[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 #10 from Alan Stern (stern@rowland.harvard.edu) ---
Figuring out how to handle isochronous transfers has been very difficult; it's
not obvious what scheme would work best.  Compounding that difficulty is the
fact that different host controller drivers store their (micro)frame numbers
differently, so drivers will never know exactly what to expect.

You say that uhci-hcd used to work "the intuitive way", but in fact experience
has shown that no way is truly intuitive.  Or to put it another differently,
what's intuitive for one person is not intuitive for another.

And don't forget that host controller drivers have to handle isochronous
endpoint queues that can be in three or four different possible states: empty,
partially or completely full, and partially full but totally expired.  It's not
at all easy to come up with a set of flags and values that can handle all the
possibilities in ways that drivers will find convenient to use.

The matter of specifying polling intervals different from what the endpoint
descriptor says is yet another can of worms.  For example, host controllers
generally can't change polling intervals in mid-stream.  And some might reserve
the necessary bandwidth at the time when the alternate setting is installed,
before any URBs have been submitted -- how would one go about changing it from
the default value then?

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