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
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.
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?
bridge->swizzle_irq = pci_common_swizzle; bridge->map_irq = of_irq_parse_and_map_pci;
-- 2.45.2