diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index e37fea6e178d..3c1ff63aa56d 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -2359,6 +2359,19 @@ void pci_enable_acs(struct pci_dev *dev)
 	pci_write_config_word(dev, pos + PCI_ACS_CTRL, ctrl);
 }
 
+static bool pci_acs_flags_enabled(struct pci_dev *pdev, u16 acs_flags)
+{
+	int pos;
+	u16 ctrl;
+
+	pos = pci_find_ext_capability(pdev, PCI_EXT_CAP_ID_ACS);
+	if (!pos)
+		return false;
+
+	pci_read_config_word(pdev, pos + PCI_ACS_CTRL, &ctrl);
+	return (ctrl & acs_flags) == acs_flags;
+}
+
 /**
  * pci_acs_enabled - test ACS against required flags for a given device
  * @pdev: device to test
@@ -2366,36 +2379,79 @@ void pci_enable_acs(struct pci_dev *dev)
  *
  * Return true if the device supports the provided flags.  Automatically
  * filters out flags that are not implemented on multifunction devices.
+ *
+ * Note that this interface checks the effective ACS capabilities of the
+ * device rather than the actual capabilities.  For instance, most single
+ * function endpoints are not required to support ACS because they have no
+ * opportunity for peer-to-peer access.  We therefore return 'true'
+ * regardless of whether the device exposes an ACS capability.  This makes
+ * it much easier for callers of this function to ignore the actual type
+ * or topology of the device when testing ACS support.
  */
 bool pci_acs_enabled(struct pci_dev *pdev, u16 acs_flags)
 {
-	int pos, ret;
-	u16 ctrl;
+	int ret;
 
 	ret = pci_dev_specific_acs_enabled(pdev, acs_flags);
 	if (ret >= 0)
 		return ret > 0;
 
+	/*
+	 * Conventional PCI and PCI-X devices never support ACS, either
+	 * effectively or actually.  The shared bus topology implies that
+	 * any device on the bus can receive or snoop DMA.
+	 */
 	if (!pci_is_pcie(pdev))
 		return false;
 
-	/* Filter out flags not applicable to multifunction */
-	if (pdev->multifunction)
+	switch (pci_pcie_type(pdev)) {
+	/*
+	 * PCI/X-to-PCIe bridges are not specifically mentioned by the spec,
+	 * but since their primary inteface is PCI/X, we conservatively
+	 * handle them as we would a non-PCIe device.
+	 */
+	case PCI_EXP_TYPE_PCIE_BRIDGE:
+	/*
+	 * PCIe 3.0, 6.12.1 excludes ACS on these devices.  "ACS is never
+	 * applicable... must never implement an ACS Extended Capability...".
+	 * This seems arbitrary, but we take a conservative interpretation
+	 * of this statement.
+	 */
+	case PCI_EXP_TYPE_PCI_BRIDGE:
+	case PCI_EXP_TYPE_RC_EC:
+		return false;
+	/*
+	 * PCIe 3.0, 6.12.1.1 specifies that downstream and root ports should
+	 * implement ACS in order to indicate their peer-to-peer capabilities,
+	 * regardless of whether they are single- or multi-function devices.
+	 */
+	case PCI_EXP_TYPE_DOWNSTREAM:
+	case PCI_EXP_TYPE_ROOT_PORT:
+		return pci_acs_flags_enabled(pdev, acs_flags);
+	/*
+	 * PCIe 3.0, 6.12.1.2 specifies ACS capabilities that should be
+	 * implemented by the remaining PCIe types to indicate peer-to-peer
+	 * capabilities, but only when they are part of a multifunciton
+	 * device.  The footnote for section 6.12 indicates the specific
+	 * PCIe types included here.
+	 */
+	case PCI_EXP_TYPE_ENDPOINT:
+	case PCI_EXP_TYPE_UPSTREAM:
+	case PCI_EXP_TYPE_LEG_END:
+	case PCI_EXP_TYPE_RC_END:
+		if (!pdev->multifunction)
+			break;
+
 		acs_flags &= (PCI_ACS_RR | PCI_ACS_CR |
 			      PCI_ACS_EC | PCI_ACS_DT);
 
-	if (pci_pcie_type(pdev) == PCI_EXP_TYPE_DOWNSTREAM ||
-	    pci_pcie_type(pdev) == PCI_EXP_TYPE_ROOT_PORT ||
-	    pdev->multifunction) {
-		pos = pci_find_ext_capability(pdev, PCI_EXT_CAP_ID_ACS);
-		if (!pos)
-			return false;
-
-		pci_read_config_word(pdev, pos + PCI_ACS_CTRL, &ctrl);
-		if ((ctrl & acs_flags) != acs_flags)
-			return false;
+		return pci_acs_flags_enabled(pdev, acs_flags);
 	}
 
+	/*
+	 * PCIe 3.0, 6.12.1.3 specifies no ACS capabilties are applicable
+	 * to single function devices with the exception of downstream ports.
+	 */
 	return true;
 }