Claude review: drm/amd/display: Wrap DCN32 phantom-plane allocation in DC_RUN_WITH_PREEMPTION_ENABLED

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

Overall Series Review

Subject: drm/amd/display: Wrap DCN32 phantom-plane allocation in DC_RUN_WITH_PREEMPTION_ENABLED
Author: Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>
Patches: 3
Reviewed: 2026-05-05T07:55:36.347956

---

This is a single well-analyzed bugfix patch targeting a real kernel BUG_ON crash in the DCN32 display mode validation path. The commit message is exceptionally thorough — it correctly identifies the root cause, provides a clear call trace, documents the reproducer, and distinguishes this fix from related prior work (Ard Biesheuvel's DC FP fixes).

The bug is real and confirmed by the code: `dc_create_plane_state()` uses `kvzalloc_obj(*plane_state, GFP_ATOMIC)` for a ~335 KiB `struct dc_plane_state`. In this kernel's `__kvmalloc_node_noprof` (slub.c:6818), the vmalloc fallback is **unconditional** when kmalloc fails for sizes > PAGE_SIZE — the `GFP_ATOMIC` flag only controls `VM_ALLOW_HUGE_VMAP`, not whether vmalloc is attempted. When this runs inside `DC_FP_START()` (which disables softirqs via `fpregs_lock()`), vmalloc's `BUG_ON(in_interrupt())` fires.

The fix concept is correct: wrap the allocation in `DC_RUN_WITH_PREEMPTION_ENABLED()` to temporarily exit the FP context. However, the fix location is suboptimal.

**Verdict: Approach is correct, but the fix should be applied in `dc_create_plane_state()` itself rather than at the call site, and the redundant preprocessor guard should be dropped.**

---

---
Generated by Claude Code Patch Reviewer