There are two main ways that GPUVM might be used:

* staged mode, where VM_BIND ioctls update the GPUVM immediately so that
  the GPUVM reflects the state of the VM *including* staged changes that
  are not yet applied to the GPU's virtual address space.
* immediate mode, where the GPUVM state is updated during run_job(),
  i.e., in the DMA fence signalling critical path, to ensure that the
  GPUVM and the GPU's virtual address space has the same state at all
  times.

Currently, only Panthor uses GPUVM in immediate mode, but the Rust
drivers Tyr and Nova will also use GPUVM in immediate mode, so it is
worth to support both staged and immediate mode well in GPUVM. To use
immediate mode, we must manage the vm_bos and vas during the fence
signalling critical path.

The first part of that work was the introduction of a fence signalling
safe mutex for the GEMs GPUVA list in commit e7fa80e2932c ("drm_gem: add
mutex to drm_gem_object.gpuva").

This is series the second part of that work: Dropping a vm_bo object in
the fence signalling critical path is problematic for two reasons:

* When using DRM_GPUVM_RESV_PROTECTED, you cannot remove the vm_bo from
  the extobj/evicted lists during the fence signalling path.
* Dropping a vm_bo could lead to the GEM object getting destroyed.
  The requirement that GEM object cleanup is fence signalling safe is
  dubious and likely to be violated in practice.

Panthor already has its own custom implementation of postponing vm_bo
cleanup. Take inspiration from that by moving the logic into GPUVM, and
adjust Panthor to use the new GPUVM logic.

Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
Changes in v2:
- Fix missing kfree in Panthor.
- Rework mutex_lock() calls to be less confusing.
- Add note about resv lock in drm_gpuvm_bo_is_dead() docs.
- Link to v1: https://lore.kernel.org/r/20250905-vmbo-defer-v1-0-7ae1a382b674@google.com

---
Alice Ryhl (2):
      drm/gpuvm: add deferred vm_bo cleanup
      panthor: use drm_gpuva_unlink_defer()

 drivers/gpu/drm/drm_gpuvm.c           | 174 ++++++++++++++++++++++++++++++++++
 drivers/gpu/drm/panthor/panthor_mmu.c | 113 ++++------------------
 include/drm/drm_gpuvm.h               |  26 +++++
 3 files changed, 219 insertions(+), 94 deletions(-)
---
base-commit: 7156602d56e5ad689ae11e03680ab6326238b5e3
change-id: 20250905-vmbo-defer-3faf90d821f5

Best regards,
-- 
Alice Ryhl <aliceryhl@google.com>