1 files changed, 15 insertions, 10 deletions

diff --git a/drivers/usb/storage/scsiglue.c b/drivers/usb/storage/scsiglue.c
index b4c9e0f18a82..09779f6a8179 100644
--- a/drivers/usb/storage/scsiglue.c
+++ b/drivers/usb/storage/scsiglue.c
@@ -71,7 +71,6 @@ static const char* host_info(struct Scsi_Host *host)
 static int slave_alloc (struct scsi_device *sdev)
 {
         struct us_data *us = host_to_us(sdev->host);
-        struct usb_host_endpoint *bulk_in_ep;
 
         /*
          * Set the INQUIRY transfer length to 36.  We don't use any of
@@ -80,16 +79,22 @@ 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.  We'll use the maxpacket value of the bulk-IN pipe
-         * to set the SCSI device queue's DMA alignment mask.
+        /* USB has unusual DMA-alignment requirements: Although the
+         * starting address of each scatter-gather element doesn't matter,
+         * the length of each element except the last must be divisible
+         * by the Bulk maxpacket value.  There's currently no way to
+         * express this by block-layer constraints, so we'll cop out
+         * and simply require addresses to be aligned at 512-byte
+         * boundaries.  This is okay since most block I/O involves
+         * hardware sectors that are multiples of 512 bytes in length,
+         * and since host controllers up through USB 2.0 have maxpacket
+         * values no larger than 512.
+         *
+         * But it doesn't suffice for Wireless USB, where Bulk maxpacket
+         * values can be as large as 2048.  To make that work properly
+         * will require changes to the block layer.
          */
-        bulk_in_ep = us->pusb_dev->ep_in[usb_pipeendpoint(us->recv_bulk_pipe)];
-        blk_queue_update_dma_alignment(sdev->request_queue,
-                        le16_to_cpu(bulk_in_ep->desc.wMaxPacketSize) - 1);
-                        /* wMaxPacketSize must be a power of 2 */
+        blk_queue_update_dma_alignment(sdev->request_queue, (512 - 1));
 
         /*
          * The UFI spec treates the Peripheral Qualifier bits in an