[PATCH] usb: xhci: Fix Short Packet handling rework ignoring errors

[Michal Pecio <michal.pecio@gmail.com>]

A Short Packet event before the last TRB of a TD is followed by another
event on the final TRB on spec-compliant HCs, which is most of them.

A 'last_td_was_short' flag was added to know if a TD has just completed
as Short Packet and another event is to come. The flag was cleared after
seeing the event (unless no TDs are pending, but that's a separate bug)
or seeing a new TD complete as something other than Short Packet.

A rework replaced the flag with an 'old_trb_comp_code' variable. When
an event doesn't match the pending TD and the previous event was Short
Packet, the new event is silently ignored.

To preserve old behavior, 'old_trb_comp_code' should be cleared at this
point, but instead it is being set to current comp code, which is often
Short Packet again. This can cause more events to be silently ignored,
even though they are no longer connected with the old TD that completed
short and indicate a serious problem with the driver or the xHC.

Common device classes like UAC in async mode, UVC, serial or the UAS
status pipe complete as Short Packet routinely and could be affected.

Clear 'old_trb_comp_code' to zero, which is an invalid completion code
and the same value the variable starts with. This restores original
behavior on Short Packet and also works for illegal Etron events, which
the code has been extended to cover too.

Fixes: b331a3d8097f ("xhci: Handle spurious events on Etron host isoc enpoints")
Signed-off-by: Michal Pecio <michal.pecio@gmail.com>
---
 drivers/usb/host/xhci-ring.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/usb/host/xhci-ring.c b/drivers/usb/host/xhci-ring.c
index 9e50cc3354cb..a35ad5489790 100644
--- a/drivers/usb/host/xhci-ring.c
+++ b/drivers/usb/host/xhci-ring.c
@@ -2916,7 +2916,7 @@ static int handle_tx_event(struct xhci_hcd *xhci,
 			if (xhci_spurious_success_tx_event(xhci, ep_ring)) {
 				xhci_dbg(xhci, "Spurious event dma %pad, comp_code %u after %u\n",
 					 &ep_trb_dma, trb_comp_code, ep_ring->old_trb_comp_code);
-				ep_ring->old_trb_comp_code = trb_comp_code;
+				ep_ring->old_trb_comp_code = 0;
 				return 0;
 			}
 
-- 
2.48.1