Claude review: drm: Add CRTC background color property

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

Patch Review

**Status: Looks good**

This is the core patch and it's well done.

**Property creation and plumbing**: The property is created as a standard property in `drm_mode_create_standard_properties()` with range `[0, U64_MAX]`, and attached to individual CRTCs via `drm_crtc_attach_background_color_property()`. This follows the established DRM pattern (similar to color management properties). The get/set in `drm_atomic_uapi.c` and state dump in `drm_atomic.c` are correct.

**Default initialization**: Setting the default to opaque black in `__drm_atomic_helper_crtc_state_reset()` is correct:

```c
+	crtc_state->background_color = DRM_ARGB64_PREP(0xffff, 0, 0, 0);
```

**ARGB64 macros**: The helper design is well thought out:

- `DRM_ARGB64_PREP()` / `DRM_ARGB64_PREP_BPC()` for packing
- `DRM_ARGB64_GET*()` / `DRM_ARGB64_GET*_BPC()` / `DRM_ARGB64_GET*_BPCS()` for extraction
- The BPC variants use proper rounding (`DIV_ROUND_CLOSEST`) for accuracy
- The BPCS variants use simple right-shift for performance

The scaling math is correct: `DIV_ROUND_CLOSEST(c * 0xFFFF, (2^bpc - 1))` properly maps the input range to 16-bit. I verified the intermediate multiplication won't overflow since the `__u16` operands promote to at least `unsigned long` when multiplied with `__GENMASK(15, 0)`.

**Documentation**: The updated kerneldoc in `drm_blend.c` clearly describes the property semantics, including the CRTC degamma/CSC/gamma pipeline placement and the note about alpha behavior. Good.

Minor: the state dump format `%llx` works but `0x%016llx` would produce more consistently readable output for a 64-bit ARGB value.

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Generated by Claude Code Patch Reviewer