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-rw-r--r--drivers/usb/storage/scsiglue.c25
1 files changed, 15 insertions, 10 deletions
diff --git a/drivers/usb/storage/scsiglue.c b/drivers/usb/storage/scsiglue.c
index b4c9e0f18a8..09779f6a817 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