Claude review: drm/fourcc: document format naming scheme

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

Patch Review

**Typo (line 137 of the diff context):**

```
+ * - The first letter indicates the number of planes, channel alignment and
+ *   ordering. It is should not be a letter already used to denote a channel.
```

"It is should not be" → "It should not be"

---

**Cb/Cr digit reversal rule is backwards:**

```
+ * - The two other digits indicate bits for the Y channel. If the Cb channel
+ *   comes before Cr, the digits are reversed.
```

This is wrong when checked against Q410/Q401:
- `DRM_FORMAT_Q410` (`'Q','4','1','0'`) = YCbCr (Cb before Cr) — digits are "10" (normal/unreversed)
- `DRM_FORMAT_Q401` (`'Q','4','0','1'`) = YCrCb (Cr before Cb) — digits are "01" (reversed)

So Q410 with Cb-before-Cr has the **normal** digits, not reversed. The rule is inverted — it should read: "If the **Cr** channel comes before Cb, the digits are reversed."

---

**Sub-sampling digit scheme doesn't cover NV formats:**

```
+ * - The first digit indicates chroma sub-sampling: 0 for 2x2, 2 for 2x1, 4 for
+ *   none.
```

This accurately describes P-series (P010→0=2x2, P210→2=2x1) and Q-series (Q410→4=none) formats. However, the NV family — the most common multi-planar YUV formats — uses a completely different scheme where the two trailing digits encode **bits-per-pixel** (NV12=12bpp, NV16=16bpp, NV24=24bpp), not "subsampling digit + Y-channel bits". While the "for the most part" qualifier hedges this, it might be worth explicitly noting that older multi-planar families (NV, YUV/YVU) predate and don't follow this convention, to avoid confusion.

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**P030 exception to "Y channel bits" claim:**

```
+ * - The two other digits indicate bits for the Y channel.
```

P030 has digits "30" but its Y channel is 10-bit (it's a packed 3×10-bit format). P010/P012/P016 all fit the rule correctly. P030 is arguably just a naming oddity but is a notable exception within the very format family (P-series) that the scheme is supposed to describe.

---

**Underscore plane delimiter claim lacks examples:**

```
+ * An underscore "_" can be used to delimit planes.
```

I couldn't find any existing format in `drm_fourcc.h` that uses an underscore to delimit planes. `YUV420_8BIT` and `YUV420_10BIT` have underscores, but those separate the format type from the bit depth, not planes. If this is forward-looking guidance, it might be clearer stated as such.

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**Minor observation — channel list completeness:**

The channel definitions are good. "D" for darkness (inverse brightness) is a nice touch documenting the D1/D2/D4/D8 formats. The single-channel "R" formats (R1, R2, R4, R8, R10, R12, R16) are covered by listing "R" as red.

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**Overall**: The RGB naming scheme documentation is accurate and useful. The YUV section needs the Cb/Cr reversal rule fixed (it's currently stating the opposite of what the code shows) and the typo corrected. Consider acknowledging that NV-family formats use a different, older convention.

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Generated by Claude Code Patch Reviewer