[PATCH 0/3] Fix import of dma-buf buffers as DRM GEM shmem objects

[Janne Grunau]

Hej,

this is triggered by the discussion of gem/shmem.rs SGTableMap issues
in [1] which has an additional issue with imported object.
Imported objects are supported since GEM shmem's AllocOps use
drm_gem_shmem_prime_import_sg_table() as gem_prime_import_sg_table which
sets the same function pointer field in struct drm_driver.

The code is broken in two ways. SGTableMap::drop() calls
__drm_gem_shmem_free_sgt_locked() uncondiftionally which it must not do
for imported objects as the object does not own `sgt`.
The second issue is that the imported object is allocated by
__drm_gem_shmem_create() with sizeof(struct drm_gem_shmem_object) as
size instead of the size of Rust's Object<T>.

The second issue is already fixed in tyr's downstream driver by adding a
`gem_create_object` callback which is used instead of the minimal
allocation in the C helper.
The first issue is fixed in the Asahi downstream driver together with a
drm_gem_is_imported() wrapper change taken from the tyr downstream
driver.

All three patches are tested in the Asahi downstream driver. As this is
unused code in-tree I marked the fixes as not suitable for stable
backports. The known downstream users of the code are aware of the
issues and carry fixes downstream. I don't expect new Rust DRM drivers
to be based on the (old) stable trees.

Link: https://lore.kernel.org/rust-for-linux/20261019-rust-shmem-fixes-v1-1-3f7f6de5b5e7@xxxxxxxxxx/ [1]
Signed-off-by: Janne Grunau <j@xxxxxxxxxx>
---
Janne Grunau (1):
      rust: drm: gem: shmem: Do not free SG table of imported objects

Sami Tolvanen (2):
      rust: drm: gem: shmem: implement gem_create_object for prime imports
      rust: drm: gem: add an is_imported() accessor

 drivers/gpu/drm/nova/gem.rs  |  4 +++
 drivers/gpu/drm/tyr/gem.rs   |  4 +++
 rust/helpers/drm.c           |  6 +++++
 rust/kernel/drm/gem/mod.rs   | 16 ++++++++++++
 rust/kernel/drm/gem/shmem.rs | 62 +++++++++++++++++++++++++++++++++++++++++++-
 5 files changed, 91 insertions(+), 1 deletion(-)
---
base-commit: 3497a403264b094ebd61b8b755165d56e7d9dc37
change-id: 20261026-rust-drm-gem-shmem-imported-sgt-85c4a45be283

Best regards,
-- 
Janne Grunau <j@xxxxxxxxxx>