blk_queue_start_tag — find a free tag and assign it
int blk_queue_start_tag (request_queue_t * q, struct request * rq);
This can either be used as a stand-alone helper, or possibly be assigned as the queue &prep_rq_fn (in which case &struct request automagically gets a tag assigned). Note that this function assumes that any type of request can be queued! if this is not true for your device, you must check the request type before calling this function. The request will also be removed from the request queue, so it's the drivers responsibility to readd it if it should need to be restarted for some reason.