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Date:   Wed, 18 May 2022 13:59:04 -0500
From:   Rob Herring <robh@kernel.org>
To:     Arnd Bergmann <arnd@arndb.de>
Cc:     Oleksandr Tyshchenko <olekstysh@gmail.com>,
        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>
Subject: Re: [PATCH V2 5/7] dt-bindings: Add xen,dev-domid property
 description for xen-grant DMA ops
Message-ID: <20220518185904.GA3685644-robh@kernel.org>
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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.

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

Rob