I believe you are asking whenever host is sending IN request for data, how does SX2 get notified of this? That will be taken care of by the SIE. If there is data in the USB domain, ready for an IN transfer, then if the host requests for data, it will be sent by the SIE in the data phase. If there is no data at the arrival of IN token, then SIE will NAK the request. Devcie just needs to commit the data to the USB domain; so that whenever host requests for it, it would be sent across by the SIE.
Dear Gayathri, thank you very much for your reply, it is realy helpful! Thanks again!