Question: request tag usage

[Hannes Reinecke <hare@suse.de>]

Hi Christoph,

as discussed it would make sense to use the request->tag in eg
scmd_printk() to identify the command.
Which I duly did, only to figure out that the tag is always '-1', ie
tagging is not in use.
(Which is okay from the SCSI side, seeing the TCQ is basically a
SCSI parallel thing).
Looking closer I found plenty of code for handling tags in the block
layer (and the blk-mq stuff, of course), but virtually none of the
non-SPI driver seems to be using them.
Which makes the original idea a bit pointless, seeing that we need
to identify the command _always_, and not just if the host happens
to support tagging.

Which leads me to some questions:
- Is the stuff in blk-mq supposed to work as a superset of SCSI TCQ?
- If so, should any HBAs with a queue depth > 1 (which does not
  support TCQ) set the tag of a command?
  (that's what I've initially thought would happen ...)
- If not (and the ->tag field is basically unused), can't we
  have the HBA to fill in a value here?

What I would like to see is to have the '->tag' field as an
identification for the commands in flight. IE every HBA with a queue
depth > 1 should be setting the tags.

Which apparently was too much to hope for ...

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		      zSeries & Storage
hare@suse.de			      +49 911 74053 688
SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg
GF: J. Hawn, J. Guild, F. Imendörffer, HRB 16746 (AG Nürnberg)
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