Claude review: Add support for a DRM backlight capability

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

Overall Series Review

Subject: Add support for a DRM backlight capability
Author: "Mario Limonciello (AMD)" <superm1@kernel.org>
Patches: 12
Reviewed: 2026-06-04T14:54:30.211806

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This is v5 of a significant series by Mario Limonciello adding DRM connector-level backlight control via a new `LUMINANCE` property, replacing the legacy sysfs-only interface. The design is solid: connectors get an atomic property, a new `DRM_CLIENT_CAP_LUMINANCE` client capability inhibits legacy sysfs writes when a compositor takes over, and the backlight notifier is restored to allow cross-subsystem coordination.

The series has working compositor support in KWin, Mutter, and wlroots, which is a strong signal. The overall architecture of a `drm_backlight` intermediary that links DRM connectors to `backlight_device` instances is clean. Driver integration for amdgpu, i915/xe, and bridge connectors is provided.

However, there are several issues ranging from potential locking bugs to incomplete implementations and UAPI concerns that should be addressed before merge.

**Key concerns:**
1. **Spinlock + sleeping calls**: `drm_backlight_lock` is a spinlock, but several paths under it call functions that may sleep (kobject_uevent_env, backlight notifier calls).
2. **Per-device vs global property range**: The luminance property is global to the DRM device but the range is updated per-connector link, meaning multi-eDP setups will clobber each other's ranges.
3. **Incomplete inhibit implementation** in patch 3 (stubs), then filled in patch 4 — the intermediate state is confusing for bisection.
4. **Notification loop risk**: `backlight_update_status` now calls `backlight_notify_brightness` which notifies DRM, which may call back into backlight.
5. **drm_atomic_connector_commit_dpms signature change** — the upstream function takes `struct drm_atomic_commit *state`, but the patch diff shows `struct drm_atomic_state *state`, suggesting this may be based on an older tree.

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