blk_queue_prep_rq - set a prepare_request function for queue
void blk_queue_prep_rq (request_queue_t * q, prep_rq_fn * pfn);
q queue pfn prepare_request function
It’s possible for a queue to register a prepare_request callback which is invoked before the request is handed to the request_fn. The goal of the function is to prepare a request for I/O, it can be used to build a cdb from the request data for instance.