Add a way for firmware to tell the OS that ATS is supported by the PCI root complex. An endpoint with ATS enabled may send Translation Requests and Translated Memory Requests, which look just like Normal Memory Requests with a non-zero AT field. So a root controller that ignores the AT field may simply forward the request to the IOMMU as a Normal Memory Request, which could end badly. In any case, the endpoint will be unusable. The ats-supported property allows the OS to only enable ATS in endpoints if the root controller can handle ATS requests. Only add the property to pcie-host-ecam-generic for the moment. For non-generic root controllers, availability of ATS can be inferred from the compatible string. Link: https://lore.kernel.org/linux-pci/20240607105415.2501934-3-jean-philippe@linaro.org Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org> Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org> Reviewed-by: Jason Gunthorpe <jgg@nvidia.com> Reviewed-by: Liviu Dudau <liviu.dudau@arm.com> Reviewed-by: Nicolin Chen <nicolinc@nvidia.com> Reviewed-by: Rob Herring <robh@kernel.org> Reviewed-by: Robin Murphy <robin.murphy@arm.com>