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Subject: Re: [PATCH V2 5/7] dt-bindings: Add xen,dev-domid property
 description for xen-grant DMA ops
To:     Rob Herring <robh@kernel.org>, Arnd Bergmann <arnd@arndb.de>
Cc:     xen-devel <xen-devel@lists.xenproject.org>,
        "open list:DRM DRIVER FOR QEMU'S CIRRUS DEVICE" 
        <virtualization@lists.linux-foundation.org>,
        DTML <devicetree@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>,
        Jason Wang <jasowang@redhat.com>,
        Krzysztof Kozlowski <krzk+dt@kernel.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>,
        Jean-Philippe Brucker <jean-philippe@linaro.org>
References: <1651947548-4055-1-git-send-email-olekstysh@gmail.com>
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 <20220518185904.GA3685644-robh@kernel.org>
From:   Oleksandr <olekstysh@gmail.com>
Message-ID: <40758f3b-cab3-2096-1711-18837bc2f73b@gmail.com>
Date:   Thu, 19 May 2022 02:48:31 +0300
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On 18.05.22 21:59, Rob Herring wrote:

Hello Rob, Arnd

> On Wed, May 18, 2022 at 03:32:27PM +0100, Arnd Bergmann wrote:
>> On Sat, May 7, 2022 at 7:19 PM Oleksandr Tyshchenko <olekstysh@gmail.com> wrote:
>>> diff --git a/Documentation/devicetree/bindings/virtio/mmio.yaml b/Documentation/devicetree/bindings/virtio/mmio.yaml
>>> index 10c22b5..29a0932 100644
>>> --- a/Documentation/devicetree/bindings/virtio/mmio.yaml
>>> +++ b/Documentation/devicetree/bindings/virtio/mmio.yaml
>>> @@ -13,6 +13,9 @@ description:
>>>     See https://www.oasis-open.org/committees/tc_home.php?wg_abbrev=virtio for
>>>     more details.
>>>
>>> +allOf:
>>> +  - $ref: /schemas/arm/xen,dev-domid.yaml#
>>> +
>>>   properties:
>>>     compatible:
>>>       const: virtio,mmio
>>> @@ -33,6 +36,10 @@ properties:
>>>       description: Required for devices making accesses thru an IOMMU.
>>>       maxItems: 1
>>>
>>> +  xen,dev-domid:
>>> +    description: Required when Xen grant mappings need to be enabled for device.
>>> +    $ref: /schemas/types.yaml#/definitions/uint32
>>> +
>>>   required:
>>>     - compatible
>>>     - reg
>> Sorry for joining the discussion late. Have you considered using the
>> generic iommu
>> binding here instead of a custom property? This would mean having a device
>> node for the grant-table mechanism that can be referred to using the 'iommus'
>> phandle property, with the domid as an additional argument.
>>
>> It does not quite fit the model that Linux currently uses for iommus,
>> as that has an allocator for dma_addr_t space, but it would think it's
>> conceptually close enough that it makes sense for the binding.
> Something common is almost always better.

agree


>
> That may also have the issue that fw_devlink will make the 'iommu'
> driver a dependency to probe.

Looks like I ran into it while experimenting. I generated the following 
nodes in guest DT using Xen toolstack:

[snip]

         xen_dummy_iommu {
                 compatible = "xen,dummy-iommu";
                 #iommu-cells = <0x01>;
                 phandle = <0xfde9>;
         };
         virtio@2000000 {
                 compatible = "virtio,mmio";
                 reg = <0x00 0x2000000 0x00 0x200>;
                 interrupts = <0x00 0x01 0xf01>;
                 interrupt-parent = <0xfde8>;
                 dma-coherent;
                 iommus = <0xfde9 0x01>;
         };

[snip]


And got:

virtio-mmio 2000000.virtio: deferred probe timeout, ignoring dependency


>
> Rob

-- 
Regards,

Oleksandr Tyshchenko