> To handle things smarter, I think I need to research how to deal with
> hubs attached to hubs attached to hubs. For instance:
>
> dwc2
> -> multi_tt hub
> -> single_tt hub
> -> device 1
> -> device 2
Keep in mind that there's always at most one (active) TT between host
and device. The TT is the point where high-speed traffic is translated
to low-/full-speed traffic, so after that you cannot translate again.
With multiple hubs you either have
-> high-speed 2.0 hub (TT inactive / irrelevant for this path)
-> multi or single TT 2.0 hub
-> device
or
-> multi or single TT 2.0 hub
-> full-speed 1.1 hub
-> device
All the information you need should already be in struct usb_device.
If udev->tt->multi == 0, then it must be scheduled in the same group
as all other devices it shares udev->tt (the same pointer address)
with. If udev->tt->multi == 1, then it belongs in the same group as
all that have the same udev->tt and the same udev->ttport. There's
even a udev->tt->hcpriv where you could link a data structure (array)
in to keep track of these matching devices.
I agree that this is a nice-to-have optimization, though... it's more
important to get the thing stable, and I think it's fine to assume
that all low-/full-speed transfers go on the same bus for the first
iteration.
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