ALSA: snd-usb: add support for implicit feedback

Implicit feedback is a streaming mode that does not rely on dedicated
sync endpoints but uses the information provided by record streams to
clock output streams. Now that the streaming logic is decoupled from the
PCM streams, this is easy to implement.

Signed-off-by: Daniel Mack <zonque@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/pcm.c

@@ -301,7 +301,7 @@ static int set_format(struct snd_usb_substream *subs, struct audioformat *fmt)
 	struct usb_interface *iface;
 	unsigned int ep, attr;
 	int is_playback = subs->direction == SNDRV_PCM_STREAM_PLAYBACK;
-	int err;
+	int err, implicit_fb = 0;
 
 	iface = usb_ifnum_to_if(dev, fmt->iface);
 	if (WARN_ON(!iface))
@@ -326,42 +326,57 @@ static int set_format(struct snd_usb_substream *subs, struct audioformat *fmt)
 	 * assume it as adaptive-out or sync-in.
 	 */
 	attr = fmt->ep_attr & USB_ENDPOINT_SYNCTYPE;
-	if (((is_playback && attr == USB_ENDPOINT_SYNC_ASYNC) ||
-	     (! is_playback && attr == USB_ENDPOINT_SYNC_ADAPTIVE)) &&
-	    altsd->bNumEndpoints >= 2) {
-		switch (subs->stream->chip->usb_id) {
-		case USB_ID(0x0763, 0x2080): /* M-Audio FastTrack Ultra */
-		case USB_ID(0x0763, 0x2081):
+
+	switch (subs->stream->chip->usb_id) {
+	case USB_ID(0x0763, 0x2080): /* M-Audio FastTrack Ultra */
+	case USB_ID(0x0763, 0x2081):
+		if (is_playback) {
+			implicit_fb = 1;
 			ep = 0x81;
 			iface = usb_ifnum_to_if(dev, 2);
+
+			if (!iface || iface->num_altsetting == 0)
+				return -EINVAL;
+
 			alts = &iface->altsetting[1];
 			goto add_sync_ep;
 		}
+	}
 
+	if (((is_playback && attr == USB_ENDPOINT_SYNC_ASYNC) ||
+	     (!is_playback && attr == USB_ENDPOINT_SYNC_ADAPTIVE)) &&
+	    altsd->bNumEndpoints >= 2) {
 		/* check sync-pipe endpoint */
 		/* ... and check descriptor size before accessing bSynchAddress
 		   because there is a version of the SB Audigy 2 NX firmware lacking
 		   the audio fields in the endpoint descriptors */
 		if ((get_endpoint(alts, 1)->bmAttributes & USB_ENDPOINT_XFERTYPE_MASK) != 0x01 ||
 		    (get_endpoint(alts, 1)->bLength >= USB_DT_ENDPOINT_AUDIO_SIZE &&
-		     get_endpoint(alts, 1)->bSynchAddress != 0)) {
+		     get_endpoint(alts, 1)->bSynchAddress != 0 &&
+		     !implicit_fb)) {
 			snd_printk(KERN_ERR "%d:%d:%d : invalid synch pipe\n",
 				   dev->devnum, fmt->iface, fmt->altsetting);
 			return -EINVAL;
 		}
 		ep = get_endpoint(alts, 1)->bEndpointAddress;
 		if (get_endpoint(alts, 0)->bLength >= USB_DT_ENDPOINT_AUDIO_SIZE &&
 		    (( is_playback && ep != (unsigned int)(get_endpoint(alts, 0)->bSynchAddress | USB_DIR_IN)) ||
-		     (!is_playback && ep != (unsigned int)(get_endpoint(alts, 0)->bSynchAddress & ~USB_DIR_IN)))) {
+		     (!is_playback && ep != (unsigned int)(get_endpoint(alts, 0)->bSynchAddress & ~USB_DIR_IN)) ||
+		     ( is_playback && !implicit_fb))) {
 			snd_printk(KERN_ERR "%d:%d:%d : invalid synch pipe\n",
 				   dev->devnum, fmt->iface, fmt->altsetting);
 			return -EINVAL;
 		}
+
+		implicit_fb = (get_endpoint(alts, 1)->bmAttributes & USB_ENDPOINT_USAGE_MASK)
+				== USB_ENDPOINT_USAGE_IMPLICIT_FB;
+
 add_sync_ep:
 		subs->sync_endpoint = snd_usb_add_endpoint(subs->stream->chip,
 							   alts, ep, !subs->direction,
-							   SND_USB_ENDPOINT_TYPE_SYNC);
-
+							   implicit_fb ?
+								SND_USB_ENDPOINT_TYPE_DATA :
+								SND_USB_ENDPOINT_TYPE_SYNC);
 		if (!subs->sync_endpoint)
 			return -EINVAL;