Re: [PATCH v8 1/5] dt-bindings: reserved-memory: Document iommu-addresses

[Robin Murphy <robin.murphy@arm.com>]

On 2022-09-05 18:08, Thierry Reding wrote:
> From: Thierry Reding <treding@nvidia.com>
> 
> 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.

This seems sufficiently robust to me, just one possibly-relevant point 
which I'll get to in the following patch...

Reviewed-by: Robin Murphy <robin.murphy@arm.com>

> Signed-off-by: Thierry Reding <treding@nvidia.com>
> ---
> Changes in v8:
> - include validation warning fixes that had crept into an unrelated patch
> 
> Changes in v7:
> - keep reserved-memory.txt to avoid broken references
> 
> Changes in v6:
> - add device phandle to iommu-addresses property in examples
> - remove now unused dt-bindings/reserved-memory.h header
> 
>   .../reserved-memory/reserved-memory.yaml      | 70 +++++++++++++++++++
>   1 file changed, 70 insertions(+)
> 
> diff --git a/Documentation/devicetree/bindings/reserved-memory/reserved-memory.yaml b/Documentation/devicetree/bindings/reserved-memory/reserved-memory.yaml
> index 44f72bcf1782..fb48d016e368 100644
> --- a/Documentation/devicetree/bindings/reserved-memory/reserved-memory.yaml
> +++ b/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: >
> @@ -97,4 +121,50 @@ oneOf:
>   
>   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>;
> +        };
> +      };
> +    };
>   ...