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EHCI spec for linking in new qTDs into an asynchronous QH. This requires that there is a qTD marked as not active and not halted at the start of the QH's list, and the hardware will know to re-fetch the qTD on each pass rather than just looking at the overlay qTD: "The host controller must be able to advance the queue from the Fetch QH state in order to avoid all hardware/software race conditions. This simple mechanism allows software to simply link qTDs to the queue head and activate them, then the host controller will always find them if/when they are reachable." This is achieved by keeping an "inactivesqtd" entry on the QH list, and re-using it each time as the start of the next transfer, and allocating a new qTD to become the next inactivesqtd. Then a new transfer can be activated by just setting its "active" flag, which avoids all the previous messing with overlay qTD state in ehci_set_qh_qtd().
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