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 07:38:21 -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 Tue, Aug 20, 2013 at 1:08 PM, Nicholas A. Bellinger
<nab@...erainc.com> wrote:
> Add a special case for COMPARE_AND_WRITE for the reverse data direction
> mapping used for pci_map_sg() + friends.

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.

 - R.
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