Re: [PATCH] USB: Fix xhci ERDP update issue

[Mathias Nyman <mathias.nyman@linux.intel.com>]

On 16.3.2022 13.57, Peter Chen wrote:
> On Mon, Mar 14, 2022 at 10:34 PM Greg KH <gregkh@linuxfoundation.org> wrote:
>>
>> On Mon, Mar 14, 2022 at 03:25:23PM +0800, WeitaoWang-oc@zhaoxin.com wrote:
>>> On some situations, software handles TRB events slower than adding TRBs,
>>> xhci_irq will not exit until all events are handled. If xhci_irq just
>>> handles 256 TRBs and exit, the temp variable(event_ring_deq) driver records
>>> in xhci irq is equal to driver current dequeue pointer. It will cause driver
>>> not update ERDP and software dequeue pointer lost sync with ERDP. On the
>>> next xhci_irq, the event ring is full but driver will not update ERDP as
>>> software dequeue pointer is equal to ERDP.
> 
> At the current driver, the ERDP is updated at most 128 TRBs, how is
> the above condition
> triggered?
> 
> Peter
> 

Before, and during _one_ interrupt handling xHC hardware writes exactly 256 events
to event ring. ring buffer size is 256 so buffer position 0 and 256 point
to the same place.

Interrupt handler stores software dequeue in a local variable "event_ring_deq".
Handler start handling events, it updates software dequeue, but not local variable.
After 128 events handler updates hardware ERDP.

So at event 128 we got:
Hardware ERDP = 128
software dequeue = 128
event_ring_deq = 0

Handler continue handling events, at event 256 try to update HW ERDP again, but fail due
to this condition in update_erst_dequeue():
      if (event_ring_deq != xhci->event_ring->dequeue)

This fails because event_ring_deq is still 0, and software deq is 256,
pointing to the same place in the event ring.

So at the end of the interrupt handler we have:
HW ERDP = 128
software dequeue = 256 (same as 0)

So in this specific case we fail to update ERDP correctly

-Mathias