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>

1 files changed, 41 insertions, 0 deletions

diff --git a/sound/usb/pcm.c b/sound/usb/pcm.c
index 776c58c7cba0..15b151ed4899 100644
--- a/sound/usb/pcm.c
+++ b/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)) &&