On Wed, Mar 16, 2022 at 8:43 PM Mathias Nyman <mathias.nyman@xxxxxxxxxxxxxxx> wrote: > > On 16.3.2022 13.57, Peter Chen wrote: > > On Mon, Mar 14, 2022 at 10:34 PM Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> wrote: > >> > >> On Mon, Mar 14, 2022 at 03:25:23PM +0800, WeitaoWang-oc@xxxxxxxxxxx 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 Cleared, thanks. Peter
