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    SubjectRe: [PATCH] usb hub: send clear_tt_buffer_complete events when canceling TT clear work
    On Mon, 1 Oct 2012, Alan Stern wrote:

    > On Mon, 1 Oct 2012, Octavian Purdila wrote:
    >
    > > There is a race condition in the USB hub code with regard to handling
    > > TT clear requests that can get the HCD driver in a deadlock. Usually
    > > when an TT clear request is scheduled it will be executed immediately:
    >
    > > However, if a suspend operation is triggered before hub_tt_work is
    > > scheduled, hub_quiesce will cancel the work without notifying the HCD
    > > driver:
    >
    > Ah, this is a very good point.
    >
    > > When the device will resume the EHCI driver will get stuck in
    > > ehci_endpoint_disable waiting for the tt_clearing flag to reset:
    >
    > > This patch notifies the HCD driver via clear_tt_buffer_complete
    > > when canceling TT clear work so that the EHCI driver has a chance to
    > > clear its tt_clearing flag.
    >
    > But I don't like the proposed fix. What we really need to do is avoid
    > suspending the device until the clear-tt-buffer operation is complete.
    > Can you write a patch to do that instead?

    Actually, it would be sufficient to avoid suspending the _hub_ until
    the clear-tt-buffer operation is complete. In short, hub_quiesce()
    should wait until all pending TT operations have finished. The
    cancel_work_sync() call should be replaced with flush_work_sync().

    Alan Stern



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