usb: dwc3: gadget: Prevent core from processing stale TRBs

With CPU re-ordering on write instructions, there might
be a chance that the HWO is set before the TRB is updated
with the new mapped buffer address.
And in the case where core is processing a list of TRBs
it is possible that it fetched the TRBs when the HWO is set
but before the buffer address is updated.
Prevent this by adding a memory barrier before the HWO
is updated to ensure that the core always process the
updated TRBs.

Fixes: f6bafc6 ("usb: dwc3: convert TRBs into bitshifts")
Cc: stable <[email protected]>
Reviewed-by: Pavankumar Kondeti <[email protected]>
Signed-off-by: Udipto Goswami <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Author: Udipto Goswami

drivers/usb/dwc3/gadget.c

@@ -1291,6 +1291,19 @@ static void __dwc3_prepare_one_trb(struct dwc3_ep *dep, struct dwc3_trb *trb,
 	if (usb_endpoint_xfer_bulk(dep->endpoint.desc) && dep->stream_capable)
 		trb->ctrl |= DWC3_TRB_CTRL_SID_SOFN(stream_id);
 
+	/*
+	 * As per data book 4.2.3.2TRB Control Bit Rules section
+	 *
+	 * The controller autonomously checks the HWO field of a TRB to determine if the
+	 * entire TRB is valid. Therefore, software must ensure that the rest of the TRB
+	 * is valid before setting the HWO field to '1'. In most systems, this means that
+	 * software must update the fourth DWORD of a TRB last.
+	 *
+	 * However there is a possibility of CPU re-ordering here which can cause
+	 * controller to observe the HWO bit set prematurely.
+	 * Add a write memory barrier to prevent CPU re-ordering.
+	 */
+	wmb();
 	trb->ctrl |= DWC3_TRB_CTRL_HWO;
 
 	dwc3_ep_inc_enq(dep);