Re: [PATCH 2/2] relax scsi dma alignment

[Pete Wyckoff <pw@osc.edu>]

James.Bottomley@HansenPartnership.com wrote on Mon, 31 Dec 2007 16:43 -0600:
> This patch relaxes the default SCSI DMA alignment from 512 bytes to 4
> bytes.  I remember from previous discussions that usb and firewire have
> sector size alignment requirements, so I upped their alignments in the
> respective slave allocs.
> 
> The reason for doing this is so that we don't get such a huge amount of
> copy overhead in bio_copy_user() for udev.  (basically all inquiries it
> issues can now be directly mapped).

Great change.  Here's another patch.  It allows a queue
dma_alignment of 0 bytes, for drivers that can move data
at byte granularity.

Two drivers try to turn off DMA alignment restrictions by
setting the dma_alignment to zero:

    drivers/scsi/iscsi_tcp.c: blk_queue_dma_alignment(sdev->request_queue, 0);
    drivers/scsi/scsi_tgt_lib.c: blk_queue_dma_alignment(q, 0);

But they end up doing copies due to the way that queue_dma_alignment()
returns 511 in this case.

		-- Pete

---

>From 90ee6d61ad71a024815eee2b416edb40c6b01256 Mon Sep 17 00:00:00 2001
From: Pete Wyckoff <pw@osc.edu>
Date: Tue, 1 Jan 2008 10:23:02 -0500
Subject: [PATCH] block: allow queue dma_alignment of zero

Let queue_dma_alignment return 0 if it was specifically set to 0.
This permits devices with no particular alignment restrictions to
use arbitrary user space buffers without copying.

Signed-off-by: Pete Wyckoff <pw@osc.edu>
---
 include/linux/blkdev.h |    7 +------
 1 files changed, 1 insertions(+), 6 deletions(-)

diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index aa3090c..eea31c2 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -860,12 +860,7 @@ static inline int bdev_hardsect_size(struct block_device *bdev)
 
 static inline int queue_dma_alignment(struct request_queue *q)
 {
-	int retval = 511;
-
-	if (q && q->dma_alignment)
-		retval = q->dma_alignment;
-
-	return retval;
+	return q ? q->dma_alignment : 511;
 }
 
 /* assumes size > 256 */
-- 
1.5.3.6