xhci: Add rmb() between reading event validity & event data access.

On weakly-ordered systems, the reading of an event's content must occur
after reading the event's validity.

Signed-off-by: Matt Evans <matt@ozlabs.org>
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>

1 files changed, 5 insertions, 0 deletions

diff --git a/drivers/usb/host/xhci-ring.c b/drivers/usb/host/xhci-ring.c
index 9b1eeb04ce69..e0f05f5abe10 100644
--- a/drivers/usb/host/xhci-ring.c
+++ b/drivers/usb/host/xhci-ring.c
@@ -2193,6 +2193,11 @@ static void xhci_handle_event(struct xhci_hcd *xhci)
         }
         xhci_dbg(xhci, "%s - OS owns TRB\n", __func__);
 
+        /*
+         * Barrier between reading the TRB_CYCLE (valid) flag above and any
+         * speculative reads of the event's flags/data below.
+         */
+        rmb();
         /* FIXME: Handle more event types. */
         switch ((le32_to_cpu(event->event_cmd.flags) & TRB_TYPE_BITMASK)) {
         case TRB_TYPE(TRB_COMPLETION):