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Date:   Tue, 31 May 2022 17:34:54 -0700 (PDT)
From:   Stefano Stabellini <sstabellini@kernel.org>
To:     Oleksandr Tyshchenko <olekstysh@gmail.com>
cc:     xen-devel@lists.xenproject.org, devicetree@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
        iommu@lists.linux-foundation.org,
        Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>,
        Rob Herring <robh+dt@kernel.org>,
        Joerg Roedel <joro@8bytes.org>, Will Deacon <will@kernel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@linaro.org>,
        Julien Grall <julien@xen.org>, Juergen Gross <jgross@suse.com>,
        Stefano Stabellini <sstabellini@kernel.org>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        Christoph Hellwig <hch@infradead.org>,
        Arnd Bergmann <arnd@arndb.de>
Subject: Re: [PATCH V3 5/8] dt-bindings: Add xen,grant-dma IOMMU description
 for xen-grant DMA ops
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On Tue, 31 May 2022, Oleksandr Tyshchenko wrote:
> From: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
> 
> The main purpose of this binding is to communicate Xen specific
> information using generic IOMMU device tree bindings (which is
> a good fit here) rather than introducing a custom property.
> 
> Introduce Xen specific IOMMU for the virtualized device (e.g. virtio)
> to be used by Xen grant DMA-mapping layer in the subsequent commit.
> 
> The reference to Xen specific IOMMU node using "iommus" property
> indicates that Xen grant mappings need to be enabled for the device,
> and it specifies the ID of the domain where the corresponding backend
> resides. The domid (domain ID) is used as an argument to the Xen grant
> mapping APIs.
> 
> This is needed for the option to restrict memory access using Xen grant
> mappings to work which primary goal is to enable using virtio devices
> in Xen guests.
> 
> Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
> ---
> Changes RFC -> V1:
>    - update commit subject/description and text in description
>    - move to devicetree/bindings/arm/
> 
> Changes V1 -> V2:
>    - update text in description
>    - change the maintainer of the binding
>    - fix validation issue
>    - reference xen,dev-domid.yaml schema from virtio/mmio.yaml
> 
> Change V2 -> V3:
>    - Stefano already gave his Reviewed-by, I dropped it due to the changes (significant)
>    - use generic IOMMU device tree bindings instead of custom property
>      "xen,dev-domid"
>    - change commit subject and description, was
>      "dt-bindings: Add xen,dev-domid property description for xen-grant DMA ops"
> ---
>  .../devicetree/bindings/iommu/xen,grant-dma.yaml   | 49 ++++++++++++++++++++++
>  1 file changed, 49 insertions(+)
>  create mode 100644 Documentation/devicetree/bindings/iommu/xen,grant-dma.yaml
> 
> diff --git a/Documentation/devicetree/bindings/iommu/xen,grant-dma.yaml b/Documentation/devicetree/bindings/iommu/xen,grant-dma.yaml
> new file mode 100644
> index 00000000..ab5765c
> --- /dev/null
> +++ b/Documentation/devicetree/bindings/iommu/xen,grant-dma.yaml
> @@ -0,0 +1,49 @@
> +# SPDX-License-Identifier: (GPL-2.0-only or BSD-2-Clause)
> +%YAML 1.2
> +---
> +$id: http://devicetree.org/schemas/iommu/xen,grant-dma.yaml#
> +$schema: http://devicetree.org/meta-schemas/core.yaml#
> +
> +title: Xen specific IOMMU for virtualized devices (e.g. virtio)
> +
> +maintainers:
> +  - Stefano Stabellini <sstabellini@kernel.org>
> +
> +description:
> +  The reference to Xen specific IOMMU node using "iommus" property indicates
> +  that Xen grant mappings need to be enabled for the device, and it specifies
> +  the ID of the domain where the corresponding backend resides.
> +  The binding is required to restrict memory access using Xen grant mappings.

I think this is OK and in line with the discussion we had on the list. I
propose the following wording instead:

"""
The Xen IOMMU represents the Xen grant table interface. Grant mappings
are to be used with devices connected to the Xen IOMMU using the
"iommus" property, which also specifies the ID of the backend domain.
The binding is required to restrict memory access using Xen grant
mappings.
"""


> +properties:
> +  compatible:
> +    const: xen,grant-dma
> +
> +  '#iommu-cells':
> +    const: 1
> +    description:
> +      Xen specific IOMMU is multiple-master IOMMU device.
> +      The single cell describes the domid (domain ID) of the domain where
> +      the backend is running.

Here I would say:

"""
The single cell is the domid (domain ID) of the domain where the backend
is running.
"""

With the two wording improvements:

Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>


> +required:
> +  - compatible
> +  - "#iommu-cells"
> +
> +additionalProperties: false
> +
> +examples:
> +  - |
> +    xen_iommu {
> +        compatible = "xen,grant-dma";
> +        #iommu-cells = <1>;
> +    };
> +
> +    virtio@3000 {
> +        compatible = "virtio,mmio";
> +        reg = <0x3000 0x100>;
> +        interrupts = <41>;
> +
> +        /* The backend is located in Xen domain with ID 1 */
> +        iommus = <&xen_iommu 1>;
> +    };
> -- 
> 2.7.4
> 
> 
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