Re: [PATCH v3 1/4] PCI: Enable ACS only after configuring IOMMU for OF platforms

Bjorn Helgaas
2026-02-04T23:33:32Z

On Fri, Jan 02, 2026 at 09:04:47PM +0530, Manivannan Sadhasivam wrote:
> For enabling ACS without the cmdline params, the platform drivers are
> expected to call pci_request_acs() API which sets a static flag,
> 'pci_acs_enable' in drivers/pci/pci.c. And this flag is used to enable ACS
> in pci_enable_acs() helper, which gets called during pci_acs_init(), as per
> this call stack:
>
> -> pci_device_add()
> -> pci_init_capabilities()
> -> pci_acs_init()
> /* check for pci_acs_enable */
> -> pci_enable_acs()
>
> For the OF platforms, pci_request_acs() is called during
> of_iommu_configure() during device_add(), as per this call stack:
>
> -> device_add()
> -> iommu_bus_notifier()
> -> iommu_probe_device()
> -> pci_dma_configure()
> -> of_dma_configure()
> -> of_iommu_configure()
> /* set pci_acs_enable */
> -> pci_request_acs()
>
> As seen from both call stacks, pci_enable_acs() is called way before the
> invocation of pci_request_acs() for the OF platforms. This means,
> pci_enable_acs() will not enable ACS for the first device that gets
> enumerated, which is usally the Root Port device. But since the static
> flag, 'pci_acs_enable' is set *afterwards*, ACS will be enabled for the
> ACS capable devices enumerated later.
>
> To fix this issue, do not call pci_enable_acs() from pci_acs_init(), but
> only from pci_dma_configure() after calling of_dma_configure(). This makes
> sure that pci_enable_acs() only gets called after the IOMMU framework has
> called pci_request_acs(). The ACS enablement flow now looks like:
>
> -> pci_device_add()
> -> pci_init_capabilities()
> /* Just store the ACS cap */
> -> pci_acs_init()
> -> device_add()
> ...
> -> pci_dma_configure()
> -> of_dma_configure()
> -> pci_request_acs()
> -> pci_enable_acs()
>
> For the ACPI platforms, pci_request_acs() is called during ACPI
> initialization time itself, independent of the IOMMU framework.

I don't think cmdline params are relevant, since I don't see any that
set pci_acs_enable or call pci_request_acs().

I propose a longer but combined call chain in the commit log to show
the old sequence vs the new one. I do notice the double call of
pci_enable_acs() (along with dev->bus->dma_configure()), which all
looks odd, but maybe it will be rationalized someday:

PCI: Enable ACS after configuring IOMMU for OF platforms

Platform, ACPI, or IOMMU drivers call pci_request_acs(), which sets
'pci_acs_enable' to request that ACS be enabled for any devices enumerated
in the future.

OF platforms called pci_enable_acs() for the first device before
of_iommu_configure() called pci_request_acs(), so ACS was never enabled for
that device (typically a Root Port).

Call pci_enable_acs() later, from pci_dma_configure(), after
of_dma_configure() has had a chance to call pci_request_acs().

Here's the call path, showing the move of pci_enable_acs() from
pci_acs_init() to pci_dma_configure(), where it always happens after
pci_request_acs():

pci_device_add
pci_init_capabilities
pci_acs_init
- pci_enable_acs
- if (pci_acs_enable) <-- previous test
- ...
device_add
bus_notify(BUS_NOTIFY_ADD_DEVICE)
iommu_bus_notifier
iommu_probe_device
iommu_init_device
dev->bus->dma_configure
pci_dma_configure # pci_bus_type.dma_configure
of_dma_configure
of_iommu_configure
pci_request_acs
pci_acs_enable = 1 <-- set
+ pci_enable_acs
+ if (pci_acs_enable) <-- new test
+ ...
bus_probe_device
device_initial_probe
...
really_probe
dev->bus->dma_configure
pci_dma_configure # pci_bus_type.dma_configure
...
pci_enable_acs

Note that we will now call pci_enable_acs() twice for every device, first
from the iommu_probe_device() path and again from the really_probe() path.
Presumably that's not an issue since we also call dev->bus->dma_configure()
twice.

For the ACPI platforms, pci_request_acs() is called during ACPI
initialization time itself, independent of the IOMMU framework.

> Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
> Tested-by: Naresh Kamboju <naresh.kamboju@linaro.org>
> Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@oss.qualcomm.com>
> ---
> drivers/pci/pci-driver.c | 8 ++++++++
> drivers/pci/pci.c | 8 --------
> drivers/pci/pci.h | 1 +
> 3 files changed, 9 insertions(+), 8 deletions(-)
>
> diff --git a/drivers/pci/pci-driver.c b/drivers/pci/pci-driver.c
> index 7c2d9d596258..301a9418e38e 100644
> --- a/drivers/pci/pci-driver.c
> +++ b/drivers/pci/pci-driver.c
> @@ -1650,6 +1650,14 @@ static int pci_dma_configure(struct device *dev)
> ret = acpi_dma_configure(dev, acpi_get_dma_attr(adev));
> }
>
> + /*
> + * Attempt to enable ACS regardless of capability because some Root
> + * Ports (e.g. those quirked with *_intel_pch_acs_*) do not have
> + * the standard ACS capability but still support ACS via those
> + * quirks.
> + */
> + pci_enable_acs(to_pci_dev(dev));
> +
> pci_put_host_bridge_device(bridge);
>
> /* @drv may not be valid when we're called from the IOMMU layer */
> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index 13dbb405dc31..2c3d0a2d6973 100644
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -3648,14 +3648,6 @@ bool pci_acs_path_enabled(struct pci_dev *start,
> void pci_acs_init(struct pci_dev *dev)
> {
> dev->acs_cap = pci_find_ext_capability(dev, PCI_EXT_CAP_ID_ACS);
> -
> - /*
> - * Attempt to enable ACS regardless of capability because some Root
> - * Ports (e.g. those quirked with *_intel_pch_acs_*) do not have
> - * the standard ACS capability but still support ACS via those
> - * quirks.
> - */
> - pci_enable_acs(dev);
> }
>
> /**
> diff --git a/drivers/pci/pci.h b/drivers/pci/pci.h
> index 0e67014aa001..4592ede0ebcc 100644
> --- a/drivers/pci/pci.h
> +++ b/drivers/pci/pci.h
> @@ -939,6 +939,7 @@ static inline resource_size_t pci_resource_alignment(struct pci_dev *dev,
> }
>
> void pci_acs_init(struct pci_dev *dev);
> +void pci_enable_acs(struct pci_dev *dev);
> #ifdef CONFIG_PCI_QUIRKS
> int pci_dev_specific_acs_enabled(struct pci_dev *dev, u16 acs_flags);
> int pci_dev_specific_enable_acs(struct pci_dev *dev);
>
> --
> 2.48.1
>