ALSA: usb: use multiple packets per urb for Wireless USB inbound audio

For Wireless USB audio devices, use multiple isoc packets per URB for
inbound endpoints with a datainterval < 5.  This allows the WUSB host
controller to take advantage of bursting to service endpoints whose
logical polling interval is less than the 4ms minimum polling interval
limit in WUSB.

Signed-off-by: Thomas Pugliese <thomas.pugliese@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/endpoint.c

@@ -636,8 +636,22 @@ static int data_ep_set_params(struct snd_usb_endpoint *ep,
 	if (usb_pipein(ep->pipe) ||
 			snd_usb_endpoint_implicit_feedback_sink(ep)) {
 
+		urb_packs = packs_per_ms;
+		/*
+		 * Wireless devices can poll at a max rate of once per 4ms.
+		 * For dataintervals less than 5, increase the packet count to
+		 * allow the host controller to use bursting to fill in the
+		 * gaps.
+		 */
+		if (snd_usb_get_speed(ep->chip->dev) == USB_SPEED_WIRELESS) {
+			int interval = ep->datainterval;
+			while (interval < 5) {
+				urb_packs <<= 1;
+				++interval;
+			}
+		}
 		/* make capture URBs <= 1 ms and smaller than a period */
-		urb_packs = min(max_packs_per_urb, packs_per_ms);
+		urb_packs = min(max_packs_per_urb, urb_packs);
 		while (urb_packs > 1 && urb_packs * maxsize >= period_bytes)
 			urb_packs >>= 1;
 		ep->nurbs = MAX_URBS;