Re: [PATCH V2 5/7] dt-bindings: Add xen,dev-domid property description for xen-grant DMA ops

[Oleksandr <olekstysh@xxxxxxxxx>]

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@xxxxxxxxx> 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