be_stream_prepare_req
req = be_stream_prepare_req(evtchnl, XENSND_OP_CLOSE);
req = be_stream_prepare_req(evtchnl, XENSND_OP_WRITE);
req = be_stream_prepare_req(evtchnl, XENSND_OP_READ);
req = be_stream_prepare_req(evtchnl, XENSND_OP_TRIGGER);
req = be_stream_prepare_req(evtchnl, XENSND_OP_HW_PARAM_QUERY);
req = be_stream_prepare_req(evtchnl, XENSND_OP_OPEN);