where does dev->archdata.iommu be set

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http://stackoverflow.com – I'm learning iommu driver architecture in linux for interest. I've see code like below in omap-iommu.c: omap_iommu_attach_dev() struct omap_iommu_arch_data *arch_data = dev->archdata.iommu; But I can't find out where to set dev->archdata.iommu . could anyone kindly tell me, how does the kernel set dev->archdata.iommu ? (HowTos)