dt-bindings: virtio: Add virtio-pci-iommu node

Some systems implement virtio-iommu as a PCI endpoint. The operating
system needs to discover the relationship between IOMMU and masters long
before the PCI endpoint gets probed. Add a PCI child node to describe the
virtio-iommu device.

The virtio-pci-iommu is conceptually split between a PCI programming
interface and a translation component on the parent bus. The latter
doesn't have a node in the device tree. The virtio-pci-iommu node
describes both, by linking the PCI endpoint to "iommus" property of DMA
master nodes and to "iommu-map" properties of bus nodes.
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

Documentation/devicetree/bindings/virtio/iommu.txt

+* virtio IOMMU PCI device
+
+When virtio-iommu uses the PCI transport, its programming interface is
+discovered dynamically by the PCI probing infrastructure. However the
+device tree statically describes the relation between IOMMU and DMA
+masters. Therefore, the PCI root complex that hosts the virtio-iommu
+contains a child node representing the IOMMU device explicitly.
+
+Required properties:
+
+- compatible:	Should be "virtio,pci-iommu"
+- reg:		PCI address of the IOMMU. As defined in the PCI Bus
+		Binding reference [1], the reg property 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.
+- #iommu-cells:	Each platform DMA master managed by the IOMMU is assigned
+		an endpoint ID, described by the "iommus" property [2].
+		For virtio-iommu, #iommu-cells must be 1.
+
+Notes:
+
+- DMA from the IOMMU device isn't managed by another IOMMU. Therefore the
+  virtio-iommu node doesn't have an "iommus" property, and is omitted from
+  the iommu-map property of the root complex.
+
+Example:
+
+pcie@10000000 {
+	compatible = "pci-host-ecam-generic";
+	...
+
+	/* The IOMMU programming interface uses slot 00:01.0 */
+	iommu0: iommu@0008 {
+		compatible = "virtio,pci-iommu";
+		reg = <0x00000800 0 0 0 0>;
+		#iommu-cells = <1>;
+	};
+
+	/*
+	 * The IOMMU manages all functions in this PCI domain except
+	 * itself. Omit BDF 00:01.0.
+	 */
+	iommu-map = <0x0 &iommu0 0x0 0x8>
+		    <0x9 &iommu0 0x9 0xfff7>;
+};
+
+pcie@20000000 {
+	compatible = "pci-host-ecam-generic";
+	...
+	/*
+	 * The IOMMU also manages all functions from this domain,
+	 * with endpoint IDs 0x10000 - 0x1ffff
+	 */
+	iommu-map = <0x0 &iommu0 0x10000 0x10000>;
+};
+
+ethernet@fe001000 {
+	...
+	/* The IOMMU manages this platform device with endpoint ID 0x20000 */
+	iommus = <&iommu0 0x20000>;
+};
+
+[1] Documentation/devicetree/bindings/pci/pci.txt
+[2] Documentation/devicetree/bindings/iommu/iommu.txt