On Thu, May 25, 2017 at 11:43:16AM -0700, Bart Van Assche wrote:

Several block drivers need to initialize the driver-private data
after having called blk_get_request() and before .prep_rq_fn() is
called, e.g. when submitting a REQ_OP_SCSI_* request. Avoid that
that initialization code has to be repeated after every
blk_get_request() call by adding a new callback function to struct
request_queue.

I still think we should do this at the end of blk_mq_alloc_request /
blk_old_get_request to exactly keep the old semantics and keep the
calls out of the hot path.