Claude review: drm: ensure blend mode supported if alpha exposed

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

Patch Review

**Verdict: Needs rework — will break existing drivers.**

The design intent is correct: if a plane exposes alpha (either through the alpha property or alpha-containing pixel formats), userspace needs to know the blend mode. Enforcing this at driver registration time is the right place.

**Problem 1: Breaks existing in-tree drivers.** As noted above, at least 6 drivers set `alpha_property` without `blend_mode_property`, and many more support ARGB formats without blend mode. These drivers currently work fine. After this patch, `drm_dev_register()` returns `-EINVAL` and the driver fails to load entirely. The commit message acknowledges this ("Drivers that hit such error must be fixed") but does not include the fixes.

This violates the kernel's regression policy — you cannot merge a change that breaks existing working drivers and expect the fixes to come later. The fixes for all affected drivers must be part of this series (or merged first).

**Problem 2: The pixel format alpha check is extremely broad.** Nearly every DRM driver supports ARGB8888 or similar formats. The `has_alpha` check in `plane_require_blend_mode_for_alpha()` will catch a very large number of drivers that don't set up blend mode. This needs careful auditing of every DRM driver in the tree.

**Problem 3: Naming convention.** The split between `drm_mode_config_validate()` (WARN-based, non-fatal) and `drm_mode_config_enforce()` (error-return, fatal) introduces a new pattern. It would be worth documenting why some checks are hard errors and others are soft warnings, or alternatively, consider making this a WARN for now and hardening it later once all drivers are fixed.

**Minor issues:**

- Typo in commit message: "relly" should be "rely", "don't" should be "doesn't", "count with" should be "count on".

- Alignment uses spaces instead of tab at line 373 of the diff:
```c
		drm_err(dev, "[PLANE:%d:%s] pixel format with alpha exposed but blend mode not setup",
		        plane->base.id, plane->name);
```
The `plane->base.id` line appears to use spaces for alignment — should use tabs per kernel coding style (compare with the `alpha_property` error message a few lines above which uses the correct tab alignment).

- The function `drm_mode_config_enforce` is not exported (`EXPORT_SYMBOL`) which is fine since it's only called from `drm_drv.c`, but it also lacks a kerneldoc comment. Since `drm_mode_config_validate()` also lacks one, this is consistent, but worth noting.

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