POLL_XFER
struct usb_xfer *sc_xfer[POLL_XFER(USBHID_N_TRANSFER)];
struct usbhid_xfer_ctx sc_xfer_ctx[POLL_XFER(USBHID_N_TRANSFER)];
xfer_idx = POLL_XFER(xfer_idx);
&sc->sc_iface_index, sc->sc_xfer + POLL_XFER(n),
(void *)(sc->sc_xfer_ctx + POLL_XFER(n)),
sc->sc_xfer[POLL_XFER(USBHID_INTR_IN_DT)]);
sc->sc_xfer, POLL_XFER(USBHID_N_TRANSFER));
sc->sc_xfer_ctx[POLL_XFER(USBHID_INTR_IN_DT)] = (struct usbhid_xfer_ctx) {
usbd_transfer_start(sc->sc_xfer[POLL_XFER(USBHID_INTR_IN_DT)]);
usbd_transfer_drain(sc->sc_xfer[POLL_XFER(USBHID_INTR_IN_DT)]);
usbd_transfer_poll(sc->sc_xfer + POLL_XFER(USBHID_INTR_IN_DT), 1);
xfer_ctx = POLL_XFER(xfer_ctx);
xfer_idx = POLL_XFER(xfer_idx);