Claude review: drm: uapi: Add macro for chipset specific event ID region

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

Overall Series Review

Subject: drm: uapi: Add macro for chipset specific event ID region
Author: Bence Csokas <bence.csokas@arm.com>
Patches: 1
Reviewed: 2026-05-25T22:50:39.045663

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This is a single-patch cleanup to the DRM UAPI headers. It introduces a `DRM_EVENT_VENDOR_SPECIFIC(_v)` macro that codifies the existing convention documented in the `struct drm_event` comment: event types `>= 0x80000000` are chipset-specific. It then converts the three existing in-tree users (exynos, virtgpu, vmwgfx) from raw hex constants to the new macro.

The patch is straightforward, correct, and all resulting values are bit-identical to the originals:
- `DRM_EXYNOS_G2D_EVENT`: `0x80000000` → `DRM_EVENT_VENDOR_SPECIFIC(0x0)` = `0x80000000` ✓
- `DRM_EXYNOS_IPP_EVENT`: `0x80000002` → `DRM_EVENT_VENDOR_SPECIFIC(0x2)` = `0x80000002` ✓
- `VIRTGPU_EVENT_FENCE_SIGNALED`: `0x90000000` → `DRM_EVENT_VENDOR_SPECIFIC(0x10000000)` = `0x90000000` ✓
- `DRM_VMW_EVENT_FENCE_SIGNALED`: `0x80000000` → `DRM_EVENT_VENDOR_SPECIFIC(0x0)` = `0x80000000` ✓

No UAPI ABI break. All three driver headers already `#include "drm.h"`, so the macro resolves correctly.

**Minor observations (not blocking):**

1. The macro uses bitwise OR (`|`), which means a caller passing a value that already has bit 31 set (e.g., `DRM_EVENT_VENDOR_SPECIFIC(0x80000001)`) would silently produce a "vendor" event indistinguishable from `DRM_EVENT_VENDOR_SPECIFIC(0x1)`. This is unlikely to happen in practice, but the macro doesn't guard against it. A comment or `BUILD_BUG_ON` for kernel-internal usage could prevent future mistakes, but for a UAPI header macro this is probably acceptable as-is.

2. `DRM_VMW_EVENT_FENCE_SIGNALED` and `DRM_EXYNOS_G2D_EVENT` share the same value `0x80000000`. This is pre-existing — different drivers handle their own event space, so there's no actual conflict. But the conversion to the macro makes this more visible. Worth noting, not a bug.

**Verdict:** Looks good. Clean mechanical conversion with no value changes.

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Generated by Claude Code Patch Reviewer