uas: Make sure sg elements are properly aligned

Copy the sg alignment trick from the usb-storage driver, without this I'm
seeing intermittent errors when using uas devices with an ehci controller.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>

1 files changed, 18 insertions, 0 deletions

diff --git a/drivers/usb/storage/uas.c b/drivers/usb/storage/uas.c
index 62086829af14..ad97615b75b1 100644
--- a/drivers/usb/storage/uas.c
+++ b/drivers/usb/storage/uas.c
@@ -925,6 +925,24 @@ static int uas_eh_bus_reset_handler(struct scsi_cmnd *cmnd)
 static int uas_slave_alloc(struct scsi_device *sdev)
 {
         sdev->hostdata = (void *)sdev->host->hostdata;
+
+        /* 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.
+         */
+        blk_queue_update_dma_alignment(sdev->request_queue, (512 - 1));
+
         return 0;
 }