Claude review: rust: drm: add base GPUVM immediate mode abstraction

[Claude Code Review Bot <claude-review@example.com>]

Patch Review

This introduces the core `GpuVm<T>`, `UniqueRefGpuVm<T>`, the `DriverGpuVm` trait, refcounting, helpers, and the MAINTAINERS/bindings plumbing.

**Observations:**

1. **`Send`/`Sync` impls for `UniqueRefGpuVm`**: The `Send` impl is automatically derived (since `UniqueRefGpuVm` contains `ARef<GpuVm<T>>` which is `Send` when `GpuVm<T>: Send+Sync`), but `Sync` is explicitly bounded on `T: Sync`:
   ```rust
   unsafe impl<T: DriverGpuVm + Sync> Sync for UniqueRefGpuVm<T> {}
   ```
   This is correct -- `&UniqueRefGpuVm` would allow calling `data_ref()` which returns `&T`, requiring `T: Sync`.

2. **Missing `Send` impl for `UniqueRefGpuVm`**: There is no explicit `Send` impl. The auto-derived one should work since `ARef<GpuVm<T>>` is `Send` when `GpuVm<T>: Send + Sync`, and both of those are implemented. This is fine.

3. **`GpuVm::new` error type signature**: The function signature is:
   ```rust
   pub fn new<E>(... ) -> Result<UniqueRefGpuVm<T>, E>
   where
       E: From<AllocError>,
       E: From<core::convert::Infallible>,
   ```
   The `E: From<Infallible>` bound is needed for the `try_pin_init!` macro. This is correct but slightly unusual -- a comment or doc note explaining why `Infallible` is needed here would help future readers.

4. **`__GFP_ZERO` for the KBox allocation**: The cover letter says `__GFP_ZERO` was added in v6 to ensure the `drm_gpuvm` struct is zeroed before `drm_gpuvm_init` is called. This is important because `drm_gpuvm_init` may not zero all fields itself. The approach of passing it to `KBox::try_pin_init` is correct.

5. **`vm_free` callback**: The `vm_free` callback reconstructs a `KBox<Self>` via `container_of!` and drops it:
   ```rust
   let me = unsafe { kernel::container_of!(Opaque::cast_from(me), Self, vm).cast_mut() };
   drop(unsafe { KBox::from_raw(me) })
   ```
   This is correct -- the C side calls `vm_free` when the refcount hits zero, and this path reclaims the Rust allocation.

6. **Minor**: The `DriverGpuVm` trait requires `Sized + Send` but not `Sync`. This is intentional -- `Sync` is only needed for `UniqueRefGpuVm<T>: Sync`, which has its own explicit bound.

No blocking issues. Clean and well-designed.

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Generated by Claude Code Patch Reviewer