usb: xhci-hcd: add a gratuitous doorbell read

This may assist in debugging #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]>
raspberrypi · Oct 22, 2024 · 29096b4 · 29096b4
1 parent 25b2b36
commit 29096b4
Showing 1 changed file with 2 additions and 0 deletions.
diff --git a/drivers/usb/host/xhci-ring.c b/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);