dt-bindings: reserved-memory: Document iommu-addresses

This adds the "iommu-addresses" property to reserved-memory nodes, which
allow describing the interaction of memory regions with IOMMUs. Two use-
cases are supported:

  1. Static mappings can be described by pairing the "iommu-addresses"
     property with a "reg" property. This is mostly useful for adopting
     firmware-allocated buffers via identity mappings. One common use-
     case where this is required is if early firmware or bootloaders
     have set up a bootsplash framebuffer that a display controller is
     actively scanning out from during the operating system boot
     process.

  2. If an "iommu-addresses" property exists without a "reg" property,
     the reserved-memory node describes an IOVA reservation. Such memory
     regions are excluded from the IOVA space available to operating
     system drivers and can be used for regions that must not be used to
     map arbitrary buffers.

Each mapping or reservation is tied to a specific device via a phandle
to the device's device tree node. This allows a reserved-memory region
to be reused across multiple devices.
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://lore.kernel.org/r/20230120174251.4004100-3-thierry.reding@gmail.comSigned-off-by: Joerg Roedel <jroedel@suse.de>

Documentation/devicetree/bindings/reserved-memory/reserved-memory.yaml

@@ -52,6 +52,30 @@ properties:
       Address and Length pairs. Specifies regions of memory that are
       acceptable to allocate from.
 
+  iommu-addresses:
+    $ref: /schemas/types.yaml#/definitions/phandle-array
+    description: >
+      A list of phandle and specifier pairs that describe static IO virtual
+      address space mappings and carveouts associated with a given reserved
+      memory region. The phandle in the first cell refers to the device for
+      which the mapping or carveout is to be created.
+
+      The specifier consists of an address/size pair and denotes the IO
+      virtual address range of the region for the given device. The exact
+      format depends on the values of the "#address-cells" and "#size-cells"
+      properties of the device referenced via the phandle.
+
+      When used in combination with a "reg" property, an IOVA mapping is to
+      be established for this memory region. One example where this can be
+      useful is to create an identity mapping for physical memory that the
+      firmware has configured some hardware to access (such as a bootsplash
+      framebuffer).
+
+      If no "reg" property is specified, the "iommu-addresses" property
+      defines carveout regions in the IOVA space for the given device. This
+      can be useful if a certain memory region should not be mapped through
+      the IOMMU.
+
   no-map:
     type: boolean
     description: >
@@ -89,12 +113,69 @@ allOf:
           - no-map
 
 oneOf:
-  - required:
-      - reg
+  - oneOf:
+      - required:
+          - reg
+
+      - required:
+          - size
+
+  - oneOf:
+      # IOMMU reservations
+      - required:
+          - iommu-addresses
 
-  - required:
-      - size
+      # IOMMU mappings
+      - required:
+          - reg
+          - iommu-addresses
 
 additionalProperties: true
 
+examples:
+  - |
+    / {
+      compatible = "foo";
+      model = "foo";
+
+      #address-cells = <2>;
+      #size-cells = <2>;
+
+      reserved-memory {
+        #address-cells = <2>;
+        #size-cells = <2>;
+        ranges;
+
+        adsp_resv: reservation-adsp {
+          /*
+           * Restrict IOVA mappings for ADSP buffers to the 512 MiB region
+           * from 0x40000000 - 0x5fffffff. Anything outside is reserved by
+           * the ADSP for I/O memory and private memory allocations.
+           */
+          iommu-addresses = <&adsp 0x0 0x00000000 0x00 0x40000000>,
+                            <&adsp 0x0 0x60000000 0xff 0xa0000000>;
+        };
+
+        fb: framebuffer@90000000 {
+          reg = <0x0 0x90000000 0x0 0x00800000>;
+          iommu-addresses = <&dc0 0x0 0x90000000 0x0 0x00800000>;
+        };
+      };
+
+      bus@0 {
+        #address-cells = <1>;
+        #size-cells = <1>;
+        ranges = <0x0 0x0 0x0 0x40000000>;
+
+        adsp: adsp@2990000 {
+          reg = <0x2990000 0x2000>;
+          memory-region = <&adsp_resv>;
+        };
+
+        dc0: display@15200000 {
+          reg = <0x15200000 0x10000>;
+          memory-region = <&fb>;
+        };
+      };
+    };
 ...