hi Mathias:
2014-06-24 16:38 GMT+08:00 Mathias Nyman <mathias.ny...@linux.intel.com>:
> On 06/23/2014 03:44 PM, David Laight wrote:
>> From: vichy
>>> hi all:
>>> when i trace xhci source, I found in skip_isoc_td, there is a
>>> possibility to increase de-queue pointer twice.
>>>
>>> in skip_isoc_td:
>>> /* Update ring dequeue pointer */
>>> while (ep_ring->dequeue != td->last_trb)
>>> inc_deq(xhci, ep_ring);
>>> inc_deq(xhci, ep_ring);
>>>
>>> why (ep_ring->dequeue != td->last_trb) we have to increase the dequeue
>>> pointer?
>>> I try to find any description in spec about this but in vain.
>>> is there any special reason to do so?
>>
>> An isoc transfer can consist of a lot of TRB.
>> The code wants to skip all of them.
>> I suspect there are faster ways to achieve the same effect!
>>
>
> As David says, the inc_dec() in the while loop will move the dequeue pointer
> to the last TRB of that TD.
> The additional inc_deq() then moves it to the next TD, (or to enqueue if no
> more TDs are on the endpoint ring)
>
> section 4.10.1 in xhci specification has some info on this:
>
> "If a Missed Service Error occurs on
did "Missed Service Error occurs" you mean is when we cannot find
match ep_dequeue_seg which contain event_buffer?
like the below code in handle_tx_event:
if (!event_seg) {
.......................
}
> an intermediate TRB of a TD of an Isoch endpoint the xHC shall advance to the
> first TRB of the next TD or
> the Enqueue Pointer (i.e.Cycle bit transition), whichever is encountered
> first, when continuing execution on
> the Transfer Ring."
>
appreciate your help,
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