usb: xhci-hcd: add a gratuitous doorbell read

This may assist in debugging raspberrypi#6141

The symptom is that the Interrupt endpoint for the dongle sometimes has
one outstanding TRB, but the lack of USB bus activity suggests that the
xHC didn't correctly register the TRB as valid when the doorbell ring
happened.

Adding this dummy read seems to fix it.

Signed-off-by: Jonathan Bell <[email protected]>

Author: P33M

drivers/usb/host/xhci-ring.c

@@ -505,6 +505,8 @@ void xhci_ring_ep_doorbell(struct xhci_hcd *xhci,
 
 	trace_xhci_ring_ep_doorbell(slot_id, DB_VALUE(ep_index, stream_id));
 
+	/* Incurs ~1.2us round-trip time (and wakes the link) */
+	readl(db_addr);
 	writel(DB_VALUE(ep_index, stream_id), db_addr);
 	/* flush the write */
 	readl(db_addr);