On Tue, Sep 23, 2025 at 03:27:01PM -0500, Bjorn Helgaas wrote:
On Wed, Sep 10, 2025 at 11:09:21PM +0530, Manivannan Sadhasivam via B4 Relay wrote:
From: Xingang Wang wangxingang5@huawei.com
When booting with devicetree, ACS is enabled for all ACS capable PCI devices except the first Root Port enumerated in the system. This is due to calling pci_request_acs() after the enumeration and initialization of the Root Port device.
I suppose you're referring to a path like below, where we *check* pci_acs_enable during PCI enumeration, but we don't *set* it until we add the device and look for a driver for it?
pci_host_common_init devm_pci_alloc_host_bridge devm_of_pci_bridge_init pci_request_acs pci_acs_enable = 1 # ++ new set here pci_host_probe pci_scan_root_bus_bridge pci_scan_device pci_init_capabilities pci_enable_acs if (pci_acs_enable) # test here ... pci_bus_add_devices driver_probe_device pci_dma_configure of_dma_configure of_dma_configure_id of_iommu_configure pci_request_acs pci_acs_enable = 1 # -- previously set here
Yes!
But afterwards, ACS is getting enabled for the rest of the PCI devices, since pci_request_acs() sets the 'pci_acs_enable' flag and the PCI core uses this flag to enable ACS for the rest of the ACS capable devices.
I don't quite understand why ACS would be enabled for *any* of the devices because we generally enumerate all of them, which includes the pci_init_capabilities() and pci_enable_acs(), before adding and attaching drivers to them.
But it does seem kind of dumb that we set the system-wide "enable ACS" property in a per-device place like an individual device probe.
I had the same opinion when I saw the 'pci_acs_enable' flag. But I think the intention was to enable ACS only if the controller is capable of assigning different IOMMU groups per device. Otherwise, ACS is more or less of no use.
Ideally, pci_request_acs() should only be called if the 'iommu-map' DT property is set for the host bridge device. Hence, call pci_request_acs() from devm_of_pci_bridge_init() if the 'iommu-map' property is present in the host bridge DT node. This aligns with the implementation of the ARM64 ACPI driver (drivers/acpi/arm64/iort.c) as well.
With this change, ACS will be enabled for all the PCI devices including the first Root Port device of the DT platforms.
Cc: stable@vger.kernel.org # 5.6 Fixes: 6bf6c24720d33 ("iommu/of: Request ACS from the PCI core when configuring IOMMU linkage") Signed-off-by: Xingang Wang wangxingang5@huawei.com Signed-off-by: Pavankumar Kondeti quic_pkondeti@quicinc.com [mani: reworded subject, description and comment] Signed-off-by: Manivannan Sadhasivam manivannan.sadhasivam@oss.qualcomm.com
drivers/iommu/of_iommu.c | 1 - drivers/pci/of.c | 8 +++++++- 2 files changed, 7 insertions(+), 2 deletions(-)
diff --git a/drivers/iommu/of_iommu.c b/drivers/iommu/of_iommu.c index 6b989a62def20ecafd833f00a3a92ce8dca192e0..c31369924944d36a3afd3d4ff08c86fc6daf55de 100644 --- a/drivers/iommu/of_iommu.c +++ b/drivers/iommu/of_iommu.c @@ -141,7 +141,6 @@ int of_iommu_configure(struct device *dev, struct device_node *master_np, .np = master_np, };
pci_request_acs();diff --git a/drivers/pci/of.c b/drivers/pci/of.c index 3579265f119845637e163d9051437c89662762f8..98c2523f898667b1618c37451d1759959d523da1 100644 --- a/drivers/pci/of.c +++ b/drivers/pci/of.c @@ -638,9 +638,15 @@ static int pci_parse_request_of_pci_ranges(struct device *dev, int devm_of_pci_bridge_init(struct device *dev, struct pci_host_bridge *bridge) {
- if (!dev->of_node)
- struct device_node *node = dev->of_node;
- if (!node) return 0;
- /* Enable ACS if IOMMU mapping is detected for the host bridge */
- if (of_property_read_bool(node, "iommu-map"))
pci_request_acs();I'm not really convinced that the existence of 'iommu-map' in devicetree is a clear signal that ACS should be enabled, so I'm a little hesitant about this part.
Is it possible to boot using a devicetree with 'iommu-map', but with the IOMMU disabled or the IOMMU driver not present? Or other situations where we don't need ACS?
Certainly possible. But the issue is, we cannot reliably detect the presence of IOMMU until the first pci_dev is created, which will be too late as pci_acs_init() is called during pci_device_add().
This seems to be the case for ACPI platforms also.
Maybe IOMMU folks Robin/Joerg/Will could comment more.
- Mani