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譬如模拟一个mass storage,当插入PC后,PC会使用bulk传输跟U盘通信。host会发出OUT令牌和IN令牌。当OUT令牌来了后发出中断,中断处理函数中读取数据发给队列,最后通过usb_ep_queue()得到PC下发的数据,但当hos发出IN令牌时,怎么通知CPU,好让我去准备数据发给PC呢?