Claude review: User readable error codes on atomic_ioctl failure

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

Overall Series Review

Subject: User readable error codes on atomic_ioctl failure
Author: Arun R Murthy <arun.r.murthy@intel.com>
Patches: 8
Reviewed: 2026-02-11T16:31:46.766206

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This v9 patch series introduces user-readable error reporting for DRM atomic ioctl failures. The goal is to provide compositors with detailed error codes instead of generic -EINVAL, enabling better error handling and recovery.

**Series-level Concerns:**

1. **UAPI Misuse of Reserved Field**: The series reuses the `reserved` field in `struct drm_mode_atomic` to pass error information back to userspace. This is problematic:
   - The field is checked to be zero on entry (patch 3 removes this check)
   - Repurposing `reserved` breaks the original design intent and may cause issues with existing userspace
   - Cover letter says "Once the series is merged the element 'reserved' used for sending the failure code in struct drm_mode_atomic is to changed to err_code" - this suggests the naming is wrong but the ABI is being established incorrectly

2. **Memory Allocation Ordering Change**: Patch 3 moves `drm_atomic_state_alloc()` to the beginning of the ioctl, which means state allocation happens before validation checks. This could impact performance in error cases and changes error paths significantly.

3. **Incomplete Coverage**: Only Intel i915 driver errors are implemented (patch 6). The TODO in the cover letter admits driver-specific errors will come later, making this series incomplete for general use.

4. **Error Code Granularity**: The error codes defined are quite coarse-grained and may not provide enough detail for compositors to make intelligent decisions.

5. **v9 Changelog Missing**: Cover letter says "EDITME: describe what is new in this series revision" - the v9 changes are not documented.

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