ARC_REG_OUTB_DOORBELL
if (arc_read(sc, ARC_REG_OUTB_DOORBELL) != 0) {
while ((reg = arc_read(sc, ARC_REG_OUTB_DOORBELL)) == 0)
arc_write(sc, ARC_REG_OUTB_DOORBELL, reg);
reg = arc_read(sc, ARC_REG_OUTB_DOORBELL);
arc_write(sc, ARC_REG_OUTB_DOORBELL, reg);