PLAYBACK_URB_PACKETS
max_packet_size * PLAYBACK_URB_PACKETS;
usb_alloc_urb(PLAYBACK_URB_PACKETS, GFP_KERNEL);
for (i = 0; i < PLAYBACK_URB_PACKETS; i++) {
total_bytes_in_urb = nominal_bytes_per_packet * PLAYBACK_URB_PACKETS;
urb->number_of_packets = PLAYBACK_URB_PACKETS;