[RFC PATCH 0/2] iommu, drm/rockchip: Map private-domain imports from DMA addresses

[Karl Mehltretter]

Diederik reported failed video-buffer imports on RK3588 with
DMABUF_DEBUG. The proposed warn-only mode [1] confirmed CPU-side
attachment access in rockchip_gem_iommu_map() during Sway video resizing.

This RFC keeps Rockchip's private scanout domain. Patch 1 copies live
iommu-dma mappings into another domain. Patch 2 uses it for PRIME
imports. Unsupported inputs keep the old page-based path, which remains
unfixed under strict DMABUF_DEBUG.

Rob's MSM approach assumes direct DMA [2]. Rockchip's attachment instead
provides IOVAs from the VOP's default domain, which cannot be mapped
unchanged into the private domain. Each import retains both mappings
and adds one reverse lookup per 4 KiB.

Christian rejected reverse translation in the earlier MSM proposal [3].
Robin NAKed exporting iommu_get_dma_domain() [4]. Keeping the helper in
dma-iommu.c does not resolve the physical-address objection.

Sharing the first VOP's default DMA domain, as Exynos does, would avoid
the translation but also change native-buffer mapping.

Is retaining the private domain and reverse-translating the DMA mappings
an acceptable direction? If so, I'll address the remaining limitations
before posting a non-RFC version.

Base: mainline 602042bf29f6. The warn-only RFC is not a prerequisite.
No stable backport requested. Strict-by-default DMABUF_DEBUG took effect
in v7.3-rc4.

Testing (builds and QEMU only):

- W=1 object and stub builds passed on arm64, ARM32 with/without LPAE,
  x86-64 GCC/Clang, i386, s390, RISC-V and UML, including dynamic SWIOTLB.
- Rockchip strict/warn A/B reproduced the control failure and warning.
  Treatments checked every page and byte in 83 imports each. Primary
  buffers had 1/127/507 DMA segments under the default 64 KiB limit.
- SMMUv3 strict/lazy tests, missing-source, bounds and rollback tests
  passed. Real bounced attachments exercised alignment and pool
  rejection with no target mapping or DMA.
- Invalid forced-SWIOTLB Rockchip provider runs are excluded. Expected
  segment-limit and unsupported-fallback diagnostics remain.

The rig uses a custom Rockchip IOMMU model and the real GEM callback in a
test module, not VOP2, Sway or a real exporter. RK3588 hardware, two-VOP
and 32-bit ARM runtime testing are outstanding.

Hardware testing is welcome, particularly with Diederik's Sway resize
workload. Compare control and both patches on the same base/config,
first with strict DMABUF_DEBUG, then with the warn-only RFC on both.
Please report full dmesg, config, exporter and visible display problems.
Keep a known-good boot kernel. DMABUF_DEBUG=n testing is welcome too.

Developed and tested with LLM assistance.

[1] https://lore.kernel.org/r/20261005064133.7305-1-kmehltretter@xxxxxxxxx/
[2] https://lore.kernel.org/r/20261006131000.81501-1-robin.clark@xxxxxxxxxxxxxxxx/
[3] https://lore.kernel.org/r/bd4e5ece-1358-4e0b-bb04-ba9de62d26f6@xxxxxxx/
[4] https://lore.kernel.org/r/47bf9a4c-2a47-4e93-bcdf-8d953c9a5ab8@xxxxxxx/

Reports:
https://lore.kernel.org/r/DLR74W1U9YPC.375IK0HOYHDIG@xxxxxxxxxxxxxx/
https://lore.kernel.org/r/DLYLA3WQNN3X.3FB9MO8ZWBQ5@xxxxxxxxxxxxxx/

Karl Mehltretter (2):
  iommu: Add iommu_map_sgtable_dma()
  drm/rockchip: Map imported buffers from DMA addresses

 drivers/gpu/drm/rockchip/rockchip_drm_gem.c |  19 ++-
 drivers/iommu/dma-iommu.c                   | 131 ++++++++++++++++++++
 include/linux/iommu.h                       |  12 ++
 3 files changed, 157 insertions(+), 5 deletions(-)


base-commit: 602042bf29f6efde39cfb5fdd9289bf4854bc0c5
-- 
2.53.0