On Mon, Nov 26, 2018 at 09:35:51AM -0700, Jens Axboe wrote:

blk-mq passes information to the hardware about any given request being
the last that we will issue in this sequence. The point is that hardware
can defer costly doorbell type writes to the last request. But if we run
into errors issuing a sequence of requests, we may never send the request
with bd->last == true set. For that case, we need a hook that tells the
hardware that nothing else is coming right now.

For failures returned by the drivers ->queue_rq() hook, the driver is
responsible for flushing pending requests, if it uses bd->last to
optimize that part. This works like before, no changes there.

Signed-off-by: Jens Axboe <axboe@kernel.dk>

This looks fine, but normally I would only add the method together with
the actual user..

Reviewed-by: Christoph Hellwig <hch@lst.de>