RE: [PATCH v3 3/5] iommu: Add iommu_get_domain_for_dev_locked() helper

Tian, Kevin
2025-08-21T08:11:09Z

> From: Nicolin Chen <nicolinc@nvidia.com>
> Sent: Tuesday, August 19, 2025 1:23 AM
>
> ... I found that in SMMUv3 driver, iommu_get_domain_for_dev() is
> used to get the RID domain for an SVA domain:
> arm_smmu_set_pasid()
> arm_smmu_blocking_set_dev_pasid()
>
> These two are already given an "old" (SVA) domain pointer, FWIW.
>
> So, we may change to passing in the old domain as you suggested,
> yet we still have to fix the iommu_get_domain_for_dev() in order
> to reflect the RID domain correctly for the driver that calls it
> (or even potentially) in some group->mutex locked context where
> the RID domain might not be naturally passed in.
>

Out of curiosity.

arm_smmu_blocking_set_dev_pasid()

/*
* When the last user of the CD table goes away downgrade the STE back
* to a non-cd_table one.
*/
if (!arm_smmu_ssids_in_use(&master->cd_table)) {
struct iommu_domain *sid_domain =
iommu_get_domain_for_dev(master->dev);

if (sid_domain->type == IOMMU_DOMAIN_IDENTITY ||
sid_domain->type == IOMMU_DOMAIN_BLOCKED)
sid_domain->ops->attach_dev(sid_domain, dev);
}

why cannot downgrade apply to the case where the RID is attached to
a DMA domain?