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.
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@arm.com/
---
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