linux-kernel - [PATCH v2 1/4] PCI: Enable ACS only after configuring IOMMU for OF platforms

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Message-Id: <20251202-pci_acs-v2-1-5d2759a71489@oss.qualcomm.com>
Date: Tue, 02 Dec 2025 19:52:48 +0530
From: Manivannan Sadhasivam <manivannan.sadhasivam@....qualcomm.com>
To: Bjorn Helgaas <bhelgaas@...gle.com>
Cc: linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
        iommu@...ts.linux.dev, Naresh Kamboju <naresh.kamboju@...aro.org>,
        Pavankumar Kondeti <quic_pkondeti@...cinc.com>,
        Xingang Wang <wangxingang5@...wei.com>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>, Jason Gunthorpe <jgg@...pe.ca>,
        Manivannan Sadhasivam <mani@...nel.org>,
        Manivannan Sadhasivam <manivannan.sadhasivam@....qualcomm.com>
Subject: [PATCH v2 1/4] PCI: Enable ACS only after configuring IOMMU for OF
 platforms

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.

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@....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 302d61783f6c..a4ee93497a06 100644
--- a/drivers/pci/pci-driver.c
+++ b/drivers/pci/pci-driver.c
@@ -1648,6 +1648,14 @@ 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 b14dd064006c..9f594fc6dade 100644
--- a/drivers/pci/pci.c
+++ b/drivers/pci/pci.c
@@ -3677,14 +3677,6 @@ 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);
 }
 
 void pci_rebar_init(struct pci_dev *pdev)
diff --git a/drivers/pci/pci.h b/drivers/pci/pci.h
index 36f8c0985430..972b28fc5455 100644
--- a/drivers/pci/pci.h
+++ b/drivers/pci/pci.h
@@ -917,6 +917,7 @@ 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