Claude review: drm/gpuvm: rust: add RUST_DRM_GPUVM option to Kconfig

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

Patch Review

**Kconfig addition** (`drivers/gpu/drm/Kconfig`):

```
+config RUST_DRM_GPUVM
+	bool
+	depends on DRM
+	select DRM_GPUVM
+	help
+	  Choose this if you need GPUVM functions in Rust
```

This correctly mirrors the existing `RUST_DRM_GEM_SHMEM_HELPER` pattern at line 278. Key points:
- `bool` (not `tristate`) — forces built-in, which is the whole point.
- `select DRM_GPUVM` — ensures the C implementation is pulled in when selected.
- No user-visible prompt (no string after `bool`) — intended to be selected by drivers, not manually enabled. This is correct.

One minor observation: `DRM_GPUVM` is `tristate`, and `RUST_DRM_GPUVM` is `bool` that selects it. When a `bool` symbol selects a `tristate`, Kconfig forces the tristate to `y` (built-in). This is exactly the desired behavior and matches the commit message explanation.

**C helper guard change** (`rust/helpers/drm_gpuvm.c`):

```c
-#ifdef CONFIG_DRM_GPUVM
+#ifdef CONFIG_RUST_DRM_GPUVM
```

Correct. The helper functions are only needed for Rust consumers, so guarding them behind `RUST_DRM_GPUVM` rather than `DRM_GPUVM` avoids compiling them when only C code uses GPUVM (where the inline/macro versions are used directly). The closing comment is also updated consistently.

**Rust cfg change** (`rust/kernel/drm/gpuvm/mod.rs`):

```rust
-#![cfg(CONFIG_DRM_GPUVM = "y")]
+#![cfg(CONFIG_RUST_DRM_GPUVM)]
```

Two things happen here:
1. The config symbol is updated to match the new Kconfig option.
2. The `= "y"` check is dropped. Since `RUST_DRM_GPUVM` is `bool` (never `m`), a bare `cfg(CONFIG_RUST_DRM_GPUVM)` is sufficient — it will be either defined or not. This is consistent with how other `bool` configs are checked elsewhere in `rust/kernel/` (e.g., `#![cfg(CONFIG_MMU)]`).

**Overall**: Clean, minimal patch. Follows established conventions. No issues found.

Reviewed-by: Claude <noreply@anthropic.com>

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