[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 #9 from Michał Pecio (michal.pecio@gmail.com) ---
Thanks Alan, that's good to know, because Documentation/ is horribly out of
date and the kerneldoc doesn't really say that this field is ignored as input.

(Also, it says that if the whole URB is already expired, it would fail to
submit, but this behavior was apparently later changed to completing with
-EXDEV).

After a bit of digging, I see that urb->start_frame and URB_ISO_ASAP used to
actually work the intuitive way on UHCI in early 2000s, but then EHCI began to
act as if ASAP is always set, xHCI got the same behavior, UHCI was converted to
EHCI behavior, and ASAP got a new and different meaning.

So today it is unrealistic to expect host controller drivers to respect
start_frame, class drivers aren't setting it to anything meaningful, and the
patch I suggested earlier would likely break some class drivers unless a new
flag separate from URB_ISO_ASAP is added to force using the requested
start_frame.

I still wonder if providing means of overriding the polling interval wouldn't
be easier overall (less changes to snd-usb-audio required to support this new
HW).

-- 
You may reply to this email to add a comment.

You are receiving this mail because:
You are watching the assignee of the bug.