[PATCH 4.9 37/62] UAS: Revert commit 3ae62a42090f ("UAS: fix alignment of scatter/gather segments")

Greg Kroah-Hartman
2019-11-04T22:26:58Z

From: Alan Stern <stern@rowland.harvard.edu>

commit 1186f86a71130a7635a20843e355bb880c7349b2 upstream.

Commit 3ae62a42090f ("UAS: fix alignment of scatter/gather segments"),
copying a similar commit for usb-storage, attempted to solve a problem
involving scatter-gather I/O and USB/IP by setting the
virt_boundary_mask for mass-storage devices.

However, it now turns out that the analogous change in usb-storage
interacted badly with commit 09324d32d2a0 ("block: force an unlimited
segment size on queues with a virt boundary"), which was added later.
A typical error message is:

ehci-pci 0000:00:13.2: swiotlb buffer is full (sz: 327680 bytes),
total 32768 (slots), used 97 (slots)

There is no longer any reason to keep the virt_boundary_mask setting
in the uas driver. It was needed in the first place only for
handling devices with a block size smaller than the maxpacket size and
where the host controller was not capable of fully general
scatter-gather operation (that is, able to merge two SG segments into
a single USB packet). But:

High-speed or slower connections never use a bulk maxpacket
value larger than 512;

The SCSI layer does not handle block devices with a block size
smaller than 512 bytes;

All the host controllers capable of SuperSpeed operation can
handle fully general SG;

Since commit ea44d190764b ("usbip: Implement SG support to
vhci-hcd and stub driver") was merged, the USB/IP driver can
also handle SG.

Therefore all supported device/controller combinations should be okay
with no need for any special virt_boundary_mask. So in order to head
off potential problems similar to those affecting usb-storage, this
patch reverts commit 3ae62a42090f.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
CC: Oliver Neukum <oneukum@suse.com>
CC: <stable@vger.kernel.org>
Acked-by: Christoph Hellwig <hch@lst.de>
Fixes: 3ae62a42090f ("UAS: fix alignment of scatter/gather segments")
Link: https://lore.kernel.org/r/Pine.LNX.4.44L0.1910231132470.1878-100000@iolanthe.rowland.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
drivers/usb/storage/uas.c | 20 --------------------
1 file changed, 20 deletions(-)

--- a/drivers/usb/storage/uas.c
+++ b/drivers/usb/storage/uas.c
@@ -796,30 +796,10 @@ static int uas_slave_alloc(struct scsi_d
{
struct uas_dev_info *devinfo =
(struct uas_dev_info *)sdev->host->hostdata;
- int maxp;

sdev->hostdata = devinfo;

/*
- * We have two requirements here. We must satisfy the requirements
- * of the physical HC and the demands of the protocol, as we
- * definitely want no additional memory allocation in this path
- * ruling out using bounce buffers.
- *
- * For a transmission on USB to continue we must never send
- * a package that is smaller than maxpacket. Hence the length of each
- * scatterlist element except the last must be divisible by the
- * Bulk maxpacket value.
- * If the HC does not ensure that through SG,
- * the upper layer must do that. We must assume nothing
- * about the capabilities off the HC, so we use the most
- * pessimistic requirement.
- */
-
- maxp = usb_maxpacket(devinfo->udev, devinfo->data_in_pipe, 0);
- blk_queue_virt_boundary(sdev->request_queue, maxp - 1);
-
- /*
* The protocol has no requirements on alignment in the strict sense.
* Controllers may or may not have alignment restrictions.
* As this is not exported, we use an extremely conservative guess.