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    SubjectRE: [PATCH v12 7/8] usb: Adding SuperSpeed support to dummy_hcd
    On Mon, 23 May 2011, Tanya Brokhman wrote:

    > I ran some more tests with xhci and I think (or hope :) ) I figured this
    > out:
    > When connecting a gadget driver that is marked as SS device (the flag
    > CONFIG_USB_GADGET_IS_SUPER_SPEED = true) to a SS port over SS cable -
    > the enumeration fails if that gadget driver doesn't provide SS descriptors.
    > BUT: if I connect the same device via HS cable to SS port - the enumeration
    > is successful. I think that this is the case where xhci-ring handles the
    > device over to the HS hcd :) (By the way, I think that in xhci the
    > shared_hcd is SS and the main_hcd is HS)
    > In conclusion it seems to me that the device speed is determined by 2
    > things:
    > 1. the cable used
    > 2. whether the device HW supports SS protocol. In our scenario it can since
    > SS support is enabled in our udc. (We haven't released it yet.)
    > So when a HS device is connected to a SS port, the xHCI checks it's speed
    > and if necessary handles it over to the SS root hub. But this is done prior
    > to the enumeration phase so if the device speed is SS but it has no SS
    > descriptors - the enumeration will fail. The enumeration itself occurs not
    > in xhci but in hub.c so the xhci isn't aware of the fact that it failed and
    > doesn't handle this.
    > Since in dummy_hcd all of this is much simpler I think that the device speed
    > should be determined by driver->speed and "which type of cable the
    > connection was made over - SS or HS". The "cable type" is exactly what the
    > module parameter is.
    > My familiarity with the Linux host isn't as good as I would like it to be
    > (still working on that) so I might be wrong with my conclusions...

    Your analysis is basically correct.

    Alan Stern

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