linux-kernel - Re: [PATCH] Promise 'stex' driver

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Message-ID: <44C037B3.4080707@garzik.org>
Date:	Thu, 20 Jul 2006 22:10:59 -0400
From:	Jeff Garzik <jeff@...zik.org>
To:	Jens Axboe <axboe@...e.de>
CC:	James Bottomley <James.Bottomley@...elEye.com>,
	Ed Lin <ed.lin@...mise.com>,
	"linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
	hch <hch@...radead.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	akpm <akpm@...l.org>, promise_linux <promise_linux@...mise.com>
Subject: Re: [PATCH] Promise 'stex' driver

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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