Re: [PATCH 2/2] iommu/of: Call pci_request_acs() before enumerating the Root Port device

[Bjorn Helgaas <helgaas@kernel.org>]

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();
>  		err = pci_for_each_dma_alias(to_pci_dev(dev),
>  					     of_pci_iommu_init, &info);
>  		of_pci_check_device_ats(dev, master_np);
> 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
> 
>