Re: [PATCH RFC v6 4/5] iommu/arm-smmu-v3: Group aliasing devices into the same IOMMU group

[Peng Fan <peng.fan@oss.nxp.com>]

On Sun, Oct 11, 2026 at 07:31:28PM +0800, Peng Fan (OSS) wrote:
>From: Peng Fan <peng.fan@nxp.com>
>
>Devices sharing a SID must share a single IOMMU domain (and
>therefore a single cd_table) because the STE can only point to one
>cd_table at a time.
>
>Detect SID aliasing at group-assignment time by looking up the RB
>tree for existing owners.  arm_smmu_device_group() is called before
>arm_smmu_insert_master(), so any RB tree hit belongs to a
>different, already-probed master.
As Sashiko pointed, I need correct as below:

Detect SID aliasing at group-assignment time by looking up the RB
tree for existing owners. Since probe_device() inserts the master
before device_group() is called, skip the device's own stream and
look for a different, already-probed master that shares the same
SID.

>
>Assisted-by: LLM
>Signed-off-by: Peng Fan <peng.fan@nxp.com>
>---
> drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c | 33 ++++++++++++++++++++---------
> 1 file changed, 23 insertions(+), 10 deletions(-)
>
>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 351ccdfa765b4..328d580935b96 100644
>--- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
>+++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
>@@ -4572,19 +4572,32 @@ static int arm_smmu_set_dirty_tracking(struct iommu_domain *domain,
> 
> static struct iommu_group *arm_smmu_device_group(struct device *dev)
> {
>-	struct iommu_group *group;
>+	struct arm_smmu_master *master = dev_iommu_priv_get(dev);
>+	struct arm_smmu_device *smmu = master->smmu;
>+	struct iommu_fwspec *fwspec = dev_iommu_fwspec_get(dev);
>+	struct iommu_group *group = NULL;
>+	int i;
>+
>+	mutex_lock(&smmu->streams_mutex);
>+	for (i = 0; i < fwspec->num_ids; i++) {
>+		struct arm_smmu_stream *stream;
>+
>+		stream = arm_smmu_find_stream(smmu, fwspec->ids[i]);
>+		if (!stream)
>+			continue;

Not very sure here, should I use below as check condition?
  if (!stream || stream->master == master)
      continue?

Per my understanding stream->master will never equals to master,
but Sashiko concerns: Could this logic be adjusted to better handle the
device's own stream?

Thanks
Peng

>+
>+		group = iommu_group_get(stream->master->dev);
>+		break;
>+	}
>+	mutex_unlock(&smmu->streams_mutex);
>+
>+	if (group)
>+		return group;
> 
>-	/*
>-	 * We don't support devices sharing stream IDs other than PCI RID
>-	 * aliases, since the necessary ID-to-device lookup becomes rather
>-	 * impractical given a potential sparse 32-bit stream ID space.
>-	 */
> 	if (dev_is_pci(dev))
>-		group = pci_device_group(dev);
>-	else
>-		group = generic_device_group(dev);
>+		return pci_device_group(dev);
> 
>-	return group;
>+	return generic_device_group(dev);
> }
> 
> static int arm_smmu_of_xlate(struct device *dev,
>
>-- 
>2.34.1
>
>