[PATCH] USB: UHCI: Improved handling of short control transfers
This patch simplies the way the UHCI driver handles short control transfers. When a transfer is short the HC will stop handling that endpoint, and it's necessary to get it going again so that the status stage of the control transfer can take place. Currently the driver does this by allocating a new QH for the transfer and setting its element pointer to point at the final status TD. The old QH is recycled. But it's not necessary to go to all that trouble; the element pointer in the original QH can be updated directly. Normally the element pointer is supposed to be owned by the HC, and it's not safe to just change its value since the HC may overwrite it at any time. But when a transfer is stopped because of a short packet, the current TD is marked inactive and the HC will not update the element pointer. To write an unchanged pointer value back to memory would be a waste of PCI bus cycles. Now the UHCI spec doesn't say explicitly that an HC _can't_ do this, but I've tested both Intel and VIA hardware and neither of them does. As a side effect of this change, some of the code for removing QHs can be eliminated.
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