On Tue, 30 Dec 2014, James Bottomley wrote:
> > _Is_ there any way to communicate the maximum transfer size? I'm not
> > aware of any SCSI command for it. It isn't part of the USB
> > mass-storage spec.
>
> For the device, it's in the Block limits VPD page. However, what the
> device supports isn't necessarily what the bridge or host bus adapter
> will support. We need to set the limit to the lowest of what the
> device, the bridge and the HBA support. We know the device (provided
> the bridge allows VPD inquiries ... not all do) and host, so we really
> need to know what the bridge will support. From the error it does look
> like we're running into a bridge limit.
I see.
> > usb-storage has no clear idea what sort of device lies on the other
> > side of the USB bridge. It might be an ATA drive, it might be a flash
> > drive, it might not be a disk at all -- usb-storage does its best not
> > to know or care.
>
> That's fine, but is there any way in USB to query the bridge to get it's
> transfer characteristics?
No, there isn't. The only query that a USB mass-storage bridge accepts
is for the maximum LUN value.
> > If you think that usb-storage needs to set a maximum transfer size for
> > disk drives, it won't be hard to write a patch. But what about all the
> > other possible transports? Will they each have to implement the same
> > transfer limit? If so, shouldn't the limit be set up from a more
> > central location, such as the sd driver?
>
> This isn't a transport problem, this is a bridge problem. T10 has
> always recognised there might be a bridge issue linking two transports,
> so it did initially come up with a bridge spec (BCC) but it was
> abandoned a decade ago in favour of transparent bridges (every switch in
> a FC topology is effectively a transparent bridge) or making them
> explicit in the standards, like SAS expanders.
>
> > Why not have sd always set max_sectors_kb to 32767 if it isn't already
> > smaller? Would that cause any problems?
>
> This wouldn't be sd ... we have lots of requirements for large transfer
> sizes for efficiency. It has to be the layer that knows there's a
> bridge, so that would make it usb.
All right. How does this patch look?
Alan Stern
Index: usb-3.18/drivers/usb/storage/scsiglue.c
===================================================================
--- usb-3.18.orig/drivers/usb/storage/scsiglue.c
+++ usb-3.18/drivers/usb/storage/scsiglue.c
@@ -114,26 +114,30 @@ static int slave_alloc (struct scsi_devi
static int slave_configure(struct scsi_device *sdev)
{
struct us_data *us = host_to_us(sdev->host);
+ unsigned int max_sectors;
- /* Many devices have trouble transferring 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).
+ /*
+ * Many devices have trouble transferring more than 32 KB at a time,
+ * while others have trouble with more than 64 KB. At this time we
+ * are limiting both to 32 KB (64 sectores).
+ * Still other devices have trouble unless the transfer size is as
+ * small as possible (one memory page).
+ *
+ * Tape drives need much higher max_sector limits, so just
+ * raise it to the maximum possible (4 GB / 512) and let the
+ * queue segment size sort out the real limit.
+ * For safety, limit all other devices to 32 MB transfer size.
*/
- if (us->fflags & (US_FL_MAX_SECTORS_64 | US_FL_MAX_SECTORS_MIN)) {
- unsigned int max_sectors = 64;
-
- if (us->fflags & US_FL_MAX_SECTORS_MIN)
- max_sectors = PAGE_CACHE_SIZE >> 9;
- if (queue_max_hw_sectors(sdev->request_queue) > max_sectors)
- blk_queue_max_hw_sectors(sdev->request_queue,
- max_sectors);
- } else if (sdev->type == TYPE_TAPE) {
- /* Tapes need much higher max_sector limits, so just
- * raise it to the maximum possible (4 GB / 512) and
- * let the queue segment size sort out the real limit.
- */
- blk_queue_max_hw_sectors(sdev->request_queue, 0x7FFFFF);
- }
+ if (us->fflags & US_FL_MAX_SECTORS_MIN)
+ max_sectors = PAGE_CACHE_SIZE >> 9;
+ else if (us->fflags & US_FL_MAX_SECTORS_64)
+ max_sectors = 64;
+ else if (sdev->type == TYPE_TAPE)
+ max_sectors = 0x7FFFFF;
+ else
+ max_sectors = 65535;
+ if (queue_max_hw_sectors(sdev->request_queue) > max_sectors)
+ blk_queue_max_hw_sectors(sdev->request_queue, max_sectors);
/* Some USB host controllers can't do DMA; they have to use PIO.
* They indicate this by setting their dma_mask to NULL. For
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