On Tue, 2008-01-01 at 11:08 -0500, Pete Wyckoff wrote:
> James.Bottomley@xxxxxxxxxxxxxxxxxxxxx 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@xxxxxxx>
> 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@xxxxxxx>
> ---
> 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 */
This is certainly a possible patch. What assurance do you have that it
doesn't upset block devices who set zero assuming they get the default
alignment?
James
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