PCI: add ACS validation utility

In a PCI environment, transactions aren't always required to reach
the root bus before being re-routed.  Intermediate switches between
an endpoint and the root bus can redirect DMA back downstream before
things like IOMMUs have a chance to intervene.  Legacy PCI is always
susceptible to this as it operates on a shared bus.  PCIe added a
new capability to describe and control this behavior, Access Control
Services, or ACS.

The utility function pci_acs_enabled() allows us to test the ACS
capabilities of an individual devices against a set of flags while
pci_acs_path_enabled() tests a complete path from a given downstream
device up to the specified upstream device.  We also include the
ability to add device specific tests as it's likely we'll see
devices that do not implement ACS, but want to indicate support
for various capabilities in this space.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>

1 files changed, 69 insertions, 0 deletions

diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
index 447e834..1ccf7d4 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -2360,6 +2360,75 @@ void pci_enable_acs(struct pci_dev *dev)
 }
 
 /**
+ * pci_acs_enabled - test ACS against required flags for a given device
+ * @pdev: device to test
+ * @acs_flags: required PCI ACS flags
+ *
+ * Return true if the device supports the provided flags.  Automatically
+ * filters out flags that are not implemented on multifunction devices.
+ */
+bool pci_acs_enabled(struct pci_dev *pdev, u16 acs_flags)
+{
+	int pos, ret;
+	u16 ctrl;
+
+	ret = pci_dev_specific_acs_enabled(pdev, acs_flags);
+	if (ret >= 0)
+		return ret > 0;
+
+	if (!pci_is_pcie(pdev))
+		return false;
+
+	/* Filter out flags not applicable to multifunction */
+	if (pdev->multifunction)
+		acs_flags &= (PCI_ACS_RR | PCI_ACS_CR |
+			      PCI_ACS_EC | PCI_ACS_DT);
+
+	if (pdev->pcie_type == PCI_EXP_TYPE_DOWNSTREAM ||
+	    pdev->pcie_type == 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 true;
+}
+
+/**
+ * pci_acs_path_enable - test ACS flags from start to end in a hierarchy
+ * @start: starting downstream device
+ * @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
+ * any step along the way does not support the required flags, return false.
+ */
+bool pci_acs_path_enabled(struct pci_dev *start,
+			  struct pci_dev *end, u16 acs_flags)
+{
+	struct pci_dev *pdev, *parent = start;
+
+	do {
+		pdev = parent;
+
+		if (!pci_acs_enabled(pdev, acs_flags))
+			return false;
+
+		if (pci_is_root_bus(pdev->bus))
+			return (end == NULL);
+
+		parent = pdev->bus->self;
+	} while (pdev != end);
+
+	return true;
+}
+
+/**
  * pci_swizzle_interrupt_pin - swizzle INTx for device behind bridge
  * @dev: the PCI device
  * @pin: the INTx pin (1=INTA, 2=INTB, 3=INTD, 4=INTD)