Re: [PATCH v2] devicetree: Add generic IOMMU device tree bindings

Arnd Bergmann
2014-05-30T19:01:32Z

On Friday 30 May 2014 12:22:32 Dave Martin wrote:
> > +
> > +Examples:
> > +=========
> > +
> > +Single-master IOMMU:
> > +--------------------
> > +
> > + iommu {
> > + #address-cells = <0>;
> > + #size-cells = <0>;
> > + };
> > +
> > + master {
> > + iommus = <&/iommu>;
> > + };
> > +
> > +Multiple-master IOMMU with fixed associations:
> > +----------------------------------------------
> > +
> > + /* multiple-master IOMMU */
> > + iommu {
> > + /*
> > + * Masters are statically associated with this IOMMU and
> > + * address translation is always enabled.
> > + */
> > + #address-cells = <0>;
> > + #size-cells = <0>;
>
> In this example, can different translations be set up for the different
> masters?
>
> With no cells available to contain any sort of ID, it looks like this
> is not possible.

Correct, this example is for an IOMMU that does not use IDs but has a
shared address space for all devices.

> > +Multiple-master IOMMU with configurable DMA window:
> > +---------------------------------------------------
> > +
> > + / {
> > + #address-cells = <1>;
> > + #size-cells = <1>;
> > +
> > + iommu {
> > + /* master ID, address of DMA window */
> > + #address-cells = <2>;
> > + #size-cells = <2>;
> > + };
> > +
> > + master {
> > + /* master ID 42, 4 GiB DMA window starting at 0 */
> > + iommus = <&/iommu 42 0 0x1 0x0>;
>
> I'm still concerned that in order to deal with future cases we will have
> to invent multiple ways to parse the "iommus" property. For example, if
> we have a PCEe RC mastering through an IOMMU, it will pass a huge set
> of possible master IDs to the IOMMU, not just noe or two.
>
> Do you have a solution in mind for that which doesn't break backwards
> compatibility?

I think we can treat PCI as a special case here and have an interface
that gets used by the PCI core code to talk to the IOMMU core code
when setting up a the dma_map_ops for a PCI function. As long as the
IOMMU driver understands what PCI is, we don't have to describe the
mapping in detail.

> One option is to include an extra cell to the IOMMUs property
> that indicates how to parse it. For now, only a single value would
> be defined. For example:
>
> iommus = <&/iommu IOMMU_SIMPLE 42>;
>
> Then maybe later
>
> iommus = <&/iommu IOMMU_RANGE 0x10000 0x10000>;
>
> (I'm not suggesting what IOMMU_RANGE might mean.)
>

This can really be left up to the specific IOMMU driver itself.
We can have drivers that support both #address-cells=<1>
and #address-cells=<2> and behave differently based on that.
I don't see a reason to define that across IOMMU implementations.

> Other options are to introduce a new property name
>
> range-iommus = <&/iommu 0x10000 0x10000>;
>
> or control the parsing of incompatible iommus properties via a compatible
> string somewhere.

Introducing a new compatible string is always an option as the last resort.

Arnd