some question about xhci skip_isoc_td

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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?

appreciate your kind help,
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