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Subject: Re: [PATCH V3 5/8] dt-bindings: Add xen,grant-dma IOMMU description
 for xen-grant DMA ops
To:     Stefano Stabellini <sstabellini@kernel.org>
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>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        Christoph Hellwig <hch@infradead.org>,
        Arnd Bergmann <arnd@arndb.de>
References: <1653944417-17168-1-git-send-email-olekstysh@gmail.com>
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From:   Oleksandr <olekstysh@gmail.com>
Message-ID: <31c21ef0-3847-a896-a387-c2e1cc0f9467@gmail.com>
Date:   Wed, 1 Jun 2022 16:32:03 +0300
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On 01.06.22 03:34, Stefano Stabellini wrote:

Hello Stefano

> 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:

I am happy with proposed wording improvements, will update.


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

Thanks!


>
>
>> +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
>>
>>
>> _______________________________________________
>> linux-arm-kernel mailing list
>> linux-arm-kernel@lists.infradead.org
>> http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
>>
-- 
Regards,

Oleksandr Tyshchenko