[SCSI] relax scsi dma alignment

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).

Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>

1 files changed, 8 insertions, 0 deletions

diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
index db52222885b7..de601370c05b 100644
--- a/drivers/scsi/scsi_lib.c
+++ b/drivers/scsi/scsi_lib.c
@@ -1668,6 +1668,14 @@ struct request_queue *__scsi_alloc_queue(struct Scsi_Host *shost,
 
         if (!shost->use_clustering)
                 clear_bit(QUEUE_FLAG_CLUSTER, &q->queue_flags);
+
+        /*
+         * set a reasonable default alignment on word boundaries: the
+         * host and device may alter it using
+         * blk_queue_update_dma_alignment() later.
+         */
+        blk_queue_dma_alignment(q, 0x03);
+
         return q;
 }
 EXPORT_SYMBOL(__scsi_alloc_queue);