[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 #14 from Michał Pecio (michal.pecio@gmail.com) ---
I skimmed the linked forum thread and it seems the issue is not so much with
"needing a specific frame interval", but not tolerating any missing packets.

https://linuxmusicians.com/viewtopic.php?p=177688#p177688

So no-gaps scheduling is already the default *without* URB_ISO_ASAP and drivers
don't need to fill sequential frame IDs themselves if that's what they want.
Note that this stops working when the queue underruns - next URB is assumed to
be a new stream and will be scheduled into the future, not into the past.

If there are gaps despite the driver resubmitting on time, that may be a bug,
but a different one than originally assumed.

If drivers want to insert a gap to resynchronize after detected lost packets,
I'm afraid that's not supported in the current model. It would be technically
possible on CFC xHCI hosts and likely EHCI. A concern is how drivers would know
when frame ID overflows back to zero and how much into the future they can
schedule.

The urb->start_frame field is now output, so drivers can use it to confirm if
the URBs are scheduled sequentially as intended. At least with CFC, it appears
that these numbers should be consistent with reality, though I have never
tested it. Debugfs/tracing/printk may be used to check them against TRBs
written to the HW.

Non-CFC looks weird, maybe it could be improved somewhat but not sure. The old
xHCI 1.0 spec is limited in what it guarantees.

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