[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>

5 files changed, 34 insertions, 11 deletions

diff --git a/drivers/ata/libata-scsi.c b/drivers/ata/libata-scsi.c
index 264ae60e3fd8..4bb268b9aaeb 100644
--- a/drivers/ata/libata-scsi.c
+++ b/drivers/ata/libata-scsi.c
@@ -824,6 +824,9 @@ static void ata_scsi_sdev_config(struct scsi_device *sdev)
          * requests.
          */
         sdev->max_device_blocked = 1;
+
+        /* set the min alignment */
+        blk_queue_update_dma_alignment(sdev->request_queue, ATA_DMA_PAD_SZ - 1);
 }
 
 static void ata_scsi_dev_config(struct scsi_device *sdev,
@@ -878,7 +881,7 @@ int ata_scsi_slave_config(struct scsi_device *sdev)
         if (dev)
                 ata_scsi_dev_config(sdev, dev);
 
-        return 0;       /* scsi layer doesn't check return value, sigh */
+        return 0;
 }
 
 /**
diff --git a/drivers/firewire/fw-sbp2.c b/drivers/firewire/fw-sbp2.c
index 624ff3e082f6..c2169d215bf7 100644
--- a/drivers/firewire/fw-sbp2.c
+++ b/drivers/firewire/fw-sbp2.c
@@ -1238,6 +1238,12 @@ static int sbp2_scsi_slave_alloc(struct scsi_device *sdev)
 
         sdev->allow_restart = 1;
 
+        /*
+         * Update the dma alignment (minimum alignment requirements for
+         * start and end of DMA transfers) to be a sector
+         */
+        blk_queue_update_dma_alignment(sdev->request_queue, 511);
+
         if (lu->tgt->workarounds & SBP2_WORKAROUND_INQUIRY_36)
                 sdev->inquiry_len = 36;
 
diff --git a/drivers/ieee1394/sbp2.c b/drivers/ieee1394/sbp2.c
index b83d254bc86e..1eda11abeb1e 100644
--- a/drivers/ieee1394/sbp2.c
+++ b/drivers/ieee1394/sbp2.c
@@ -1963,6 +1963,12 @@ static int sbp2scsi_slave_alloc(struct scsi_device *sdev)
         lu->sdev = sdev;
         sdev->allow_restart = 1;
 
+        /*
+         * Update the dma alignment (minimum alignment requirements for
+         * start and end of DMA transfers) to be a sector
+         */
+        blk_queue_update_dma_alignment(sdev->request_queue, 511);
+
         if (lu->workarounds & SBP2_WORKAROUND_INQUIRY_36)
                 sdev->inquiry_len = 36;
         return 0;
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);
diff --git a/drivers/usb/storage/scsiglue.c b/drivers/usb/storage/scsiglue.c
index 7c9593b7b04e..dd8b13ef2414 100644
--- a/drivers/usb/storage/scsiglue.c
+++ b/drivers/usb/storage/scsiglue.c
@@ -81,6 +81,16 @@ static int slave_alloc (struct scsi_device *sdev)
          */
         sdev->inquiry_len = 36;
 
+        /* Scatter-gather buffers (all but the last) must have a length
+         * divisible by the bulk maxpacket size.  Otherwise a data packet
+         * would end up being short, causing a premature end to the data
+         * transfer.  Since high-speed bulk pipes have a maxpacket size
+         * of 512, we'll use that as the scsi device queue's DMA alignment
+         * mask.  Guaranteeing proper alignment of the first buffer will
+         * have the desired effect because, except at the beginning and
+         * the end, scatter-gather buffers follow page boundaries. */
+        blk_queue_update_dma_alignment(sdev->request_queue, (512 - 1));
+
         /*
          * The UFI spec treates the Peripheral Qualifier bits in an
          * INQUIRY result as reserved and requires devices to set them
@@ -100,16 +110,6 @@ static int slave_configure(struct scsi_device *sdev)
 {
         struct us_data *us = host_to_us(sdev->host);
 
-        /* Scatter-gather buffers (all but the last) must have a length
-         * divisible by the bulk maxpacket size.  Otherwise a data packet
-         * would end up being short, causing a premature end to the data
-         * transfer.  Since high-speed bulk pipes have a maxpacket size
-         * of 512, we'll use that as the scsi device queue's DMA alignment
-         * mask.  Guaranteeing proper alignment of the first buffer will
-         * have the desired effect because, except at the beginning and
-         * the end, scatter-gather buffers follow page boundaries. */
-        blk_queue_dma_alignment(sdev->request_queue, (512 - 1));
-
         /* Many devices have trouble transfering more than 32KB at a time,
          * while others have trouble with more than 64K. At this time we
          * are limiting both to 32K (64 sectores).