Comment 41 for bug 986817

*
  * Synchronizing the playback rate to the capture rate is done by using
  * the same sequence of packet sizes for both streams.
  * Submitting a playback URB therefore requires both a ready URB and
  * the size of the corresponding capture packet, i.e., both playback
  * and capture URBs must have been completed. Since the USB core does
  * not guarantee that playback and capture complete callbacks are
  * called alternately, we use two FIFOs for packet sizes and read URBs;
  * submitting playback URBs is possible as long as both FIFOs are
  * nonempty.
  */

the driver seem expect same rate for playback and capture