pci_for_each_dma_alias() normally iterates all the way up to the root of a PCI hierarchy. But on some systems the IOMMU is attached below the root, and DMA aliases generated above the IOMMU are useless.
Jayachandran C <email@example.com> has proposed patches to limit the DMA alias search on ThunderX2. These are planned for v4.12.
The purpose of this bugzilla is only to archive lspci output showing the unusual topology of ThunderX2.
Created attachment 255905 [details]
lspci -tvn output
Created attachment 255907 [details]
lspci -vv output