GPN_SOF
CTASSERT((GPN_SOF | GPN_EOF) == GPN_FRAME);
CTASSERT((GPN_SOF & GPN_EOF) == 0);
if ((subtype & GPN_SOF) == 0 && sc->sc_rxmbuf == NULL) {
if ((subtype & GPN_SOF) && sc->sc_rxmbuf != NULL) {
gd.gd_subtype = GPN_SOF;
case GPN_SOF: