diff --git a/drivers/pci/dmar.c b/drivers/pci/dmar.c
index 0a19708..f183ac9 100644
--- a/drivers/pci/dmar.c
+++ b/drivers/pci/dmar.c
@@ -532,7 +532,7 @@ static int dmar_pci_device_match(struct pci_dev *devices[], int cnt,
 struct dmar_drhd_unit *
 dmar_find_matched_drhd_unit(struct pci_dev *dev)
 {
-	struct dmar_drhd_unit *dmaru = NULL;
+	struct dmar_drhd_unit *dmaru, *found = NULL;
 	struct acpi_dmar_hardware_unit *drhd;
 
 	dev = pci_physfn(dev);
@@ -544,14 +544,38 @@ dmar_find_matched_drhd_unit(struct pci_dev *dev)
 
 		if (dmaru->include_all &&
 		    drhd->segment == pci_domain_nr(dev->bus))
-			return dmaru;
-
-		if (dmar_pci_device_match(dmaru->devices,
+			found = dmaru;
+		else if (dmar_pci_device_match(dmaru->devices,
 					  dmaru->devices_cnt, dev))
-			return dmaru;
+			found = dmaru;
+
+
+		if (found)
+			break;
+	}
+
+	/* We know that this device only exists on this chipset, has its
+	 * own IOMMU, and is uniquely identified by bit 54 being set in
+	 * its capability mask.  Catch BIOSes that specify the incorrect
+	 * IOMMU unit.
+	 */
+	if (found &&
+	    dev->vendor == PCI_VENDOR_ID_INTEL &&
+	    dev->device == PCI_DEVICE_ID_INTEL_IOAT_SNB &&
+	    !test_bit(54, (unsigned long *) &found->iommu->cap)) {
+		struct intel_iommu *iommu = found->iommu;
+
+		WARN_TAINT_ONCE(1, TAINT_FIRMWARE_WORKAROUND,
+				"BIOS wrongly assigned I/OAT IOMMU "
+				"%d: reg_base_addr %llx cap %llx ecap %llx\n",
+				iommu->seq_id,
+				(unsigned long long)found->reg_base_addr,
+				(unsigned long long)iommu->cap,
+				(unsigned long long)iommu->ecap);
+		found = NULL;
 	}
 
-	return NULL;
+	return found;
 }
 
 int __init dmar_dev_scope_init(void)