scrn_timer
static timeout_t scrn_timer;
scrn_timer(NULL);
callout_reset(&sc->scrn_timer_ch, hz / 10, scrn_timer, sc);
callout_reset(&sc->scrn_timer_ch, hz / 10, scrn_timer, sc);
callout_reset(&sc->scrn_timer_ch, hz / 10, scrn_timer, sc);
callout_reset(&sc->scrn_timer_ch, hz / 25, scrn_timer, sc);
scrn_timer(sc);