From a1d5ce866f2e2062246343af757c22e9abe4e844 Mon Sep 17 00:00:00 2001
From: Jonathan Bell <jonathan@raspberrypi.com>
Date: Mon, 13 Dec 2021 16:04:03 +0000
Subject: [PATCH] usb: xhci: add VLI_TRB_CACHE_BUG quirk

The VL805 fetches up to 4 transfer TRBs at a time. TRB reads don't cross
a 64B boundary, and if a TRB is fetched and is not on a 64B boundary,
the read is sized up to the next 64B boundary.

However the VL805 implements a readahead prefetch for TRBs on a transfer
ring. This fetches the next 64B after any TRB read has happened. Near
the end of a ring segment, the prefetcher can read the first 64B of the
next page in physical memory and this is where the behaviour causes a
bug.

The controller does not tag reads with which endpoint they are for, so
if the start of the next page is a ring segment used by a victim
endpoint, and the victim endpoint is about to fetch TRBs from the start
of the segment, the victim endpoint will read from the prefetched data
and not perform a read to main memory. If the data is stale, the ring
cycle state bit may not be correct and the endpoint will silently halt.

Adjust trbs_per_seg for transfer rings allocated for this controller.

See https://github.com/raspberrypi/linux/issues/4685

Signed-off-by: Jonathan Bell <jonathan@raspberrypi.com>
---
 drivers/usb/host/xhci-mem.c | 11 +++++++++++
 drivers/usb/host/xhci-pci.c |  1 +
 drivers/usb/host/xhci.h     |  1 +
 3 files changed, 13 insertions(+)

--- a/drivers/usb/host/xhci-mem.c
+++ b/drivers/usb/host/xhci-mem.c
@@ -392,6 +392,17 @@ struct xhci_ring *xhci_ring_alloc(struct
 		return ring;
 
 	ring->trbs_per_seg = TRBS_PER_SEGMENT;
+	/*
+	 * The Via VL805 has a bug where cache readahead will fetch off the end
+	 * of a page if the Link TRB of a transfer ring is in the last 4 slots.
+	 * Where there are consecutive physical pages containing ring segments,
+	 * this can cause a desync between the controller's view of a ring
+	 * and the host.
+	 */
+	if (xhci->quirks & XHCI_VLI_TRB_CACHE_BUG &&
+	    type != TYPE_EVENT && type != TYPE_COMMAND)
+		ring->trbs_per_seg -= 4;
+
 	ret = xhci_alloc_segments_for_ring(xhci, &ring->first_seg,
 			&ring->last_seg, num_segs, ring->trbs_per_seg,
 			cycle_state, type, max_packet, flags);
--- a/drivers/usb/host/xhci-pci.c
+++ b/drivers/usb/host/xhci-pci.c
@@ -308,6 +308,7 @@ static void xhci_pci_quirks(struct devic
 	if (pdev->vendor == PCI_VENDOR_ID_VIA && pdev->device == 0x3483) {
 		xhci->quirks |= XHCI_LPM_SUPPORT;
 		xhci->quirks |= XHCI_AVOID_DQ_ON_LINK;
+		xhci->quirks |= XHCI_VLI_TRB_CACHE_BUG;
 	}
 
 	if (pdev->vendor == PCI_VENDOR_ID_ASMEDIA &&
--- a/drivers/usb/host/xhci.h
+++ b/drivers/usb/host/xhci.h
@@ -1667,6 +1667,7 @@ struct xhci_hcd {
 #define XHCI_WRITE_64_HI_LO	BIT_ULL(47)
 #define XHCI_CDNS_SCTX_QUIRK	BIT_ULL(48)
 #define XHCI_AVOID_DQ_ON_LINK	BIT_ULL(49)
+#define XHCI_VLI_TRB_CACHE_BUG	BIT_ULL(50)
 
 	unsigned int		num_active_eps;
 	unsigned int		limit_active_eps;
