From: Lu Baolu baolu.lu@linux.intel.com Sent: Wednesday, September 4, 2024 2:07 PM
SOC-integrated devices on some platforms require their PCI ATS enabled for operation when the IOMMU is in scalable mode. Those devices are reported via ACPI/SATC table with the ATC_REQUIRED bit set in the Flags field.
The PCI subsystem offers the 'pci=noats' kernel command to disable PCI ATS on all devices. Using 'pci=noat' with devices that require PCI ATS can cause a conflict, leading to boot failure, especially if the device is a graphics device.
To prevent this issue, check PCI ATS support before enumerating the IOMMU devices. If any device requires PCI ATS, but PCI ATS is disabled by 'pci=noats', switch the IOMMU to operate in legacy mode to ensure successful booting.
I guess the reason of switching to legacy mode is because the platform automatically enables ATS in this mode, as the comment says in dmar_ats_supported(). This should be explained otherwise it's unclear why switching the mode can make ATS working for those devices.
But then doesn't it break the meaning of 'pci=noats' which means disabling ATS physically? It's described as "do not use PCIe ATS and IOMMU device IOTLB" in kernel doc, which is not equivalent to "leave PCIe ATS to be managed by HW".
and why would one want to use 'pci=noats' on a platform which requires ats?