ALSA: usb: handle descriptor with SYNC_NONE illegal value

The M-Audio Transit exposes an interface with a SYNC_NONE attribute.
This is not a valid value according to the USB audio classspec. However
there is a sync endpoint associated to this record. Changing the logic to
try to use this sync endpoint allows for seamless transitions between
altset 2 and altset 3. If any errors happen, the behavior remains the same.

$ more /proc/asound/card1/stream0
M-Audio Transit USB at usb-0000:00:14.0-2, full speed : USB Audio

Playback:
  Status: Stop
  Interface 1
    Altset 1
    Format: S24_3LE
    Channels: 2
    Endpoint: 3 OUT (ADAPTIVE)
    Rates: 48001 - 96000 (continuous)
  Interface 1
    Altset 2
    Format: S24_3LE
    Channels: 2
    Endpoint: 3 OUT (NONE)
    Rates: 8000 - 48000 (continuous)
  Interface 1
    Altset 3
    Format: S16_LE
    Channels: 2
    Endpoint: 3 OUT (ASYNC)
    Rates: 8000 - 48000 (continuous)
Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/pcm.c

@@ -412,10 +412,17 @@ static int set_sync_endpoint(struct snd_usb_substream *subs,
 	if (altsd->bNumEndpoints < 2)
 		return 0;
 
-	if ((is_playback && attr != USB_ENDPOINT_SYNC_ASYNC) ||
+	if ((is_playback && (attr == USB_ENDPOINT_SYNC_SYNC ||
+			     attr == USB_ENDPOINT_SYNC_ADAPTIVE)) ||
 	    (!is_playback && attr != USB_ENDPOINT_SYNC_ADAPTIVE))
 		return 0;
 
+	/*
+	 * In case of illegal SYNC_NONE for OUT endpoint, we keep going to see
+	 * if we don't find a sync endpoint, as on M-Audio Transit. In case of
+	 * error fall back to SYNC mode and don't create sync endpoint
+	 */
+
 	/* check sync-pipe endpoint */
 	/* ... and check descriptor size before accessing bSynchAddress
 	   because there is a version of the SB Audigy 2 NX firmware lacking
@@ -429,6 +436,8 @@ static int set_sync_endpoint(struct snd_usb_substream *subs,
 			   get_endpoint(alts, 1)->bmAttributes,
 			   get_endpoint(alts, 1)->bLength,
 			   get_endpoint(alts, 1)->bSynchAddress);
+		if (is_playback && attr == USB_ENDPOINT_SYNC_NONE)
+			return 0;
 		return -EINVAL;
 	}
 	ep = get_endpoint(alts, 1)->bEndpointAddress;
@@ -439,6 +448,8 @@ static int set_sync_endpoint(struct snd_usb_substream *subs,
 			"%d:%d : invalid sync pipe. is_playback %d, ep %02x, bSynchAddress %02x\n",
 			   fmt->iface, fmt->altsetting,
 			   is_playback, ep, get_endpoint(alts, 0)->bSynchAddress);
+		if (is_playback && attr == USB_ENDPOINT_SYNC_NONE)
+			return 0;
 		return -EINVAL;
 	}
 
@@ -450,8 +461,11 @@ static int set_sync_endpoint(struct snd_usb_substream *subs,
 						   implicit_fb ?
 							SND_USB_ENDPOINT_TYPE_DATA :
 							SND_USB_ENDPOINT_TYPE_SYNC);
-	if (!subs->sync_endpoint)
+	if (!subs->sync_endpoint) {
+		if (is_playback && attr == USB_ENDPOINT_SYNC_NONE)
+			return 0;
 		return -EINVAL;
+	}
 
 	subs->data_endpoint->sync_master = subs->sync_endpoint;