linux-kernel - Re: [PATCH 9/9] tcm_qla2xxx: Add special case for COMPARE_AND_WRITE data

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Date:	Wed, 21 Aug 2013 09:47:14 -0700
From:	Roland Dreier <roland@...estorage.com>
To:	"Nicholas A. Bellinger" <nab@...erainc.com>
Cc:	target-devel <target-devel@...r.kernel.org>,
	lkml <linux-kernel@...r.kernel.org>,
	linux-scsi <linux-scsi@...r.kernel.org>,
	Christoph Hellwig <hch@....de>, Hannes Reinecke <hare@...e.de>,
	Martin Petersen <martin.petersen@...cle.com>,
	Chris Mason <chris.mason@...ionio.com>,
	James Bottomley <JBottomley@...allels.com>,
	Nicholas Bellinger <nab@...ux-iscsi.org>,
	Giridhar Malavali <giridhar.malavali@...gic.com>,
	Chad Dupuis <chad.dupuis@...gic.com>
Subject: Re: [PATCH 9/9] tcm_qla2xxx: Add special case for COMPARE_AND_WRITE data_direction

On Wed, Aug 21, 2013 at 7:38 AM, Roland Dreier <roland@...estorage.com> wrote:
> I don't understand this.  In fact the whole patch series looks quite
> confused.  COMPARE AND WRITE is a normal Data-Out command, with no
> requirement for special bidirectional handling or anything like that.
> The only slightly unusual thing is that a CAW command with a NUMBER OF
> LOGICAL BLOCKS equal to N will actually transfer 2*N worth of data --
> one set of data for the compare operation and a second set to write if
> the compare succeeds.  But just to be clear, the transfer of those 2*N
> blocks happens as a single transfer during the Data-Out phase.

OK, I understand the patch set a bit better.  You're using the bidi
infrastructure to have a place to stick the data that you internally
read to implement the compare, but then you end up having places like
this where you have to say, "oh it's not really a bidi command, it's
just a compare and write."

Shouldn't there be a way to confine the COMPARE AND WRITE handling to
the actual implementation of that command?  Or maybe make the bidi
handling more generic so that this becomes clearer?

 - R.
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