Re: [PATCH 1/5] usb: xhci: Prefer endpoint context dequeue pointer over stopped_trb

[Greg KH <gregkh@linuxfoundation.org>]

On Tue, Apr 22, 2014 at 03:22:58PM +0300, Mathias Nyman wrote:
> From: Julius Werner <jwerner@chromium.org>
> 
> 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."

Ok, but:

> 
> Signed-off-by: Julius Werner <jwerner@chromium.org>
> Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>

You don't actually add the stable@ tag here, why not?

You have read Documentation/stable_kernel_rules.txt for how stable
kernel trees work, so why not add the label here?

greg k-h