linux-kernel - [RFC PATCH] iommu/arm-smmu-v3: Allow stream ID sharing for devices behind PCIe-to-PCI bridges

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Message-ID: <20251211133933.4128100-1-nirmoyd@nvidia.com>
Date: Thu, 11 Dec 2025 05:39:33 -0800
From: Nirmoy Das <nirmoyd@...dia.com>
To: <will@...nel.org>, <robin.murphy@....com>, <joro@...tes.org>,
	<nicolinc@...dia.com>
CC: <jgg@...pe.ca>, <linux-arm-kernel@...ts.infradead.org>,
	<iommu@...ts.linux.dev>, <linux-kernel@...r.kernel.org>, Nirmoy Das
	<nirmoyd@...dia.com>
Subject: [RFC PATCH] iommu/arm-smmu-v3: Allow stream ID sharing for devices behind PCIe-to-PCI bridges

ASPEED BMC controllers have VGA and USB functions behind a PCIe-to-PCI
bridge that causes them to share the same stream ID:

  \-[0005:e0]---00.0-[e1-e2]----00.0-[e2]--+-00.0  ASPEED Graphics Family
                                           \-02.0  ASPEED USB Controller

Both devices get stream ID(0x5e200) due to bridge aliasing. Currently this
USB controller gets rejected with "Aliasing StreamID unsupported", causing
USB timeout errors. Allow stream ID sharing when both devices are on
the same bus behind a PCIe-to-PCI bridge to fix this issue.

Signed-off-by: Nirmoy Das <nirmoyd@...dia.com>
---
 drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
index 2a8b46b948f05..411b8803eb46f 100644
--- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
+++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
@@ -3480,6 +3480,17 @@ static int arm_smmu_insert_master(struct arm_smmu_device *smmu,
 			if (existing_master == master)
 				continue;

+			/* Allow PCI devices on the same bus to share stream IDs
+			 * due to PCIe-to-PCI bridge aliasing.
+			 */
+			if (dev_is_pci(master->dev) && dev_is_pci(existing_master->dev)) {
+				struct pci_dev *pdev = to_pci_dev(master->dev);
+				struct pci_dev *existing_pdev = to_pci_dev(existing_master->dev);
+
+				if (pdev->bus == existing_pdev->bus)
+					continue;
+			}
+
 			dev_warn(master->dev,
 				 "Aliasing StreamID 0x%x (from %s) unsupported, expect DMA to be broken\n",
 				 sid, dev_name(existing_master->dev));
--
2.43.0