Re: [PATCH v9 1/5] dt-bindings: reserved-memory: Document iommu-addresses

[Thierry Reding <thierry.reding@gmail.com>]

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On Fri, Oct 07, 2022 at 02:45:31PM +0100, Robin Murphy wrote:
> On 2022-09-23 13:35, Thierry Reding wrote:
> > From: Thierry Reding <treding@nvidia.com>
> > 
> > This adds the "iommu-addresses" property to reserved-memory nodes, which
> > allow describing the interaction of memory regions with IOMMUs. Two use-
> > cases are supported:
> > 
> >    1. Static mappings can be described by pairing the "iommu-addresses"
> >       property with a "reg" property. This is mostly useful for adopting
> >       firmware-allocated buffers via identity mappings. One common use-
> >       case where this is required is if early firmware or bootloaders
> >       have set up a bootsplash framebuffer that a display controller is
> >       actively scanning out from during the operating system boot
> >       process.
> > 
> >    2. If an "iommu-addresses" property exists without a "reg" property,
> >       the reserved-memory node describes an IOVA reservation. Such memory
> >       regions are excluded from the IOVA space available to operating
> >       system drivers and can be used for regions that must not be used to
> >       map arbitrary buffers.
> 
> Bah, I've only just realised: don't we also need to change the "oneOf:
> required: ..." schema to permit "iommu-addresses" without "reg" or "size"?

Hm... good point. I think at least we'll want another:

     - required:
         - iommu-addresses

in there. I wonder if we also need to avoid the combination of "size"
and "iommu-addresses". When "size" is specified, is it guaranteed that
those regions will be allocated before the direct mapping needs to be
created?

Thierry

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