diff --git a/drivers/thunderbolt/domain.c b/drivers/thunderbolt/domain.c
index 7018d959f775..2889a214dadc 100644
--- a/drivers/thunderbolt/domain.c
+++ b/drivers/thunderbolt/domain.c
@@ -7,9 +7,7 @@
  */
 
 #include <linux/device.h>
-#include <linux/dmar.h>
 #include <linux/idr.h>
-#include <linux/iommu.h>
 #include <linux/module.h>
 #include <linux/pm_runtime.h>
 #include <linux/slab.h>
@@ -257,13 +255,9 @@ static ssize_t iommu_dma_protection_show(struct device *dev,
 					 struct device_attribute *attr,
 					 char *buf)
 {
-	/*
-	 * Kernel DMA protection is a feature where Thunderbolt security is
-	 * handled natively using IOMMU. It is enabled when IOMMU is
-	 * enabled and ACPI DMAR table has DMAR_PLATFORM_OPT_IN set.
-	 */
-	return sprintf(buf, "%d\n",
-		       iommu_present(&pci_bus_type) && dmar_platform_optin());
+	struct tb *tb = container_of(dev, struct tb, dev);
+
+	return sysfs_emit(buf, "%d\n", tb->nhi->iommu_dma_protection);
 }
 static DEVICE_ATTR_RO(iommu_dma_protection);
 
diff --git a/drivers/thunderbolt/nhi.c b/drivers/thunderbolt/nhi.c
index c73da0532be4..9e396e283792 100644
--- a/drivers/thunderbolt/nhi.c
+++ b/drivers/thunderbolt/nhi.c
@@ -14,6 +14,7 @@
 #include <linux/errno.h>
 #include <linux/pci.h>
 #include <linux/interrupt.h>
+#include <linux/iommu.h>
 #include <linux/module.h>
 #include <linux/delay.h>
 #include <linux/property.h>
@@ -1102,6 +1103,45 @@ static void nhi_check_quirks(struct tb_nhi *nhi)
 		nhi->quirks |= QUIRK_AUTO_CLEAR_INT;
 }
 
+static int nhi_check_iommu_pdev(struct pci_dev *pdev, void *data)
+{
+	if (!pdev->untrusted ||
+	    !dev_iommu_capable(&pdev->dev, IOMMU_CAP_PRE_BOOT_PROTECTION))
+		return 0;
+	*(bool *)data = true;
+	return 1; /* Stop walking */
+}
+
+static void nhi_check_iommu(struct tb_nhi *nhi)
+{
+	struct pci_bus *bus = nhi->pdev->bus;
+	bool port_ok = false;
+
+	/*
+	 * Ideally what we'd do here is grab every PCI device that
+	 * represents a tunnelling adapter for this NHI and check their
+	 * status directly, but unfortunately USB4 seems to make it
+	 * obnoxiously difficult to reliably make any correlation.
+	 *
+	 * So for now we'll have to bodge it... Hoping that the system
+	 * is at least sane enough that an adapter is in the same PCI
+	 * segment as its NHI, if we can find *something* on that segment
+	 * which meets the requirements for Kernel DMA Protection, we'll
+	 * take that to imply that firmware is aware and has (hopefully)
+	 * done the right thing in general. We need to know that the PCI
+	 * layer has seen the ExternalFacingPort property and propagated
+	 * it to the "untrusted" flag that the IOMMU layer will then
+	 * enforce, but also that the IOMMU driver itself can be trusted
+	 * not to have been subverted by a pre-boot DMA attack.
+	 */
+	while (bus->parent)
+		bus = bus->parent;
+
+	pci_walk_bus(bus, nhi_check_iommu_pdev, &port_ok);
+
+	nhi->iommu_dma_protection = port_ok;
+}
+
 static int nhi_init_msi(struct tb_nhi *nhi)
 {
 	struct pci_dev *pdev = nhi->pdev;
@@ -1219,6 +1259,7 @@ static int nhi_probe(struct pci_dev *pdev, const struct pci_device_id *id)
 		return -ENOMEM;
 
 	nhi_check_quirks(nhi);
+	nhi_check_iommu(nhi);
 
 	res = nhi_init_msi(nhi);
 	if (res) {
diff --git a/include/linux/thunderbolt.h b/include/linux/thunderbolt.h
index 124e13cb1469..7a8ad984e651 100644
--- a/include/linux/thunderbolt.h
+++ b/include/linux/thunderbolt.h
@@ -465,6 +465,7 @@ static inline struct tb_xdomain *tb_service_parent(struct tb_service *svc)
  * @msix_ida: Used to allocate MSI-X vectors for rings
  * @going_away: The host controller device is about to disappear so when
  *		this flag is set, avoid touching the hardware anymore.
+ * @iommu_dma_protection: An IOMMU will isolate external-facing ports.
  * @interrupt_work: Work scheduled to handle ring interrupt when no
  *		    MSI-X is used.
  * @hop_count: Number of rings (end point hops) supported by NHI.
@@ -479,6 +480,7 @@ struct tb_nhi {
 	struct tb_ring **rx_rings;
 	struct ida msix_ida;
 	bool going_away;
+	bool iommu_dma_protection;
 	struct work_struct interrupt_work;
 	u32 hop_count;
 	unsigned long quirks;
