xhci: add a quirk to work around a suspected cache bug on VLI controllers

Certain transfer ring access patterns can cause the controller to hang
fetching TRBs for a USB2.0 endpoint.

- If two USB2.0 endpoints are active at once and
- Both endpoints are traversing a Link TRB where the following segment
  has a lower page address and
- One of the endpoints is a Bulk IN and
- The other endpoint is an Interrupt IN

Then the Interrupt IN endpoint can end up not getting polled.
It is unclear what the precise failure mode is, as the controller seems to
haphazardly and repeatedly fetch TRBs for both endpoints but does not
advance the Interrupt endpoint transfer.

As a workaround, add a quirk that initially constrains all USB2.0 transfer
rings to a single segment in size. If for any reason a device driver queues
up enough outstanding transfers to fill the ring segment, then the ring
will be expanded. This has not been seen to occur with UMS or UVC drivers,
which aggressively queue buffers.

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

Author: P33M

drivers/usb/host/xhci-mem.c

@@ -1445,6 +1445,7 @@ int xhci_endpoint_init(struct xhci_hcd *xhci,
 	unsigned int mult;
 	unsigned int avg_trb_len;
 	unsigned int err_count = 0;
+	unsigned int nsegs = 2;
 
 	ep_index = xhci_get_endpoint_index(&ep->desc);
 	ep_ctx = xhci_get_ep_ctx(xhci, virt_dev->in_ctx, ep_index);
@@ -1455,6 +1456,10 @@ int xhci_endpoint_init(struct xhci_hcd *xhci,
 
 	ring_type = usb_endpoint_type(&ep->desc);
 
+	if (xhci->quirks & XHCI_VLI_TRB_CACHE_BUG &&
+	    udev->speed != USB_SPEED_SUPER) {
+		nsegs = 1;
+	}
 	/*
 	 * Get values to fill the endpoint context, mostly from ep descriptor.
 	 * The average TRB buffer lengt for bulk endpoints is unclear as we
@@ -1502,7 +1507,7 @@ int xhci_endpoint_init(struct xhci_hcd *xhci,
 
 	/* Set up the endpoint ring */
 	virt_dev->eps[ep_index].new_ring =
-		xhci_ring_alloc(xhci, 2, 1, ring_type, max_packet, mem_flags);
+		xhci_ring_alloc(xhci, nsegs, 1, ring_type, max_packet, mem_flags);
 	if (!virt_dev->eps[ep_index].new_ring)
 		return -ENOMEM;
 

drivers/usb/host/xhci-pci.c

@@ -287,6 +287,7 @@ static void xhci_pci_quirks(struct device *dev, struct xhci_hcd *xhci)
 		xhci->quirks |= XHCI_LPM_SUPPORT;
 		xhci->quirks |= XHCI_EP_CTX_BROKEN_DCS;
 		xhci->quirks |= XHCI_AVOID_DQ_ON_LINK;
+		xhci->quirks |= XHCI_VLI_TRB_CACHE_BUG;
 	}
 
 	if (pdev->vendor == PCI_VENDOR_ID_ASMEDIA &&

drivers/usb/host/xhci.h

@@ -1886,6 +1886,7 @@ struct xhci_hcd {
 #define XHCI_NO_SOFT_RETRY	BIT_ULL(40)
 #define XHCI_EP_CTX_BROKEN_DCS	BIT_ULL(41)
 #define XHCI_AVOID_DQ_ON_LINK	BIT_ULL(42)
+#define XHCI_VLI_TRB_CACHE_BUG	BIT_ULL(43)
 
 	unsigned int		num_active_eps;
 	unsigned int		limit_active_eps;