Messages in this thread | | | Date | Thu, 20 Jul 2006 22:10:59 -0400 | From | Jeff Garzik <> | Subject | Re: [PATCH] Promise 'stex' driver |
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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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