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 03:21:46PM +0100, Robin Murphy wrote:
> On 2022-10-07 14:54, Thierry Reding wrote:
> > 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?
> 
> Well, it couldn't really be a direct mapping anyway. In general I don't
> think that combination makes any sense, since the presence of
> "iommu-addresses" means one of two things; either it says the IOVA range is
> carved out for some special purpose or just unusable, in which case
> allocating any memory to back it would surely be pointless, or it's saying
> don't touch these addresses because the device is already accessing them,
> thus the underlying physical memory must be allocated somewhere already.

I thought perhaps there could be cases where it is known that a
controller needs to access memory in a certain I/O virtual region but
doesn't actually care where that lives in physical memory and also does
not rely on that memory have been previously set up (pre-filled, or
whatever). Say you've got a micro-controller in a system that needs its
firmware in a given region, but the OS can set up that region without
any other limitations. One could use "size" and "iommu-addresses" to
make sure the region is allocated with a specific size and located in a
specific I/O virtual region. Not sure if that's perhaps a bit exotic,
though.

Thierry

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