Re: [PATCH] Promise 'stex' driver

From: Jeff Garzik
Date: Thu Jul 20 2006 - 22:09:21 EST

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
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.


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