Fix user-pinned DMA for Thunderbolt-connected devices

The kernel marks Thunderbolt-connected devices as "untrusted", forcing
DMA through SWIOTLB bounce buffering. This breaks user-pinned DMA: the
SWIOTLB path allocates IOVAs per scatter-gather entry rather than as a
contiguous range, so PIN_PAGES fails the contiguity check. Even if it
didn't fail there, bounce buffering is fundamentally incompatible with
user-pinned memory since the device would DMA to bounce buffers rather
than the user's pages.

Clear the untrusted flag during probe. With IOMMU active, the device is
already restricted to explicitly-mapped addresses and cannot access
arbitrary system memory.

Also add fix-tt-hotplug-bars, a helper script to work around BAR
allocation failures on Thunderbolt hotplug. Requires "pci=realloc" on
the kernel command line.

Author: joelsmithTT

enumerate.c

@@ -278,6 +278,21 @@ static int tenstorrent_pci_probe(struct pci_dev *dev, const struct pci_device_id
 	dma_set_max_seg_size(&dev->dev, UINT_MAX);
 	dma_set_seg_boundary(&dev->dev, ULONG_MAX);
 
+	// Thunderbolt-connected devices are marked "untrusted" by the kernel,
+	// which forces SWIOTLB bounce buffering even with IOMMU enabled. The
+	// IOMMU itself provides DMA isolation; clear the flag only when IOMMU
+	// is active and the untrust is due to an external-facing port.
+	// pci_dev.external_facing was added in v5.9.
+#if LINUX_VERSION_CODE >= KERNEL_VERSION(5, 9, 0)
+	if (dev->untrusted && is_iommu_translated(&dev->dev)) {
+		struct pci_dev *root = pcie_find_root_port(dev);
+		if (root && root->external_facing) {
+			dev_info(&dev->dev, "Clearing untrusted flag (external-facing port, IOMMU active)\n");
+			dev->untrusted = 0;
+		}
+	}
+#endif
+
 	pci_set_master(dev);
 	pci_enable_pcie_error_reporting(dev);
 

tools/fix-tt-hotplug-bars

@@ -0,0 +1,111 @@
+#!/bin/bash
+# SPDX-FileCopyrightText: © 2025 Tenstorrent Inc.
+# SPDX-License-Identifier: GPL-2.0-only
+#
+# Fix BAR allocation for Thunderbolt-connected Tenstorrent devices.
+#
+# When a Tenstorrent device is connected via Thunderbolt, the kernel may fail
+# to allocate sufficient memory for the device's BARs. This script fixes that
+# by removing the Thunderbolt bridge and rescanning, which allows the kernel's
+# pci=realloc logic to properly size the bridge windows.
+#
+# PREREQUISITE: Boot with "pci=realloc" on the kernel command line.
+#   1. Edit /etc/default/grub
+#   2. Add "pci=realloc" to GRUB_CMDLINE_LINUX
+#   3. Run: sudo update-grub && sudo reboot
+#
+# Usage: fix-tt-hotplug-bars [--dry-run]
+
+set -euo pipefail
+
+TENSTORRENT_VENDOR="0x1e52"
+DRY_RUN=0
+
+die() { echo "fix-tt-hotplug-bars: $*" >&2; exit 1; }
+log() { echo "fix-tt-hotplug-bars: $*" >&2; }
+
+# Check if device has unassigned BARs
+has_unassigned_bars() {
+    lspci -s "${1#0000:}" -v 2>/dev/null | grep -q '<unassigned>'
+}
+
+# Walk up the PCI tree to find the Thunderbolt Host bridge.
+# "Host" = in your computer, "Hub" = in external docks.
+# We want the first Host bridge - the downstream port connecting to the dock.
+find_thunderbolt_bridge() {
+    local dev="$1"
+    local current="$dev"
+    
+    while [[ -d "/sys/bus/pci/devices/$current" ]]; do
+        local real_path parent_path parent
+        real_path=$(readlink -f "/sys/bus/pci/devices/$current")
+        parent_path=$(dirname "$real_path")
+        parent=$(basename "$parent_path")
+        
+        # Stop at root
+        [[ "$parent" == pci* || "$parent" == "devices" ]] && break
+        
+        # Look for Thunderbolt "Host" bridge (not "Hub" which is in docks)
+        if [[ -f "/sys/bus/pci/devices/$parent/vendor" ]]; then
+            if [[ "$(cat "/sys/bus/pci/devices/$parent/vendor")" == "0x8086" ]]; then
+                if lspci -s "${parent#0000:}" 2>/dev/null | grep -qiE "thunderbolt.*host|usb4.*host"; then
+                    echo "$parent"
+                    return 0
+                fi
+            fi
+        fi
+        
+        current="$parent"
+    done
+    
+    return 1
+}
+
+main() {
+    [[ "${1:-}" == "--dry-run" ]] && DRY_RUN=1
+    [[ $EUID -eq 0 ]] || die "must be run as root"
+    
+    local fixed=0
+
+    echo 1 > /sys/bus/pci/rescan
+    
+    for dev_path in /sys/bus/pci/devices/*; do
+        [[ -f "$dev_path/vendor" ]] || continue
+        [[ "$(cat "$dev_path/vendor")" == "$TENSTORRENT_VENDOR" ]] || continue
+        
+        local dev=$(basename "$dev_path")
+        
+        has_unassigned_bars "$dev" || continue
+        
+        log "device $dev has unassigned BARs"
+        
+        local bridge
+        if ! bridge=$(find_thunderbolt_bridge "$dev"); then
+            log "warning: $dev not behind a Thunderbolt bridge, skipping"
+            continue
+        fi
+        
+        log "removing Thunderbolt bridge $bridge and rescanning..."
+        
+        if [[ $DRY_RUN -eq 1 ]]; then
+            log "[dry-run] would remove $bridge and rescan"
+        else
+            echo 1 > "/sys/bus/pci/devices/$bridge/remove"
+            sleep 2
+            echo 1 > /sys/bus/pci/rescan
+            sleep 3
+
+            if has_unassigned_bars "$dev"; then
+                log "warning: $dev still has unassigned BARs after rescan"
+                continue
+            fi
+        fi
+        
+        ((fixed++)) || true
+    done
+    
+    [[ $fixed -eq 0 ]] && log "no Tenstorrent devices with BAR issues found"
+    [[ $fixed -gt 0 ]] && log "fixed $fixed device(s)"
+}
+
+main "$@"