patch "usb-storage: Revert commit 747668dbc061 ("usb-storage: Set" added to usb-linus

This is a note to let you know that I've just added the patch titled

    usb-storage: Revert commit 747668dbc061 ("usb-storage: Set

to my usb git tree which can be found at
git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/usb.git
in the usb-linus branch.

The patch will show up in the next release of the linux-next tree (usually sometime within the next 24 hours during the week.)

The patch will hopefully also be merged in Linus's tree for the next -rc kernel release.

If you have any questions about this process, please let me know.

From 9a976949613132977098fc49510b46fa8678d864 Mon Sep 17 00:00:00 2001
From: Alan Stern <[email protected]>
Date: Mon, 21 Oct 2019 11:48:06 -0400
Subject: usb-storage: Revert commit 747668dbc061 ("usb-storage: Set
virt_boundary_mask to avoid SG overflows")

Commit 747668dbc061 ("usb-storage: Set virt_boundary_mask to avoid SG overflows") 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 this interacts 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 for usb-storage. 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 fix the swiotlb problem, this patch reverts commit 747668dbc061.

---
 drivers/usb/storage/scsiglue.c | 10 ----------
 1 file changed, 10 deletions(-)

index 6737fab94959..54a3c8195c96 100644
@@ -68,7 +68,6 @@ 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);
-	int maxp;
 
 	/*
 	 * Set the INQUIRY transfer length to 36.  We don't use any of

@@ -77,15 +76,6 @@ static int slave_alloc (struct scsi_device *sdev)
 	 */
 	sdev->inquiry_len = 36;
 
-	/*
-	 * USB has unusual scatter-gather requirements: the length of each
-	 * scatterlist element except the last must be divisible by the
-	 * Bulk maxpacket value.  Fortunately this value is always a
-	 * power of 2.  Inform the block layer about this requirement.
-	 */
-	maxp = usb_maxpacket(us->pusb_dev, us->recv_bulk_pipe, 0);
-	blk_queue_virt_boundary(sdev->request_queue, maxp - 1);
-
 	/*
 	 * Some host controllers may have alignment requirements.
 	 * We'll play it safe by requiring 512-byte alignment always.

2.23.0