TAIL_ADDR
ENGINE_READ_FW(engine, RING_TAIL) & TAIL_ADDR);
(ENGINE_READ_FW(engine, RING_TAIL) & TAIL_ADDR))
(ENGINE_READ(engine, RING_TAIL) & TAIL_ADDR))
ENGINE_READ(engine, RING_TAIL) & TAIL_ADDR);
ENGINE_READ(engine, RING_TAIL) & TAIL_ADDR,
(head & HEAD_ADDR) != (tail & TAIL_ADDR))