Subject: [PATCH RFCv1 0/3] Allow ATS to be always on for certain ATS-capable devices

From: Nicolin Chen <[email-masked]>
Date: Fri, 16 Jan 2026 20:56:39 -0800

PCI ATS function is controlled by IOMMU driver calling pci_enable_ats() and
pci_disable_ats() helpers. In general, IOMMU driver only enables ATS, when
a translation channel is enabled on a PASID, typically for an SVA use case.
When a device's RID is IOMMU bypassed and there is no active PASID running
SVA use case, ATS is always disabled.

However, certain pcie devices support non-PASID ATS on its RID, even if the
RID is IOMMU bypassed. E.g. CXL.cache capability requires ATS to access the
physical memory; some NVIDIA GPUs in non-CXL configuration also support ATS
on a bypassed RID.

Provide a helper function to detect CXL.cache capability and scan through a
device ID list.

As the initial use case, call the helper in ARM SMMUv3 driver and adapt the
driver accordingly with a per-device ats_always_on flag.

This is on Github:
https://github.com/nicolinc/iommufd/commits/pci_ats_always_on-rfcv1/

Nicolin Chen (3):
  PCI: Allow ATS to be always on for CXL.cache capable devices
  PCI: Allow ATS to be always on for non-CXL NVIDIA GPUs
  iommu/arm-smmu-v3: Allow ATS to be always on

 drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.h |  1 +
 drivers/pci/pci.h                           |  9 +++
 include/linux/pci-ats.h                     |  3 +
 include/uapi/linux/pci_regs.h               |  5 ++
 drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c | 74 ++++++++++++++++++---
 drivers/pci/ats.c                           | 45 +++++++++++++
 drivers/pci/quirks.c                        | 23 +++++++
 7 files changed, 149 insertions(+), 11 deletions(-)

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2.43.0