[Bug 220748] usb: xhci_queue_isoc_tx_prepare ignore start_frame and always assumes URB_ISO_ASAP is set

[bugzilla-daemon@kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=220748

--- Comment #26 from dylan_robinson@motu.com ---
(In reply to Michał Pecio from comment #21)
>> If, for any reason, a service interval is missed, the alternate setting must
>> be toggled back to zero and then re-enabled, and the stream restarted.
>
> Does the same apply to missed service on IN endpoints?

If the stream is interrupted, the alternate setting should be toggled for the
input streaming interface as well.

> Or IN endpoints with lost/corrupted packets?

This would only affect the ability to maintain implicit feedback
synchronization. As long as the endpoint is serviced during the service
interval, the device's buffer will not overrun. However, if a packet is lost or
corrupted, the driver cannot determine the payload size, which means it cannot
correctly derive the corresponding OUT transfer sizes.

> And for the record, are you saying there is no hope to predict what the HW
> expects to send and receive in the next interval and handle it right?

Correct, the hardware offers no mechanism to predict this. The device relies on
implicit feedback, so the correct OUT transfer size depends on the actual
length received on the corresponding IN endpoint.

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