Claude review: drm/i915/modeset: fix NULL/ctx handling in lock paths

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Overall Series Review

Subject: drm/i915/modeset: fix NULL/ctx handling in lock paths
Author: george.d.sworo@intel.com
Patches: 6
Reviewed: 2026-05-05T11:50:03.943890

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**NAK.** This series adds defensive NULL checks for `ctx` inside the static helper `modeset_lock()`, but this function **can never be called with a NULL `ctx`** in the existing code. The two callers are:

1. `drm_modeset_lock()` (line 394) — which already handles `ctx == NULL` at line 397 and only calls `modeset_lock()` when `ctx` is non-NULL.
2. `drm_modeset_backoff()` (line 360) — which dereferences `ctx->contended` before calling `modeset_lock()`, so `ctx` is necessarily non-NULL.

The cover letter claims this is "seen in HDCP/MST paths" but provides no crash log, stack trace, or reproducer. If there is genuinely a code path reaching this function with a NULL context, the correct fix is to fix the broken caller, not to silently paper over it in a core locking primitive. Adding a silent fallback to plain `ww_mutex_lock(..., NULL)` would bypass deadlock detection and lock tracking (`ctx->locked` list), which would mask real bugs and cause worse problems downstream (e.g., `drm_modeset_drop_locks()` wouldn't know about the lock).

The commit messages also claim `drm_modeset_drop_locks()` has the same problem, but neither patch touches that function.

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