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

Manivannan Sadhasivam via B4 Relay
2026-01-02T15:35:04Z

From: Manivannan Sadhasivam <manivannan.sadhasivam@oss.qualcomm.com>

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.

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