Index: sys/dev/usb/serial/usb_serial.c
===================================================================
--- sys/dev/usb/serial/usb_serial.c
+++ sys/dev/usb/serial/usb_serial.c
@@ -154,7 +154,8 @@
 static void	ucom_detach_tty(struct ucom_super_softc *, struct ucom_softc *);
 static void	ucom_queue_command(struct ucom_softc *,
 		    usb_proc_callback_t *, struct termios *pt,
-		    struct usb_proc_msg *t0, struct usb_proc_msg *t1);
+		    struct usb_proc_msg *t0, struct usb_proc_msg *t1,
+		    bool sync);
 static void	ucom_shutdown(struct ucom_softc *);
 static void	ucom_ring(struct ucom_softc *, uint8_t);
 static void	ucom_break(struct ucom_softc *, uint8_t);
@@ -598,7 +599,8 @@
 static void
 ucom_queue_command(struct ucom_softc *sc,
     usb_proc_callback_t *fn, struct termios *pt,
-    struct usb_proc_msg *t0, struct usb_proc_msg *t1)
+    struct usb_proc_msg *t0, struct usb_proc_msg *t1,
+    bool sync)
 {
 	struct ucom_super_softc *ssc = sc->sc_super;
 	struct ucom_param_task *task;
@@ -629,9 +631,11 @@
 		task->termios_copy = *pt;
 
 	/*
-	 * Closing the device should be synchronous.
+	 * Synchronous execution will mostly be used for closing the device or
+	 * param changes.  Caller should make that decision, though, rather than
+	 * assuming that here that the caller wants to block on it.
 	 */
-	if (fn == ucom_cfg_close)
+	if (sync)
 		usb_proc_mwait(&ssc->sc_tq, t0, t1);
 
 	/*
@@ -789,12 +793,12 @@
 
 	ucom_queue_command(sc, ucom_cfg_open, NULL,
 	    &sc->sc_open_task[0].hdr,
-	    &sc->sc_open_task[1].hdr);
+	    &sc->sc_open_task[1].hdr, false);
 
 	/* Queue transfer enable command last */
 	ucom_queue_command(sc, ucom_cfg_start_transfers, NULL,
 	    &sc->sc_start_task[0].hdr, 
-	    &sc->sc_start_task[1].hdr);
+	    &sc->sc_start_task[1].hdr, false);
 
 	ucom_modem(tp, SER_DTR | SER_RTS, 0);
 
@@ -842,7 +846,7 @@
 
 	ucom_queue_command(sc, ucom_cfg_close, NULL,
 	    &sc->sc_close_task[0].hdr,
-	    &sc->sc_close_task[1].hdr);
+	    &sc->sc_close_task[1].hdr, true);
 
 	sc->sc_flag &= ~(UCOM_FLAG_HL_READY | UCOM_FLAG_RTS_IFLOW);
 
@@ -1085,7 +1089,7 @@
 	/* defer driver programming */
 	ucom_queue_command(sc, ucom_cfg_line_state, NULL,
 	    &sc->sc_line_state_task[0].hdr, 
-	    &sc->sc_line_state_task[1].hdr);
+	    &sc->sc_line_state_task[1].hdr, false);
 }
 
 static void
@@ -1247,7 +1251,7 @@
 
 	ucom_queue_command(sc, ucom_cfg_status_change, NULL,
 	    &sc->sc_status_task[0].hdr,
-	    &sc->sc_status_task[1].hdr);
+	    &sc->sc_status_task[1].hdr, false);
 }
 
 static void
@@ -1322,12 +1326,12 @@
 	/* Queue baud rate programming command first */
 	ucom_queue_command(sc, ucom_cfg_param, t,
 	    &sc->sc_param_task[0].hdr,
-	    &sc->sc_param_task[1].hdr);
+	    &sc->sc_param_task[1].hdr, true);
 
 	/* Queue transfer enable command last */
 	ucom_queue_command(sc, ucom_cfg_start_transfers, NULL,
 	    &sc->sc_start_task[0].hdr, 
-	    &sc->sc_start_task[1].hdr);
+	    &sc->sc_start_task[1].hdr, false);
 
 	if (t->c_cflag & CRTS_IFLOW) {
 		sc->sc_flag |= UCOM_FLAG_RTS_IFLOW;