Re: Question: request tag usage

[Hannes Reinecke <hare@suse.de>]

On 09/26/2014 10:03 AM, Christoph Hellwig wrote:
> On Fri, Sep 26, 2014 at 08:29:29AM +0200, Hannes Reinecke wrote:
>> 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).
> 
> tag are still a live part of SAM for every transport, they've only
> been renamed to "command identifier" in SAM-4 to confuse everyone.
> 
>> 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.
> 
> A quick grep for scsi_activate_tcq disagrees with you.
> 
Yeah, I've noticed after I've written the mail.
However, main point still stands: using 'tag' to identify commands
is pointless if not all of the LLDDs use tagging ...

>> 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?
> 
> Yes.
> 
>> - 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?
> 
> blk-mq will always provide, and does rely on a valid request->tag.
> A LLDD can still use it's own internal tagging or mess with scmd->tag,
> although in general it should benefit from using the block layer
> tagging.
> 
I know. But I was asking about non-mq LLDDs.

>> Which apparently was too much to hope for ...
> 
> I guess for now we'll need to stay with the command pointer address.
> We can revisit this once the old request layer is gone.
> 
Too bad. The tags would have provided a really nice concise way
of identifying the command...

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		      zSeries & Storage
hare@suse.de			      +49 911 74053 688
SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg
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