usb_transfer_complete: pipe=%p reqh=%p actlen=%d\n",
-		     pipe, reqh, reqh->actlen));
+	SPLUSBCHECK;
+
+	DPRINTFN(5, ("usb_transfer_complete: pipe=%p reqh=%p status=%d actlen=%d\n",
+		     pipe, reqh, reqh->status, reqh->actlen));
 
 #ifdef DIAGNOSTIC
 	if (!pipe) {
@@ -776,9 +781,14 @@ usb_transfer_complete(reqh)
 	if ((reqh->flags & USBD_SYNCHRONOUS) && !polling)
 		wakeup(reqh);
 
-	if (!pipe->repeat && 
-	    reqh->status != USBD_CANCELLED && reqh->status != USBD_TIMEOUT)
-		usbd_start_next(pipe);
+	if (!pipe->repeat) {
+		/* XXX should we stop the queue on all errors? */
+		if (reqh->status == USBD_CANCELLED ||
+		    reqh->status == USBD_TIMEOUT)
+			pipe->running = 0;
+		else
+			usbd_start_next(pipe);
+	}
 }
 
 usbd_status
@@ -789,8 +799,8 @@ usb_insert_transfer(reqh)
 	usbd_status r;
 	int s;
 
-	DPRINTFN(5,("usb_insert_transfer: pipe=%p running=%d\n", pipe,
-		    pipe->running));
+	DPRINTFN(5,("usb_insert_transfer: pipe=%p running=%d timeout=%d\n", 
+		    pipe, pipe->running, reqh->timeout));
 	s = splusb();
 	SIMPLEQ_INSERT_TAIL(&pipe->queue, reqh, next);
 	if (pipe->running)
@@ -803,6 +813,7 @@ usb_insert_transfer(reqh)
 	return (r);
 }
 
+/* Called at splusb() */
 void
 usbd_start_next(pipe)
 	usbd_pipe_handle pipe;
@@ -810,6 +821,8 @@ usbd_start_next(pipe)
 	usbd_request_handle reqh;
 	usbd_status r;
 
+	SPLUSBCHECK;
+
 	DPRINTFN(10, ("usbd_start_next: pipe=%p\n", pipe));
 	
 #ifdef DIAGNOSTIC
diff --git a/sys/dev/usb/usbdivar.h b/sys/dev/usb/usbdivar.h
index e09c8ff9913e..40680dc7343c 100644
--- a/sys/dev/usb/usbdivar.h
+++ b/sys/dev/usb/usbdivar.h
@@ -1,4 +1,4 @@
-/*	$NetBSD: usbdivar.h,v 1.32 1999/09/13 19:49:41 augustss Exp $	*/
+/*	$NetBSD: usbdivar.h,v 1.33 1999/09/13 21:33:25 augustss Exp $	*/
 
 /*
  * Copyright (c) 1998 The NetBSD Foundation, Inc.
@@ -221,6 +221,16 @@ void		usb_transfer_complete __P((usbd_request_handle reqh));
 int		usb_bus_count __P((void));
 void		usb_needs_explore __P((usbd_bus_handle));
 
+#ifdef DIAGNOSTIC
+#define SPLUSBCHECK \
+	do { int _s = splusb(), _su = splusb(); \
+             if (_s != _su) printf("SPLUSBCHECK failed %d!=%d, %s:%d\n", \
+				   _s, _su, __FILE__, __LINE__); \
+        } while (0)
+#else
+#define SPLUSBCHECK
+#endif
+
 /* Locator stuff. */
 
 #if defined(__NetBSD__)