On 06/13/2016 09:21 AM, Christoph Hellwig wrote:

blk_get_request is used for BLOCK_PC and similar pass through requests.
Currently we always need to call blk_rq_set_block_pc or an opencoded
version of it to allow appending bios using the request mapping helpers
later on, which is a somewhat awkward API.  Instead move the
initialization part of blk_rq_set_block_pc into blk_get_request, so that
we always have a safe to use request.  blk_rq_set_block_pc now goes away
in favor of just setting cmd_type to REQ_TYPE_BLOCK_PC, or nothing in case
it was overriden with a different type a little later (or earlier in case
of the SCSI OSD code..)

It may be awkward, but we have those to avoid doing things like this:

+	rq->__data_len = 0;
+	rq->__sector = (sector_t) -1;
+	rq->bio = rq->biotail = NULL;
+	memset(rq->__cmd, 0, sizeof(rq->__cmd));

for every request we allocate, when we don't use ->cmd at all. Honest, 
I'd rather have

struct request *blk_get_pc_request();

and similar helpers around this, so we don't have to do extra 
initialization when we don't need it.

-- 
Jens Axboe