US_IOBUF_SIZE
BUG_ON(num_registers > US_IOBUF_SIZE/2);
BUG_ON(num_registers > US_IOBUF_SIZE/2);
us->iobuf = usb_alloc_coherent(us->pusb_dev, US_IOBUF_SIZE,
usb_free_coherent(us->pusb_dev, US_IOBUF_SIZE, us->iobuf, us->iobuf_dma);