Revert "usb: xhci: Link TRB must not occur within a USB payload burst"

This reverts commit 35773dac5f862cb1c82ea151eba3e2f6de51ec3e.  It's a
hack that caused regressions in the usb-storage and userspace USB
drivers that use usbfs and libusb.  Commit 70cabb7d992f "xhci 1.0: Limit
arbitrarily-aligned scatter gather." should fix the issues seen with the
ax88179_178a driver on xHCI 1.0 hosts, without causing regressions.

Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Cc: stable@vger.kernel.org # 3.12

2 files changed, 2 insertions, 54 deletions

diff --git a/drivers/usb/host/xhci-ring.c b/drivers/usb/host/xhci-ring.c
index 0272450a1667..0ed64eb68e48 100644
--- a/drivers/usb/host/xhci-ring.c
+++ b/drivers/usb/host/xhci-ring.c
@@ -2967,58 +2967,8 @@ static int prepare_ring(struct xhci_hcd *xhci, struct xhci_ring *ep_ring,
         }
 
         while (1) {
-                if (room_on_ring(xhci, ep_ring, num_trbs)) {
-                        union xhci_trb *trb = ep_ring->enqueue;
-                        unsigned int usable = ep_ring->enq_seg->trbs +
-                                        TRBS_PER_SEGMENT - 1 - trb;
-                        u32 nop_cmd;
-
-                        /*
-                         * Section 4.11.7.1 TD Fragments states that a link
-                         * TRB must only occur at the boundary between
-                         * data bursts (eg 512 bytes for 480M).
-                         * While it is possible to split a large fragment
-                         * we don't know the size yet.
-                         * Simplest solution is to fill the trb before the
-                         * LINK with nop commands.
-                         */
-                        if (num_trbs == 1 || num_trbs <= usable || usable == 0)
-                                break;
-
-                        if (ep_ring->type != TYPE_BULK)
-                                /*
-                                 * While isoc transfers might have a buffer that
-                                 * crosses a 64k boundary it is unlikely.
-                                 * Since we can't add NOPs without generating
-                                 * gaps in the traffic just hope it never
-                                 * happens at the end of the ring.
-                                 * This could be fixed by writing a LINK TRB
-                                 * instead of the first NOP - however the
-                                 * TRB_TYPE_LINK_LE32() calls would all need
-                                 * changing to check the ring length.
-                                 */
-                                break;
-
-                        if (num_trbs >= TRBS_PER_SEGMENT) {
-                                xhci_err(xhci, "Too many fragments %d, max %d\n",
-                                                num_trbs, TRBS_PER_SEGMENT - 1);
-                                return -ENOMEM;
-                        }
-
-                        nop_cmd = cpu_to_le32(TRB_TYPE(TRB_TR_NOOP) |
-                                        ep_ring->cycle_state);
-                        ep_ring->num_trbs_free -= usable;
-                        do {
-                                trb->generic.field[0] = 0;
-                                trb->generic.field[1] = 0;
-                                trb->generic.field[2] = 0;
-                                trb->generic.field[3] = nop_cmd;
-                                trb++;
-                        } while (--usable);
-                        ep_ring->enqueue = trb;
-                        if (room_on_ring(xhci, ep_ring, num_trbs))
-                                break;
-                }
+                if (room_on_ring(xhci, ep_ring, num_trbs))
+                        break;
 
                 if (ep_ring == xhci->cmd_ring) {
                         xhci_err(xhci, "Do not support expand command ring\n");
diff --git a/include/linux/usb.h b/include/linux/usb.h
index c716da18c668..7f6eb859873e 100644
--- a/include/linux/usb.h
+++ b/include/linux/usb.h
@@ -1265,8 +1265,6 @@ typedef void (*usb_complete_t)(struct urb *);
  * @sg: scatter gather buffer list, the buffer size of each element in
  *      the list (except the last) must be divisible by the endpoint's
  *      max packet size if no_sg_constraint isn't set in 'struct usb_bus'
- *      (FIXME: scatter-gather under xHCI is broken for periodic transfers.
- *      Do not use urb->sg for interrupt endpoints for now, only bulk.)
  * @num_mapped_sgs: (internal) number of mapped sg entries
  * @num_sgs: number of entries in the sg list
  * @transfer_buffer_length: How big is transfer_buffer.  The transfer may