[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 #13 from Alan Stern (stern@rowland.harvard.edu) ---
The solution that's in uhci-hcd is specific to that driver.  It won't
necessarily work with other drivers.  Or rather, it might work but probably not
in the same way (since qh->bandwidth_reserved will be set and cleared at
different times in different host controller drivers).  It wouldn't be
surprising if it doesn't work at all for some drivers (for example, if
bandwidth_reserved gets set when the altsetting is installed, before any URBs
can be submitted).  The scheme simply is not usable as a general-purpose
approach.

Ignoring bandwidth reservation is the same as ignoring attempts to change the
polling interval, since reserved bandwidth = maxpacket size / polling interval
(roughly speaking).  Besides, the USB spec does not allow USB-2 drivers to
ignore bandwidth reservation, and xHCI hardware enforces bandwidth reservation.

Now, if this turns out to be sufficiently important to enough people, we could
always add a new kernel API specifically meant for changing an endpoint's
interval.  Maybe by adding a new ioctl to usbfs -- I don't know.  This would
complicate the existing usbfs API for reporting endpoint descriptors: Should
the new interval be stored in the descriptor or should it be reported
separately somehow?

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