Re: [PATCH] Promise 'stex' driver

[Jeff Garzik <jeff@garzik.org>]

Jens Axboe wrote:
> On Thu, Jul 20 2006, Jeff Garzik wrote:
> > Jens Axboe wrote:
> > > On Thu, Jul 20 2006, Jeff Garzik wrote:
> > > > James Bottomley wrote:
> > > > > On Thu, 2006-07-20 at 17:27 -0400, Jeff Garzik wrote:
> > > > > > Since _no individual SCSI driver_ uses the block layer
> > > > > > tagging, it is likely that some instability and core kernel
> > > > > > development
> > > > > > will occur, in order to make that work.
> > > > > That's not quite true: 53c700 and tmscsim both use it ... I could with
> > > > > the usage were wider, but at least 53c700 has pretty regular and
> > > > > constant usage ... enough I think to validate the block tag code (it's
> > > > > been using it for the last three years).
> > > > Not for the case being discussed in this thread, adapter-wide tags.
> > > That just means the map is shared, otherwise there should be little if
> > > any difference.
> > >
> > > > AFAICS, no file in include/scsi/* or drivers/scsi/* ever calls 
> > > > blk_queue_init_tags() with a non-NULL third arg.
> > > grpe again, it's in scsi_tcq.h.
> > What tree are you looking at?
> >
> > There is only one user in the entire tree, and NULL is hardcoded as the 
> > third arg.  This is 2.6.18-rc2:
>
> Sorry, missed your non-NULL statement, I thought you meant in generel.
>
> As long as you get the locking right for the map access, there's really
> nothing that seperates shared vs non-shared tag mappings. So I don't
> think it's a big deal.
>
> If we don't encourage new drivers to use the block layer tagging, we
> might as well not bother with it.

I don't disagree it's a good thing to have.

I only assert that a --completely unused-- sub-feature cannot be a merge 
requirement for a new driver.  That's an unreasonable standard.  We have 
a driver that is clean and works with all well-known and well-used APIs.

As I discovered painfully via libata, using an unused API upstream 
(->eh_strategy_handler) can often be a destabilizing or limiting factor.

You _hope_ that this feature works, but it's far better IMO to debug an 
unused core feature upstream.  If the upstream driver breaks when 
host-wide blktag support is added, then it's trivial to find where the 
breakage occurred: the stex blktag-update patch.

	Jeff



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