PCI: Revert "Enable ACS after configuring IOMMU for OF platforms"

REGRESSION Applied John Hancock 7 participants 11 messages Mar 20 – Apr 20

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John Hancock

author

2026-03-20 17:24 UTC #0 lore raw

Commit 7a126c1b6cfa ("PCI: Enable ACS after configuring IOMMU for OF platforms") introduced a regression affecting AMD IOMMU group isolation on x86 systems, making PCIe passthrough non-functional.

While the commit addresses a legitimate ordering issue on OF/Device Tree platforms, the fix modifies pci_dma_configure(), which executes on all platforms regardless of firmware interface. On AMD systems with IOMMU, moving pci_enable_acs() from pci_acs_init() to pci_dma_configure() alters the point at which ACS is evaluated relative to IOMMU group assignment. The result is that devices which previously occupied individual, exclusive IOMMU groups are merged into a single group containing both passthrough and non-passthrough members, violating IOMMU isolation requirements.

The commit author notes that pci_enable_acs() is now called twice per device and that this is "presumably not an issue." On AMD IOMMU hardware this assumption does not hold -- the change in call ordering has observable and breaking consequences for group topology.

It is worth noting that this is a stable/LTS series (6.12.y), where changes to fundamental PCI initialization ordering carry significant risk for production and specialized workloads that depend on stable IOMMU behavior across kernel updates. A regression of this nature -- silently breaking PCIe passthrough without any configuration change on the part of the user -- is particularly disruptive in a series that users reasonably expect to be conservative.

This revert restores pci_enable_acs() to pci_acs_init() and marks it static again, fully restoring correct IOMMU group topology on affected hardware.

Regression introduced in: 6.12.75
Tested on: 6.12.77 with this revert applied

Hardware:
CPU: AMD Ryzen Threadripper 2990WX (family 23h, Zen+)
IOMMU: AMD-Vi

Bisect:
6.12.74: GOOD -- IOMMU groups correct, passthrough functional

  6.12.75: BAD  -- IOMMU groups collapsed, passthrough broken
  6.12.76: BAD  -- still broken
  6.12.77: BAD  -- still broken

Fixes 7a126c1b6cfa ("PCI: Enable ACS after configuring IOMMU for OF platforms")

Signed-off-by John Hancock <[email protected]>

---
 drivers/pci/pci-driver.c | 8 --------
 drivers/pci/pci.c        | 8 ++++----
 drivers/pci/pci.h        | 1 -
 3 files changed, 4 insertions(+), 13 deletions(-)

drivers/pci/pci-driver.c +0 -8

index 9846ab70c..a00a2ce01 100644
@@ -1656,14 +1656,6 @@ 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);
 
 	if (!ret && !driver->driver_managed_dma) {

drivers/pci/pci.c +9 -1

index 040c0be2d..3eb878781 100644
@@ -1072,7 +1072,7 @@ static void pci_std_enable_acs(struct pci_dev *dev, struct pci_acs *caps)
  * pci_enable_acs - enable ACS if hardware support it
  * @dev: the PCI device
  */
-void pci_enable_acs(struct pci_dev *dev)
+static void pci_enable_acs(struct pci_dev *dev)
 {
 	struct pci_acs caps;
 	bool enable_acs = false;

@@ -3718,6 +3718,14 @@ 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);
 }
 
 /**

drivers/pci/pci.h +0 -1

index b9b97ec0f..b1f393a42 100644
@@ -653,7 +653,6 @@ 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_cfg);
 int pci_dev_specific_enable_acs(struct pci_dev *dev);

[email protected]

2026-03-23 12:31 UTC #1 lore raw

Hi,

Thanks for submitting this fix/revert -- I spent a lot of time today figuring out why my VM didn't start anymore, and finally came here.

Seeing that it's been a few days since this was posted, I figured I'd add this message as a "ping" to the committers, in hopes of getting this merged soon. (This regression is hitting end users now.)

Best regards,
Bjørn Forsman

Manivannan Sadhasivam

Acked-by

2026-03-23 13:54 UTC #2 lore raw

In reply to John Hancock

+ Robin

Quoted 4 lines · John Hancock

Commit 7a126c1b6cfa ("PCI: Enable ACS after configuring IOMMU for OF platforms") introduced a regression affecting AMD IOMMU group isolation on x86 systems, making PCIe passthrough non-functional.

Quoted 9 lines

While the commit addresses a legitimate ordering issue on OF/Device Tree platforms, the fix modifies pci_dma_configure(), which executes on all platforms regardless of firmware interface. On AMD systems with IOMMU, moving pci_enable_acs() from pci_acs_init() to pci_dma_configure() alters the point at which ACS is evaluated relative to IOMMU group assignment. The result is that devices which previously occupied individual, exclusive IOMMU groups are merged into a single group containing both passthrough and non-passthrough members, violating IOMMU isolation requirements.

Ouch! Sorry for the breakage.

Quoted 5 lines

The commit author notes that pci_enable_acs() is now called twice per device and that this is "presumably not an issue." On AMD IOMMU hardware this assumption does not hold -- the change in call ordering has observable and breaking consequences for group topology.

Quoted 8 lines

It is worth noting that this is a stable/LTS series (6.12.y), where changes to fundamental PCI initialization ordering carry significant risk for production and specialized workloads that depend on stable IOMMU behavior across kernel updates. A regression of this nature -- silently breaking PCIe passthrough without any configuration change on the part of the user -- is particularly disruptive in a series that users reasonably expect to be conservative.

I still haven't investigated this failure deeply, but it is also worth noting that this regression only happens with v6.12 and earlier stable kernels as mentioned in [1].

Quoted 11 lines

This revert restores pci_enable_acs() to pci_acs_init() and marks it static again, fully restoring correct IOMMU group topology on affected hardware.
Regression introduced in: 6.12.75
Tested on: 6.12.77 with this revert applied
Hardware:
CPU: AMD Ryzen Threadripper 2990WX (family 23h, Zen+)
IOMMU: AMD-Vi

Quoted 8 lines

Bisect:
6.12.74: GOOD -- IOMMU groups correct, passthrough functional
  6.12.75: BAD  -- IOMMU groups collapsed, passthrough broken
  6.12.76: BAD  -- still broken
  6.12.77: BAD  -- still broken
Fixes 7a126c1b6cfa ("PCI: Enable ACS after configuring IOMMU for OF platforms")
Signed-off-by John Hancock <[email protected]>

Acked-by Manivannan Sadhasivam <[email protected]>

- Mani

[1] https://lore.kernel.org/all/[email protected]/

Signature · 1 line

மணிவண்ணன் சதாசிவம்

Robin Murphy

2026-03-23 15:06 UTC #3 lore raw

Quoted 33 lines · Manivannan Sadhasivam

+ Robin
Quoted 4 lines · John Hancock
Commit 7a126c1b6cfa ("PCI: Enable ACS after configuring IOMMU for OF platforms") introduced a regression affecting AMD IOMMU group isolation on x86 systems, making PCIe passthrough non-functional.
Quoted 9 lines
While the commit addresses a legitimate ordering issue on OF/Device Tree platforms, the fix modifies pci_dma_configure(), which executes on all platforms regardless of firmware interface. On AMD systems with IOMMU, moving pci_enable_acs() from pci_acs_init() to pci_dma_configure() alters the point at which ACS is evaluated relative to IOMMU group assignment. The result is that devices which previously occupied individual, exclusive IOMMU groups are merged into a single group containing both passthrough and non-passthrough members, violating IOMMU isolation requirements.
Ouch! Sorry for the breakage.
Quoted 5 lines
The commit author notes that pci_enable_acs() is now called twice per device and that this is "presumably not an issue." On AMD IOMMU hardware this assumption does not hold -- the change in call ordering has observable and breaking consequences for group topology.
Quoted 8 lines
It is worth noting that this is a stable/LTS series (6.12.y), where changes to fundamental PCI initialization ordering carry significant risk for production and specialized workloads that depend on stable IOMMU behavior across kernel updates. A regression of this nature -- silently breaking PCIe passthrough without any configuration change on the part of the user -- is particularly disruptive in a series that users reasonably expect to be conservative.

Quoted 3 lines

I still haven't investigated this failure deeply, but it is also worth noting that this regression only happens with v6.12 and earlier stable kernels as mentioned in [1].

Oops, indeed, relying on pci_dma_configure() to be called prior to group assignment in iommu_init_device() only works since bcb81ac6ae3c ("iommu: Get DT/ACPI parsing into the proper probe path") added that call path in 6.15 - thus the backport probably doesn't actually work for OF platforms either.

Dropping this from 6.12.y and earlier stable branches seems like the correct action to me (but not a mainline revert, obviously). ACS had essentially *never* worked properly on OF platforms prior to 6.15, but that was more down to fundamental design flaws in the OF-based IOMMU probe path (dating back to 4.12) rather than any easily-fixable bug as such, so realistically I think we just leave it that way.

Thanks,
Robin.

Quoted 26 lines

Quoted 11 lines
This revert restores pci_enable_acs() to pci_acs_init() and marks it static again, fully restoring correct IOMMU group topology on affected hardware.
Regression introduced in: 6.12.75
Tested on: 6.12.77 with this revert applied
Hardware:
CPU: AMD Ryzen Threadripper 2990WX (family 23h, Zen+)
IOMMU: AMD-Vi
Quoted 8 lines
Bisect:
6.12.74: GOOD -- IOMMU groups correct, passthrough functional
   6.12.75: BAD  -- IOMMU groups collapsed, passthrough broken
   6.12.76: BAD  -- still broken
   6.12.77: BAD  -- still broken
Fixes 7a126c1b6cfa ("PCI: Enable ACS after configuring IOMMU for OF platforms")
Signed-off-by John Hancock <[email protected]>
Acked-by Manivannan Sadhasivam <[email protected]>
- Mani
[1] https://lore.kernel.org/all/[email protected]/

Manivannan Sadhasivam

2026-03-23 15:14 UTC #4 lore raw

Quoted 34 lines · Robin Murphy

Quoted 33 lines · Manivannan Sadhasivam
+ Robin
Quoted 4 lines · John Hancock
Commit 7a126c1b6cfa ("PCI: Enable ACS after configuring IOMMU for OF platforms") introduced a regression affecting AMD IOMMU group isolation on x86 systems, making PCIe passthrough non-functional.
Quoted 9 lines
While the commit addresses a legitimate ordering issue on OF/Device Tree platforms, the fix modifies pci_dma_configure(), which executes on all platforms regardless of firmware interface. On AMD systems with IOMMU, moving pci_enable_acs() from pci_acs_init() to pci_dma_configure() alters the point at which ACS is evaluated relative to IOMMU group assignment. The result is that devices which previously occupied individual, exclusive IOMMU groups are merged into a single group containing both passthrough and non-passthrough members, violating IOMMU isolation requirements.
Ouch! Sorry for the breakage.
Quoted 13 lines
The commit author notes that pci_enable_acs() is now called twice per device and that this is "presumably not an issue." On AMD IOMMU hardware this assumption does not hold -- the change in call ordering has observable and breaking consequences for group topology.
It is worth noting that this is a stable/LTS series (6.12.y), where changes to fundamental PCI initialization ordering carry significant risk for production and specialized workloads that depend on stable IOMMU behavior across kernel updates. A regression of this nature -- silently breaking PCIe passthrough without any configuration change on the part of the user -- is particularly disruptive in a series that users reasonably expect to be conservative.

Quoted 9 lines

Quoted 3 lines
I still haven't investigated this failure deeply, but it is also worth noting that this regression only happens with v6.12 and earlier stable kernels as mentioned in [1].
Oops, indeed, relying on pci_dma_configure() to be called prior to group assignment in iommu_init_device() only works since bcb81ac6ae3c ("iommu: Get DT/ACPI parsing into the proper probe path") added that call path in 6.15 - thus the backport probably doesn't actually work for OF platforms either.

Ah, that makes sense. Thanks for finding the root cause. It might be very obvious to you, but still... ;)

Quoted 7 lines

Dropping this from 6.12.y and earlier stable branches seems like the correct action to me (but not a mainline revert, obviously). ACS had essentially *never* worked properly on OF platforms prior to 6.15, but that was more down to fundamental design flaws in the OF-based IOMMU probe path (dating back to 4.12) rather than any easily-fixable bug as such, so realistically I think we just leave it that way.

That's my opinion as well. I guess I need to send reverts for rest of the older stable kernels as well.

Thanks once again, Robin!

- Mani

Signature · 1 line

மணிவண்ணன் சதாசிவம்

Akemi Yagi

2026-03-23 22:32 UTC #5 lore raw

Quoted 9 lines · Manivannan Sadhasivam

Quoted 7 lines
Dropping this from 6.12.y and earlier stable branches seems like the correct action to me (but not a mainline revert, obviously). ACS had essentially *never* worked properly on OF platforms prior to 6.15, but that was more down to fundamental design flaws in the OF-based IOMMU probe path (dating back to 4.12) rather than any easily-fixable bug as such, so realistically I think we just leave it that way.
That's my opinion as well. I guess I need to send reverts for rest of the older stable kernels as well.

This is a short note to say that the 6.1 kernel was affected and the issue was fixed by reverting the referenced commit ( https://elrepo.org/
bugs/view.php?id=1587 ).

Relevant bugzilla entry is here:

https://bugzilla.kernel.org/show_bug.cgi?id=221234

Akemi

[email protected]

2026-03-27 17:48 UTC #6 lore raw

In reply to Manivannan Sadhasivam

Hi Mani,

[CC Sasha Levin as git committer of all the cherry-picks to the LTS kernels.]

What's the status of the reverts/fixes?

I tested even older LTS kernels (5.15.x and 5.10.x) and they're affected too. IOW, the only LTS kernel that isn't affected is 6.18.x.

I hope this can be fixed soon.

Best regards,
Bjørn Forsman

Thorsten Leemhuis

2026-03-31 08:32 UTC #7 lore raw

In reply to Manivannan Sadhasivam

Quoted 27 lines · Manivannan Sadhasivam

Quoted 11 lines · Robin Murphy
Quoted 10 lines · Manivannan Sadhasivam
Quoted 3 lines · John Hancock
Commit 7a126c1b6cfa ("PCI: Enable ACS after configuring IOMMU for OF platforms") introduced a regression affecting AMD IOMMU group isolation on x86 systems, making PCIe passthrough non-functional.
[...]
Ouch! Sorry for the breakage.
[...]
I still haven't investigated this failure deeply, but it is also worth noting that this regression only happens with v6.12 and earlier stable kernels as mentioned in [1].
Quoted 4 lines
Oops, indeed, relying on pci_dma_configure() to be called prior to group assignment in iommu_init_device() only works since bcb81ac6ae3c ("iommu: Get DT/ACPI parsing into the proper probe path") added that call path in 6.15 - thus the backport probably doesn't actually work for OF platforms either.
Ah, that makes sense. Thanks for finding the root cause. It might be very obvious to you, but still... ;)
Quoted 6 lines
Dropping this from 6.12.y and earlier stable branches seems like the correct action to me (but not a mainline revert, obviously). ACS had essentially *never* worked properly on OF platforms prior to 6.15, but that was more down to fundamental design flaws in the OF-based IOMMU probe path (dating back to 4.12) rather than any easily-fixable bug as such, so realistically I think we just leave it that way.

Quoted 2 lines

That's my opinion as well. I guess I need to send reverts for rest of the older stable kernels as well.

Mani, did you send those reverts? I could not find any on lore. And the one at the start of the thread likely won't work, as it doesn't state that c41e2fb67e26b0 ("PCI: Enable ACS after configuring IOMMU for OF platforms") needs to be reverted for 6.12.y and all earlier series. So to speed things up:

Greg, Sasha, could you maybe simply revert that backported commit directly in 6.12.y and all earlier series?

Ciao, Thorsten

Manivannan Sadhasivam

2026-03-31 09:19 UTC #8 lore raw

Quoted 31 lines · Thorsten Leemhuis

Quoted 27 lines · Manivannan Sadhasivam
Quoted 11 lines · Robin Murphy
Quoted 10 lines · Manivannan Sadhasivam
Quoted 3 lines · John Hancock
Commit 7a126c1b6cfa ("PCI: Enable ACS after configuring IOMMU for OF platforms") introduced a regression affecting AMD IOMMU group isolation on x86 systems, making PCIe passthrough non-functional.
[...]
Ouch! Sorry for the breakage.
[...]
I still haven't investigated this failure deeply, but it is also worth noting that this regression only happens with v6.12 and earlier stable kernels as mentioned in [1].
Quoted 4 lines
Oops, indeed, relying on pci_dma_configure() to be called prior to group assignment in iommu_init_device() only works since bcb81ac6ae3c ("iommu: Get DT/ACPI parsing into the proper probe path") added that call path in 6.15 - thus the backport probably doesn't actually work for OF platforms either.
Ah, that makes sense. Thanks for finding the root cause. It might be very obvious to you, but still... ;)
Quoted 6 lines
Dropping this from 6.12.y and earlier stable branches seems like the correct action to me (but not a mainline revert, obviously). ACS had essentially *never* worked properly on OF platforms prior to 6.15, but that was more down to fundamental design flaws in the OF-based IOMMU probe path (dating back to 4.12) rather than any easily-fixable bug as such, so realistically I think we just leave it that way.
Quoted 2 lines
That's my opinion as well. I guess I need to send reverts for rest of the older stable kernels as well.

Quoted 9 lines

Mani, did you send those reverts? I could not find any on lore. And the one at the start of the thread likely won't work, as it doesn't state that c41e2fb67e26b0 ("PCI: Enable ACS after configuring IOMMU for OF platforms") needs to be reverted for 6.12.y and all earlier series. So to speed things up:
Greg, Sasha, could you maybe simply revert that backported commit directly in 6.12.y and all earlier series?

Sorry for the delay. I just sent the reverts for v5.10, v5.15, v6.1 and v6.6. This patch is for v6.12, so we've covered all affected stable kernels prior to v6.15.

- Mani

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மணிவண்ணன் சதாசிவம்

Manivannan Sadhasivam

2026-03-23 15:09 UTC #9 lore raw

In reply to Manivannan Sadhasivam

+ Jason

Quoted 65 lines · Manivannan Sadhasivam

+ Robin
Quoted 4 lines · John Hancock
Commit 7a126c1b6cfa ("PCI: Enable ACS after configuring IOMMU for OF platforms") introduced a regression affecting AMD IOMMU group isolation on x86 systems, making PCIe passthrough non-functional.
Quoted 9 lines
While the commit addresses a legitimate ordering issue on OF/Device Tree platforms, the fix modifies pci_dma_configure(), which executes on all platforms regardless of firmware interface. On AMD systems with IOMMU, moving pci_enable_acs() from pci_acs_init() to pci_dma_configure() alters the point at which ACS is evaluated relative to IOMMU group assignment. The result is that devices which previously occupied individual, exclusive IOMMU groups are merged into a single group containing both passthrough and non-passthrough members, violating IOMMU isolation requirements.
Ouch! Sorry for the breakage.
Quoted 5 lines
The commit author notes that pci_enable_acs() is now called twice per device and that this is "presumably not an issue." On AMD IOMMU hardware this assumption does not hold -- the change in call ordering has observable and breaking consequences for group topology.
Quoted 8 lines
It is worth noting that this is a stable/LTS series (6.12.y), where changes to fundamental PCI initialization ordering carry significant risk for production and specialized workloads that depend on stable IOMMU behavior across kernel updates. A regression of this nature -- silently breaking PCIe passthrough without any configuration change on the part of the user -- is particularly disruptive in a series that users reasonably expect to be conservative.
I still haven't investigated this failure deeply, but it is also worth noting that this regression only happens with v6.12 and earlier stable kernels as mentioned in [1].
Quoted 11 lines
This revert restores pci_enable_acs() to pci_acs_init() and marks it static again, fully restoring correct IOMMU group topology on affected hardware.
Regression introduced in: 6.12.75
Tested on: 6.12.77 with this revert applied
Hardware:
CPU: AMD Ryzen Threadripper 2990WX (family 23h, Zen+)
IOMMU: AMD-Vi
Quoted 8 lines
Bisect:
6.12.74: GOOD -- IOMMU groups correct, passthrough functional
  6.12.75: BAD  -- IOMMU groups collapsed, passthrough broken
  6.12.76: BAD  -- still broken
  6.12.77: BAD  -- still broken
Fixes 7a126c1b6cfa ("PCI: Enable ACS after configuring IOMMU for OF platforms")
Signed-off-by John Hancock <[email protected]>
Acked-by Manivannan Sadhasivam <[email protected]>
- Mani
[1] https://lore.kernel.org/all/[email protected]/
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மணிவண்ணன் சதாசிவம்

[email protected]

maintainer Applied

2026-04-20 15:33 UTC #10 lore raw

Patch "PCI: Revert "Enable ACS after configuring IOMMU for OF platforms"" has been added to the 6.12-stable tree

In reply to John Hancock

This is a note to let you know that I've just added the patch titled

PCI: Revert "Enable ACS after configuring IOMMU for OF platforms"

to the 6.12-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
pci-revert-enable-acs-after-configuring-iommu-for-of-platforms.patch
and it can be found in the queue-6.12 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From [email protected] Fri Mar 20 18:29:23 2026
From: John Hancock <[email protected]>
Date: Fri, 20 Mar 2026 13:23:35 -0400
Subject: PCI: Revert "Enable ACS after configuring IOMMU for OF platforms"
To: [email protected]
Cc: [email protected], [email protected], [email protected], [email protected], [email protected], John Hancock <[email protected]>
Message-ID: <[email protected]>

From: John Hancock <[email protected]>

This reverts 7a126c1b6cfa2c4b5a7013164451ecddd210110d which is commit c41e2fb67e26b04d919257875fa954aa5f6e392e upstream.

Commit 7a126c1b6cfa ("PCI: Enable ACS after configuring IOMMU for OF platforms") introduced a regression affecting AMD IOMMU group isolation on x86 systems, making PCIe passthrough non-functional.

This revert restores pci_enable_acs() to pci_acs_init() and marks it static again, fully restoring correct IOMMU group topology on affected hardware.

Regression introduced in: 6.12.75
Tested on: 6.12.77 with this revert applied

Hardware:
CPU: AMD Ryzen Threadripper 2990WX (family 23h, Zen+)
IOMMU: AMD-Vi

Bisect:
6.12.74: GOOD -- IOMMU groups correct, passthrough functional

  6.12.75: BAD  -- IOMMU groups collapsed, passthrough broken
  6.12.76: BAD  -- still broken
  6.12.77: BAD  -- still broken

Fixes 7a126c1b6cfa ("PCI: Enable ACS after configuring IOMMU for OF platforms")

Signed-off-by John Hancock <[email protected]>

Signed-off-by Greg Kroah-Hartman <[email protected]>

---
 drivers/pci/pci-driver.c |    8 --------
 drivers/pci/pci.c        |   10 +++++++++-
 drivers/pci/pci.h        |    1 -
 3 files changed, 9 insertions(+), 10 deletions(-)

drivers/pci/pci-driver.c +0 -8

@@ -1656,14 +1656,6 @@ static int pci_dma_configure(struct devi
 		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);
 
 	if (!ret && !driver->driver_managed_dma) {

drivers/pci/pci.c +9 -1

@@ -1072,7 +1072,7 @@ static void pci_std_enable_acs(struct pc
  * pci_enable_acs - enable ACS if hardware support it
  * @dev: the PCI device
  */
-void pci_enable_acs(struct pci_dev *dev)
+static void pci_enable_acs(struct pci_dev *dev)
 {
 	struct pci_acs caps;
 	bool enable_acs = false;

@@ -3718,6 +3718,14 @@ bool pci_acs_path_enabled(struct pci_dev
 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);
 }
 
 /**

drivers/pci/pci.h +0 -1

@@ -653,7 +653,6 @@ static inline resource_size_t pci_resour
 }
 
 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);

Patches currently in stable-queue which might be from [email protected] are

queue-6.12/pci-revert-enable-acs-after-configuring-iommu-for-of-platforms.patch