dt-bindings: Add "external-facing" PCIe port property

Provide a way for the firmware to tell the OS which devices are external to
the machine and therefore untrusted.  The property can describe for example
Thunderbolt and other user-accessible ports, which should always have the
strongest IOMMU protection.

Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Grant Likely <grant.likely@arm.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>

Documentation/devicetree/bindings/pci/pci.txt

@@ -24,3 +24,53 @@ driver implementation may support the following properties:
    unsupported link speed, for instance, trying to do training for
    unsupported link speed, etc.  Must be '4' for gen4, '3' for gen3, '2'
    for gen2, and '1' for gen1. Any other values are invalid.
+
+PCI-PCI Bridge properties
+-------------------------
+
+PCIe root ports and switch ports may be described explicitly in the device
+tree, as children of the host bridge node. Even though those devices are
+discoverable by probing, it might be necessary to describe properties that
+aren't provided by standard PCIe capabilities.
+
+Required properties:
+
+- reg:
+   Identifies the PCI-PCI bridge. As defined in the IEEE Std 1275-1994
+   document, it is a five-cell address encoded as (phys.hi phys.mid
+   phys.lo size.hi size.lo). phys.hi should contain the device's BDF as
+   0b00000000 bbbbbbbb dddddfff 00000000. The other cells should be zero.
+
+   The bus number is defined by firmware, through the standard bridge
+   configuration mechanism. If this port is a switch port, then firmware
+   allocates the bus number and writes it into the Secondary Bus Number
+   register of the bridge directly above this port. Otherwise, the bus
+   number of a root port is the first number in the bus-range property,
+   defaulting to zero.
+
+   If firmware leaves the ARI Forwarding Enable bit set in the bridge
+   above this port, then phys.hi contains the 8-bit function number as
+   0b00000000 bbbbbbbb ffffffff 00000000. Note that the PCIe specification
+   recommends that firmware only leaves ARI enabled when it knows that the
+   OS is ARI-aware.
+
+Optional properties:
+
+- external-facing:
+   When present, the port is external-facing. All bridges and endpoints
+   downstream of this port are external to the machine. The OS can, for
+   example, use this information to identify devices that cannot be
+   trusted with relaxed DMA protection, as users could easily attach
+   malicious devices to this port.
+
+Example:
+
+pcie@10000000 {
+	compatible = "pci-host-ecam-generic";
+	...
+	pcie@0008 {
+		/* Root port 00:01.0 is external-facing */
+		reg = <0x00000800 0 0 0 0>;
+		external-facing;
+	};
+};