The Microsoft LifeChat 3000 USB headset was causing a very reproducible
hang whenever it was plugged in.  At first, I thought the host
controller was producing bad transfer events, because the log was filled
with errors like:

xhci_hcd 0000:00:14.0: ERROR Transfer event TRB DMA ptr not part of current TD

However, it turned out to be an xHCI driver bug in the ring expansion
patches.  The bug is triggered When there are two ring segments, and a
TD that ends just before a link TRB, like so:

 ______________                     _____________
|              |              ---> | setup TRB B |
 ______________               |     _____________
|              |              |    |  data TRB B |
 ______________               |     _____________
| setup TRB A  | <-- deq      |    |  data TRB B |
 ______________               |     _____________
| data TRB A   |              |    |             | <-- enq, deq''
 ______________               |     _____________
| status TRB A |              |    |             |
 ______________               |     _____________
|  link TRB    |---------------    |  link TRB   |
 _____________  <--- deq'           _____________

TD A (the first control transfer) stalls on the data phase.  That halts
the ring.  The xHCI driver moves the hardware dequeue pointer to the
first TRB after the stalled transfer, which happens to be the link TRB.

Once the Set TR dequeue pointer command completes, the function
update_ring_for_set_deq_completion runs.  That function is supposed to
update the xHCI driver's dequeue pointer to match the internal hardware
dequeue pointer.  On the first call this would work fine, and the
software dequeue pointer would move to deq'.

However, if the transfer immediately after that stalled (TD B in this
case), another Set TR Dequeue command would be issued.  That would move
the hardware dequeue pointer to deq''.  Once that command completed,
update_ring_for_set_deq_completion would run again.

The original code would unconditionally increment the software dequeue
pointer, which moved the pointer off the ring segment into la-la-land.
The while loop would happy increment the dequeue pointer (possibly
wrapping it) until it matched the hardware pointer value.

The while loop would also access all the memory in between the first
ring segment and the second ring segment to determine if it was a link
TRB.  This could cause general protection faults, although it was
unlikely because the ring segments came from a DMA pool, and would often
have consecutive memory addresses.

If nothing in that space looked like a link TRB, the deq_seg pointer for
the ring would remain on the first segment.  Thus, the deq_seg and the
software dequeue pointer would get out of sync.

When the next transfer event came in after the stalled transfer, the
xHCI driver code would attempt to convert the software dequeue pointer
into a DMA address in order to compare the DMA address for the completed
transfer.  Since the deq_seg and the dequeue pointer were out of sync,
xhci_trb_virt_to_dma would return NULL.

The transfer event would get ignored, the transfer would eventually
timeout, and we would mistakenly convert the finished transfer to no-op
TRBs.  Some kernel driver (maybe xHCI?) would then get stuck in an
infinite loop in interrupt context, and the whole machine would hang.

This patch should be backported to kernels as old as 3.4, that contain
the commit b008df60 "xHCI: count free
TRBs on transfer ring"
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Cc: Andiry Xu <andiry.xu@amd.com>
Cc: stable@vger.kernel.org