Some devices are required to use a specific type (identity or dma) of
default domain when they are used with a vendor iommu. When the system
level default domain type is different from it, the vendor iommu driver
has to request a new default domain with either
iommu_request_dma_domain_for_dev() or iommu_request_dm_for_dev() in the
add_dev() callback. Unfortunately, these two helpers only work when the
group hasn't been assigned to any other devices, hence, some vendor iommu
driver has to use a private domain if it fails to request a new default
one.
Joerg proposed an on-going proposal which makes the default domain
framework to support configuring per-group default domain during boot
process.
https://lkml.org/lkml/2020/4/14/616
[This has been applied in iommu/next.]
Hence, there is no need to keep the private domain implementation
in the Intel IOMMU driver. This patch series aims to remove it.