Index: sys/dev/sound/pcm/channel.h
===================================================================
--- sys/dev/sound/pcm/channel.h
+++ sys/dev/sound/pcm/channel.h
@@ -258,6 +258,7 @@
 int chn_reinit(struct pcm_channel *c);
 int chn_write(struct pcm_channel *c, struct uio *buf);
 int chn_read(struct pcm_channel *c, struct uio *buf);
+u_int32_t chn_prepare(struct pcm_channel *c, int enable);
 u_int32_t chn_start(struct pcm_channel *c, int force);
 int chn_sync(struct pcm_channel *c, int threshold);
 int chn_flush(struct pcm_channel *c);
@@ -367,6 +368,7 @@
 #define CHN_F_VCHAN_ADAPTIVE	0x00008000  /* adaptive format/rate selection */
 #define CHN_F_VCHAN_DYNAMIC	(CHN_F_VCHAN_PASSTHROUGH | CHN_F_VCHAN_ADAPTIVE)
 
+#define CHN_F_READY		0x01000000
 #define	CHN_F_VIRTUAL		0x10000000  /* not backed by hardware */
 #define CHN_F_BITPERFECT	0x20000000  /* un-cooked, Heh.. */
 #define CHN_F_PASSTHROUGH	0x40000000  /* passthrough re-config */
Index: sys/dev/sound/pcm/channel.c
===================================================================
--- sys/dev/sound/pcm/channel.c
+++ sys/dev/sound/pcm/channel.c
@@ -670,6 +670,15 @@
 	CHN_UNLOCK(c);
 }
 
+u_int32_t
+chn_prepare(struct pcm_channel *c, int enable)
+{
+	int ret = 0;
+
+	ret = CHANNEL_PREPARE(c->methods, c->devinfo, enable);
+	return ret;
+}
+
 u_int32_t
 chn_start(struct pcm_channel *c, int force)
 {
Index: sys/dev/sound/pcm/channel_if.m
===================================================================
--- sys/dev/sound/pcm/channel_if.m
+++ sys/dev/sound/pcm/channel_if.m
@@ -153,6 +153,12 @@
 	int go;
 };
 
+METHOD int prepare {
+	kobj_t obj;
+	void *data;
+	int go;
+};
+
 METHOD u_int32_t getptr {
 	kobj_t obj;
 	void *data;
Index: sys/dev/sound/pcm/dsp.c
===================================================================
--- sys/dev/sound/pcm/dsp.c
+++ sys/dev/sound/pcm/dsp.c
@@ -1085,7 +1085,6 @@
 	ret = 0;
 	xcmd = 0;
 	chn = NULL;
-
 	if (IOCGROUP(cmd) == 'M') {
 		if (cmd == OSS_GETVERSION) {
 			*arg_i = SOUND_VERSION;
@@ -1734,28 +1733,34 @@
 
     	case SNDCTL_DSP_SETTRIGGER:
 		if (rdch) {
-			CHN_LOCK(rdch);
-			rdch->flags &= ~CHN_F_NOTRIGGER;
-		    	if (*arg_i & PCM_ENABLE_INPUT)
+		    	if (*arg_i & PCM_ENABLE_INPUT) {
+				chn_prepare(rdch, 1);
+				CHN_LOCK(rdch);
 				chn_start(rdch, 1);
-			else {
+				rdch->flags &= ~CHN_F_NOTRIGGER;
+				CHN_UNLOCK(rdch);
+			} else {
+				CHN_LOCK(rdch);
 				chn_abort(rdch);
 				chn_resetbuf(rdch);
 				rdch->flags |= CHN_F_NOTRIGGER;
+				CHN_UNLOCK(rdch);
 			}
-			CHN_UNLOCK(rdch);
 		}
 		if (wrch) {
-			CHN_LOCK(wrch);
-			wrch->flags &= ~CHN_F_NOTRIGGER;
-		    	if (*arg_i & PCM_ENABLE_OUTPUT)
+		    	if (*arg_i & PCM_ENABLE_OUTPUT) {
+				chn_prepare(wrch, 1);
+				CHN_LOCK(wrch);
 				chn_start(wrch, 1);
-			else {
+				wrch->flags &= ~CHN_F_NOTRIGGER;
+				CHN_UNLOCK(wrch);
+			} else {
+				CHN_LOCK(wrch);
 				chn_abort(wrch);
 				chn_resetbuf(wrch);
 				wrch->flags |= CHN_F_NOTRIGGER;
+				CHN_UNLOCK(wrch);
 			}
-			CHN_UNLOCK(wrch);
 		}
 		break;
 
@@ -1789,11 +1794,17 @@
 
     	case SNDCTL_DSP_POST:
 		if (wrch) {
+			chn_prepare(wrch, 1);
 			CHN_LOCK(wrch);
 			wrch->flags &= ~CHN_F_NOTRIGGER;
 			chn_start(wrch, 1);
 			CHN_UNLOCK(wrch);
 		}
+#if 0
+		if (rdch) {
+			chn_prepare(rdch, 1);
+		}
+#endif
 		break;
 
 	case SNDCTL_DSP_SETDUPLEX:
Index: sys/dev/sound/pcm/vchan.c
===================================================================
--- sys/dev/sound/pcm/vchan.c
+++ sys/dev/sound/pcm/vchan.c
@@ -184,6 +184,21 @@
 	return (ret);
 }
 
+static int
+vchan_prepare(kobj_t obj, void *data, int go)
+{
+	struct vchan_info *info;
+	struct pcm_channel *c, *p;
+
+	info = data;
+	c = info->channel;
+	p = c->parentchannel;
+
+	if (p)
+		chn_prepare(p, go);
+	return 1;
+}
+
 static struct pcmchan_caps *
 vchan_getcaps(kobj_t obj, void *data)
 {
@@ -244,6 +259,7 @@
 	KOBJMETHOD(channel_setformat,		vchan_setformat),
 	KOBJMETHOD(channel_setspeed,		vchan_setspeed),
 	KOBJMETHOD(channel_trigger,		vchan_trigger),
+	KOBJMETHOD(channel_prepare,		vchan_prepare),
 	KOBJMETHOD(channel_getcaps,		vchan_getcaps),
 	KOBJMETHOD(channel_getmatrix,		vchan_getmatrix),
 	KOBJMETHOD_END
Index: sys/dev/sound/usb/uaudio.h
===================================================================
--- sys/dev/sound/usb/uaudio.h
+++ sys/dev/sound/usb/uaudio.h
@@ -56,6 +56,7 @@
 		    uint32_t format);
 extern int	uaudio_chan_set_param_format(struct uaudio_chan *ch,
 		    uint32_t format);
+extern void	uaudio_chan_prepare(struct uaudio_chan *ch, int enable);
 extern void	uaudio_chan_start(struct uaudio_chan *ch);
 extern void	uaudio_chan_stop(struct uaudio_chan *ch);
 extern int	uaudio_mixer_init_sub(struct uaudio_softc *, struct snd_mixer *);
Index: sys/dev/sound/usb/uaudio.c
===================================================================
--- sys/dev/sound/usb/uaudio.c
+++ sys/dev/sound/usb/uaudio.c
@@ -267,6 +267,7 @@
 #define	CHAN_OP_DRAIN 3
 
 	uint8_t iface_index;
+	uint8_t cur_alt_index;
 };
 
 #define	UMIDI_EMB_JACK_MAX   16		/* units */
@@ -1342,6 +1343,8 @@
 		goto error;
 	}
 
+	chan->cur_alt_index = (operation == CHAN_OP_START) ?
+			chan_alt->iface_alt_index : 0;
 	/*
 	 * Only set the sample rate if the channel reports that it
 	 * supports the frequency control.
@@ -2775,6 +2778,22 @@
 	    rchan->running == 0);
 }
 
+void
+uaudio_chan_prepare(struct uaudio_chan *ch, int enable)
+{
+	struct uaudio_softc *sc = ch->priv_sc;
+
+	if (ch->cur_alt_index == ch->usb_alt[ch->cur_alt].iface_alt_index) {
+		return;
+	}
+	usbd_req_set_alt_interface_no(sc->sc_udev,
+		NULL,
+		ch->iface_index,
+		ch->usb_alt[ch->cur_alt].iface_alt_index);
+
+	ch->cur_alt_index = ch->usb_alt[ch->cur_alt].iface_alt_index;
+}
+
 void
 uaudio_chan_start(struct uaudio_chan *ch)
 {
@@ -2836,8 +2855,20 @@
 		}
 	}
 	if (do_stop) {
-		usbd_transfer_stop(ch->xfer[0]);
-		usbd_transfer_stop(ch->xfer[1]);
+		usb_proc_explore_unlock(sc->sc_udev);
+		CHN_UNLOCK(ch->pcm_ch);
+
+		usbd_transfer_drain(ch->xfer[0]);
+		usbd_transfer_drain(ch->xfer[1]);
+
+		usbd_req_set_alt_interface_no(sc->sc_udev,
+			NULL,
+			ch->iface_index,
+			0);
+		ch->cur_alt_index = 0;
+		CHN_LOCK(ch->pcm_ch);
+		usb_proc_explore_lock(sc->sc_udev);
+
 	}
 }
 
Index: sys/dev/sound/usb/uaudio_pcm.c
===================================================================
--- sys/dev/sound/usb/uaudio_pcm.c
+++ sys/dev/sound/usb/uaudio_pcm.c
@@ -79,6 +79,13 @@
 	return (uaudio_chan_set_param_fragments(data, blocksize, blockcount));
 }
 
+static int
+ua_chan_prepare(kobj_t obj, void *data, int go)
+{
+	uaudio_chan_prepare(data, go);
+	return 0;
+}
+
 static int
 ua_chan_trigger(kobj_t obj, void *data, int go)
 {
@@ -118,6 +125,7 @@
 	KOBJMETHOD(channel_setblocksize, ua_chan_setblocksize),
 	KOBJMETHOD(channel_setfragments, ua_chan_setfragments),
 	KOBJMETHOD(channel_trigger, ua_chan_trigger),
+	KOBJMETHOD(channel_prepare, ua_chan_prepare),
 	KOBJMETHOD(channel_getptr, ua_chan_getptr),
 	KOBJMETHOD(channel_getcaps, ua_chan_getcaps),
 	KOBJMETHOD(channel_getmatrix, ua_chan_getmatrix),