Re: [PATCH 00/11] Make the 32-bit ARM DMA API work with normal IOMMU DMA default domains (+ Tegra PoC)

[Marek Szyprowski]

On 15.09.2026 04:14, Mikko Perttunen wrote:
> Depending on the probe ordering, TegraDRM does not currently probe on 
> kernels with CONFIG_ARM_DMA_USE_IOMMU enabled, such as with 
> multi_v7_defconfig. The Tegra SMMU puts several display and 2D/3D 
> clients into one IOMMU group, and the legacy 32-bit ARM DMA API path 
> creates one UNMANAGED domain per device and attaches it with 
> iommu_attach_device(). That only works for one device per group, so 
> every client after the first fails to attach, 
> iommu_device_use_default_domain() rejects the group with -EBUSY, and the 
> driver goes down with it. tegra_defconfig is currently working by virtue 
> of not enabling CONFIG_ARM_DMA_USE_IOMMU.
>
> Robin suggested [1] that instead of teaching the legacy path about 
> groups, we should explore modernizing the 32-bit ARM DMA code a bit by 
> making it use domain cookies and have the core code own the domains, as 
> is the case on other architectures. Still, ARM's own DMA code is kept to 
> support existing drivers and APIs that expect it, and because dma-iommu 
> is considered too big and unnecessary for 32-bit ARM.


I agree that such little modernization is the right step to unify a bit
ARM 32bit with modern archs and get rid of some workaround in the drivers.

It was me who raised the concern about the drivers depending on the first-fit
allocation algorithm used in the32-bit ARM DMA code. This is an issue for some
legacy Samsung Exynos drivers. I not aware of any other driver used on ARM 32bit
SoCs that depends on this behavior.

If one is still interested in switching ARM 32bit to generic IOMMU-DMA code I
can prepare a workaround for those Exynos drivers. I still have it on my todo
list, but in meantime there was no interest in updating ARM 32bit DMA code.


> The series is in four parts:
>
> - Patches 1-2 are preparation in the IOMMU core:
>   1. Add iommu_teardown_dma_ops, counterpart to iommu_setup_dma_ops.
>   2. Add cookie type for ARM's DMA domains.
> - Patches 3-6 implement the DMA API side on 32-bit ARM:
>   3. Split up domain and address space management code in arch/arm/.
>   4. Implement parallel dma_ops / cookie APIs on ARM side.
>   5. Avoid domain allocation when IOMMU core does it.
>   6. Support foreign-allocated sg_tables.
> - Patch 7 enables 32-bit ARM IOMMU drivers to provide a DMA default 
>   domain.
> - Patches 8-11 convert Tegra:
>   8. Allows deferring translation enable until driver handshake for
>      selected devices.
>   9-10. Removes code detaching ARM UNMANAGED DMA domains in
>         Host1x/TegraDRM.
>   11. Enables DMA default domain for selected Tegra devices.
>
> None of this should have any functional effect on any device other than 
> the Tegra devices enabled in patches 8-11. Other IOMMU drivers can opt 
> in later. The new DMA default domains are enabled only when the IOMMU 
> driver explicitly specifies a DMA domain type, just returning 0 for the 
> default is not enough (unlike on arm64), keeping the current behavior.
>
> Patches 8-11 are provided for demonstration but we can deal with them 
> separately if the rest of the series is accepted.
>
> Patches 1,2,3 are applicable independently.
> Patches 4,5 depend on the previous patches.
> Patch 6 can be applied independently but is only required once
>   the whole series lands (by TegraDRM).
> Patch 7 depends on patch 4.
>
> Patches 1-7 only touch drivers/iommu/ and arch/arm/mm/ and associated 
> headers.
>
> Best regards,
> Mikko
>
> [1] https://lore.kernel.org/linux-tegra/910d1492-ad3b-4201-93c6-208be1dcde7d@xxxxxxx/
>
> ---
> Mikko Perttunen (11):
>       iommu: Add iommu_teardown_dma_ops
>       iommu: Add cookie type for ARM_DMA_USE_IOMMU
>       ARM: dma-mapping: Split mapping allocation from domain allocation
>       ARM: dma-mapping: Implement and expose DMA setup functions
>       ARM: dma-mapping: Bypass the legacy path for core-owned DMA API domains
>       ARM: dma-mapping: Handle foreign buffers in arm_iommu_get_sgtable
>       iommu: Let 32-bit ARM drivers opt into a DMA API default domain
>       iommu/tegra-smmu: Defer display client translation to driver handover
>       gpu: host1x: Drop release of legacy 32-bit ARM DMA domain
>       drm/tegra: Drop release of legacy 32-bit ARM DMA domain
>       iommu/tegra-smmu: Use a DMA API default domain per swgroup
>
>  arch/arm/include/asm/dma-iommu.h |  17 ++++
>  arch/arm/mm/dma-mapping.c        | 209 ++++++++++++++++++++++++++++++++-------
>  drivers/gpu/drm/tegra/dc.c       |   8 ++
>  drivers/gpu/drm/tegra/drm.c      |  15 ---
>  drivers/gpu/host1x/dev.c         |  15 ---
>  drivers/iommu/Kconfig            |   1 +
>  drivers/iommu/dma-iommu.c        |   5 +
>  drivers/iommu/dma-iommu.h        |  52 +++++++---
>  drivers/iommu/iommu.c            |  24 ++---
>  drivers/iommu/tegra-smmu.c       | 109 +++++++++++++++++++-
>  drivers/memory/tegra/tegra114.c  |  15 ++-
>  drivers/memory/tegra/tegra124.c  |  12 ++-
>  drivers/memory/tegra/tegra210.c  |   6 +-
>  drivers/memory/tegra/tegra30.c   |  18 ++--
>  include/linux/iommu.h            |   3 +
>  include/soc/tegra/mc.h           |  20 ++++
>  16 files changed, 410 insertions(+), 119 deletions(-)
> ---
> base-commit: f5098b6bae761e346ebcd9da7f95622c04733cff
> change-id: 20260728-b4-arm32-iommu-dma-cookie-3170e07f9125
>
>
Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland