Re: [PATCH v2 3/3] libata: use blk taging

[Christoph Hellwig <hch@infradead.org>]

On Wed, Jan 14, 2015 at 09:37:30AM -0800, Dan Williams wrote:
> libsas uses the libata allocated tag for NCQ.  libsas first asks
> libata to allocate a tag/qc for ncq, then it creates a sas_task and
> assigns the scsi tag.  To use a generic allocator I assume you would
> need to flip that ordering to allocate the sas_task tag first and then
> use that plus another key to allocate a sub-tag for ata to use for
> NCQ.

As far as I can tell all libsas drivers as well as ipr use block
layer tagging, so they always get requests/scsi_cmnds with a tag
already assigned. For libsas drivers currently use per-LUN tags,
although with scsi-mq we will get per-host tags.

libata on the other hand at the moment assigns it's own tags,
which are only stored in struct ata_queued_cmd, but never in the
request. libata assigns tags per ata_host.  Each ata_host
might have multiple ports, which each get their own Scsi_Host
allocated.

Given that ATA NCQ has a fairly low queue depth I assume we want
to stick to assigning ATA tags per-device, but due to the host
per device scheme libata that even works using scsi-mq. (Q: how do tags
work when port mulipliers are involved?)

So for SAS driver using libata we will need a separate tag allocator,
but that one might as well be in libsas instead of keeping it in libata.