Commits
Julius Werner committed 1f81b6d22a5
usb: xhci: Prefer endpoint context dequeue pointer over stopped_trb We have observed a rare cycle state desync bug after Set TR Dequeue Pointer commands on Intel LynxPoint xHCs (resulting in an endpoint that doesn't fetch new TRBs and thus an unresponsive USB device). It always triggers when a previous Set TR Dequeue Pointer command has set the pointer to the final Link TRB of a segment, and then another URB gets enqueued and cancelled again before it can be completed. Further investigation showed that the xHC had returned the Link TRB in the TRB Pointer field of the Transfer Event (CC == Stopped -- Length Invalid), but when xhci_find_new_dequeue_state() later accesses the Endpoint Context's TR Dequeue Pointer field it is set to the first TRB of the next segment. The driver expects those two values to be the same in this situation, and uses the cycle state of the latter together with the address of the former. This should be fine according to the XHCI specification, since the endpoint ring should be stopped when returning the Transfer Event and thus should not advance over the Link TRB before it gets restarted. However, real-world XHCI implementations apparently don't really care that much about these details, so the driver should follow a more defensive approach to try to work around HC spec violations. This patch removes the stopped_trb variable that had been used to store the TRB Pointer from the last Transfer Event of a stopped TRB. Instead, xhci_find_new_dequeue_state() now relies only on the Endpoint Context, requiring a small amount of additional processing to find the virtual address corresponding to the TR Dequeue Pointer. Some other parts of the function were slightly rearranged to better fit into this model. This patch should be backported to kernels as old as 2.6.31 that contain the commit ae636747146ea97efa18e04576acd3416e2514f5 "USB: xhci: URB cancellation support." Signed-off-by: Julius Werner <jwerner@chromium.org> Cc: stable@vger.kernel.org Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>