This sounds like a mirror of the problem of finding the IOMMU on parisc (there
can be more than one).
The way parisc solves this is to look in dev->platform_data and if that's null
walk up the dev->parent until the IOMMU is found and then cache the IOMMU ops
in the current dev->platform_data. Obviously, you can't use platform_data,
but you could use driver_data for this. The IOMMU's actually lie on a parisc
specific bus, so the ability to walk up the device tree without having to know
the device types was crucial to implementing this.
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