Re: [PATCH] megaraid_sas: Enable shared host tag map

[Christoph Hellwig <hch@lst.de>]

On Mon, Nov 24, 2014 at 04:51:14PM +0100, Hannes Reinecke wrote:
> It is useful as is, as we'll be getting prefixed logging output :-)

Use the blk-mq code path if you care :)

> Which I didn't do yet as the driver is using a larger tag map than
> that one announced to the block layer.
> This is to facilitate internal command submission, which should
> always work independent on any tag starvation issues from the
> upper layers.

This is an "issue" for a lot of drivers.  blk-mq provides a reserved_tags
pool for that, which reserves a number of tags for internal use, those
must be allocated using blk_mq_alloc_request with the reserved argument
set to true.

The lockless hpsa patches expose this to SCSI, which I'm generally
fine with, but we need to find a way to transparently make this work
for the old code path, too.  This might be as simple as embedding a
second blk_queue_tag structure into the Scsi_Host, adding a constant
prefix to the tag and providing some wrappes in scsi that allow
allocating a struct request (or rather scsi_cmnd) for internal use.