On Sat, 1 Dec 2018, Anurag Kumar Vulisha wrote: > In some corner cases the gadget controller may get out of sync > with host and may get into hang state, thus creating a dealock. > For example when bulk streams are enabled for an endpoint, there > can be a condition where the gadget controller waits for the host > to issue prime transaction and the host controller waits for the > gadget to issue ERDY. This condition could create a deadlock. > > To avoid such potential deadlocks, a timer is started after queuing > any request for the endpoint in usb_ep_queue(). The gadget driver > is expected to stop the timer if a valid event is found (ex: stream > event for stream capable endpoints). If no valid event is found, the > timer expires after the programmed timeout value and a timeout > callback function registered would be called. This callback function > dequeues the request and re-queues it again, doing so makes the > controller restart the transfer, thus avoiding deadlocks. > > This kind of behaviour is observed in dwc3 controller and expected > to be generic issue with other controllers supporting bulk streams. I find this whole approach rather dubious. First of all, if some sort of deadlock causes a transfer to fail to complete, the host is expected to cancel and restart it. Not the gadget. Second, if a request timer expires and the request is cancelled, the gadget driver's completion handler will be called. This is not what you want if the UDC core is going to resubmit the request automatically. Third, if a request timer expires and the timer handler calls usb_ep_dequeue() followed immediately by usb_ep_queue_timeout(), the resubmit will probably fail because the dequeue won't have completed yet. Fourth, the patch contains a race between the timer expiring and the request completing. Alan Stern
