[PATCH 1/5 v2] blk-mq: Add prep/unprep support

[Christoph Hellwig]

On Wed, Apr 15, 2015 at 02:34:40PM +0200, Matias Bj??rling wrote:
> Allow users to hook into prep/unprep functions just before an IO is
> dispatched to the device driver. This is necessary for request-based
> logic to take place at upper layers.

I don't think any of this logic belongs into the block layer.  All this
should be library functions called by the drivers.