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

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Message-ID: <20130821155314.GA17850@infradead.org>
Date:	Wed, 21 Aug 2013 08:53:14 -0700
From:	Christoph Hellwig <hch@...radead.org>
To:	Roland Dreier <roland@...estorage.com>
Cc:	"Nicholas A. Bellinger" <nab@...erainc.com>,
	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 07:38:21AM -0700, Roland Dreier 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.

I think the confusion is that the implementation of COMPARE AND WRITE
obviously requires a read and a write phase, and the implementation
tries to mix this up with an actual bidirectional scsi command.

If the core stopped keying off t_bidi_data_sg and used better flag
this could be easily solved.
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