[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 #5 from Alan Stern (stern@rowland.harvard.edu) ---
For anyone who wants to know more about how isochronous scheduling is intended
to work in the Linux USB stack, see the description in the kerneldoc for
usb_submit_urb() in drivers/usb/core/urb.c.

In short, the start_frame field in struct urb is output-only.  The host
controller driver doesn't pay any attention to it when an URB is submitted, but
the frame assigned to the URB's first isochronous packet gets stored in
start_frame before the URB completes.  Furthermore, the URB_ISO_ASAP flag only
affects what happens to an URB that was submitted too late to keep the
endpoint's queue going (say, because interrupts were blocked for too long).  If
the queue is just starting up, or the URB isn't already too late, URB_ISO_ASAP
is ignored.

Also, host controller drivers are not likely to pay attention to the interval
field.  They will use the interval specified in the endpoint's descriptor.

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