ENVY_MT_PBUFSZ
sc->intrs[sc->nintr].opos = envy_mt_read_2(sc, ENVY_MT_PBUFSZ);
reg = envy_mt_read_2(sc, ENVY_MT_PBUFSZ);
envy_mt_write_2(sc, ENVY_MT_PBUFSZ, bufsz / 4 - 1);