oops in pci_acs_path_enabled

index 7469b53..27d8c97 100644
@@ -4133,6 +4133,7 @@ static int intel_iommu_add_device(struct device *dev)
                                          PCI_DEVFN(PCI_SLOT(dma_pdev->devfn),
                                          0)));
 
+#if 0
        while (!pci_is_root_bus(dma_pdev->bus)) {
                if (pci_acs_path_enabled(dma_pdev->bus->self,
                                         NULL, REQ_ACS_FLAGS))

@@ -4140,6 +4141,7 @@ static int intel_iommu_add_device(struct device *dev)
 
                swap_pci_ref(&dma_pdev, pci_dev_get(dma_pdev->bus->self));
        }
+#endif
 
        group = iommu_group_get(&dma_pdev->dev);
        pci_dev_put(dma_pdev);

index 7469b53..4e37e9b 100644
@@ -4124,8 +4124,14 @@ static int intel_iommu_add_device(struct device *dev)
 	} else
 		dma_pdev = pci_dev_get(pdev);
 
+acs_retest:
+	/* Account for quirked devices */
 	swap_pci_ref(&dma_pdev, pci_get_dma_source(dma_pdev));
 
+	/*
+	 * If it's a multifunction device that does not support our
+	 * required ACS flags, add to the same group as function 0.
+	 */
 	if (dma_pdev->multifunction &&
 	    !pci_acs_enabled(dma_pdev, REQ_ACS_FLAGS))
 		swap_pci_ref(&dma_pdev,

@@ -4133,14 +4139,29 @@ static int intel_iommu_add_device(struct device *dev)
 					  PCI_DEVFN(PCI_SLOT(dma_pdev->devfn),
 					  0)));
 
-	while (!pci_is_root_bus(dma_pdev->bus)) {
-		if (pci_acs_path_enabled(dma_pdev->bus->self,
-					 NULL, REQ_ACS_FLAGS))
-			break;
+	/*
+	 * Test ACS support from our current DMA device up to the top of the
+	 * hierarchy.  If the test fails, go to the next upstream device and
+	 * try again.  Devices on the root bus always go through the iommu.
+	 */
+	if (!pci_is_root_bus(dma_pdev->bus)) {
+		struct pci_bus *bus = dma_pdev->bus;
+
+		if (pci_acs_path_enabled(bus, NULL, REQ_ACS_FLAGS))
+			goto done;
+
+		while (!bus->self) {
+			if (!pci_is_root_bus(bus))
+				bus = bus->parent;
+			else
+				goto done;
+		}
 
-		swap_pci_ref(&dma_pdev, pci_dev_get(dma_pdev->bus->self));
+		swap_pci_ref(&dma_pdev, pci_dev_get(bus->self));
+		goto acs_retest;
 	}
 
+done:
 	group = iommu_group_get(&dma_pdev->dev);
 	pci_dev_put(dma_pdev);
 	if (!group) {

index f3ea977..995c13f 100644
@@ -2475,21 +2475,28 @@ bool pci_acs_enabled(struct pci_dev *pdev, u16 acs_flags)
 }
 
 /**
- * pci_acs_path_enable - test ACS flags from start to end in a hierarchy
- * @start: starting downstream device
+ * pci_acs_path_enabled - test ACS flags from a starting bus to an end device
+ * @bus: starting downstream bus
  * @end: ending upstream device or NULL to search to the root bus
  * @acs_flags: required flags
  *
- * Walk up a device tree from start to end testing PCI ACS support.  If
+ * Walk up a PCI hiearchy from bus to end testing PCI ACS support.  If
  * any step along the way does not support the required flags, return false.
  */
-bool pci_acs_path_enabled(struct pci_dev *start,
+bool pci_acs_path_enabled(struct pci_bus *bus,
 			  struct pci_dev *end, u16 acs_flags)
 {
-	struct pci_dev *pdev, *parent = start;
+	struct pci_dev *pdev;
 
 	do {
-		pdev = parent;
+		while (!bus->self) {
+			if (!pci_is_root_bus(bus))
+				bus = bus->parent;
+			else
+				return (end == NULL);
+		}
+
+		pdev = bus->self;
 
 		if (!pci_acs_enabled(pdev, acs_flags))
 			return false;

@@ -2497,7 +2504,7 @@ bool pci_acs_path_enabled(struct pci_dev *start,
 		if (pci_is_root_bus(pdev->bus))
 			return (end == NULL);
 
-		parent = pdev->bus->self;
+		bus = bus->self->bus;
 	} while (pdev != end);
 
 	return true;

index 5faa831..eb9773c 100644
@@ -1652,7 +1652,7 @@ static inline bool pci_is_pcie(struct pci_dev *dev)
 
 void pci_request_acs(void);
 bool pci_acs_enabled(struct pci_dev *pdev, u16 acs_flags);
-bool pci_acs_path_enabled(struct pci_dev *start,
+bool pci_acs_path_enabled(struct pci_bus *bus,
 			  struct pci_dev *end, u16 acs_flags);
 
 #define PCI_VPD_LRDT			0x80	/* Large Resource Data Type */