ALSA: usb-audio: detect implicit feedback on Roland devices

All the Roland/Edirol/BOSS USB audio devices that need implicit feedback
show this unambiguously in their descriptors, so it might be a good idea
to let the driver detect this.

This should make playback work correctly (at least with Jack) with the
following devices:
- BOSS GT-100
- BOSS JS-8 Jam Station
- Edirol M-16DX
- Roland GAIA SH-01
Signed-off-by: Clemens Ladisch <clemens@ladisch.de>

sound/usb/pcm.c

@@ -298,6 +298,35 @@ static int deactivate_endpoints(struct snd_usb_substream *subs)
 	return 0;
 }
 
+static int search_roland_implicit_fb(struct usb_device *dev, int ifnum,
+				     unsigned int altsetting,
+				     struct usb_host_interface **alts,
+				     unsigned int *ep)
+{
+	struct usb_interface *iface;
+	struct usb_interface_descriptor *altsd;
+	struct usb_endpoint_descriptor *epd;
+
+	iface = usb_ifnum_to_if(dev, ifnum);
+	if (!iface || iface->num_altsetting < altsetting + 1)
+		return -ENOENT;
+	*alts = &iface->altsetting[altsetting];
+	altsd = get_iface_desc(*alts);
+	if (altsd->bAlternateSetting != altsetting ||
+	    altsd->bInterfaceClass != USB_CLASS_VENDOR_SPEC ||
+	    (altsd->bInterfaceSubClass != 2 &&
+	     altsd->bInterfaceProtocol != 2   ) ||
+	    altsd->bNumEndpoints < 1)
+		return -ENOENT;
+	epd = get_endpoint(*alts, 0);
+	if (!usb_endpoint_is_isoc_in(epd) ||
+	    (epd->bmAttributes & USB_ENDPOINT_USAGE_MASK) !=
+					USB_ENDPOINT_USAGE_IMPLICIT_FB)
+		return -ENOENT;
+	*ep = epd->bEndpointAddress;
+	return 0;
+}
+
 /*
  * find a matching format and set up the interface
  */
@@ -393,6 +422,18 @@ static int set_format(struct snd_usb_substream *subs, struct audioformat *fmt)
 			goto add_sync_ep;
 		}
 	}
+	if (is_playback &&
+	    attr == USB_ENDPOINT_SYNC_ASYNC &&
+	    altsd->bInterfaceClass == USB_CLASS_VENDOR_SPEC &&
+	    altsd->bInterfaceProtocol == 2 &&
+	    altsd->bNumEndpoints == 1 &&
+	    USB_ID_VENDOR(subs->stream->chip->usb_id) == 0x0582 /* Roland */ &&
+	    search_roland_implicit_fb(dev, altsd->bInterfaceNumber + 1,
+				      altsd->bAlternateSetting,
+				      &alts, &ep) >= 0) {
+		implicit_fb = 1;
+		goto add_sync_ep;
+	}
 
 	if (((is_playback && attr == USB_ENDPOINT_SYNC_ASYNC) ||
 	     (!is_playback && attr == USB_ENDPOINT_SYNC_ADAPTIVE)) &&