Claude review: dma-buf/udmabuf: skip redundant cpu sync to fix cacheline EEXIST warning

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

Patch Review

**Analysis of the change:**

The patch changes two lines in `get_sg_table()` and `put_sg_table()`:

```c
-	ret = dma_map_sgtable(dev, sg, direction, 0);
+	ret = dma_map_sgtable(dev, sg, direction, DMA_ATTR_SKIP_CPU_SYNC);
```

```c
-	dma_unmap_sgtable(dev, sg, direction, 0);
+	dma_unmap_sgtable(dev, sg, direction, DMA_ATTR_SKIP_CPU_SYNC);
```

**Correctness:** The change is correct. `get_sg_table()` is called from two places:
1. `map_udmabuf()` (line 186) — the dma-buf `map_dma_buf` op, called by importers. CPU sync at map time is redundant; importers manage their own cache coherency.
2. `begin_cpu_udmabuf()` (line 252) — on the first call, creates the sg table. Subsequent calls use `dma_sync_sgtable_for_cpu()` explicitly (line 258). The initial map doesn't need a sync either, as the pages are freshly mapped.

Similarly, `put_sg_table()` is called from `unmap_udmabuf()` and `release_udmabuf()`, where the CPU sync at unmap time is unnecessary.

**Precedent:** I confirmed that both `drm_gem_map_dma_buf()` (`drm_prime.c:684`) and `amdgpu_dma_buf_map()` (`amdgpu_dma_buf.c:213`) already pass `DMA_ATTR_SKIP_CPU_SYNC`, matching the commit message's claim.

**One minor observation:** The commit message says the flag "suppresses this check in `add_dma_entry()`" — that's slightly imprecise. `DMA_ATTR_SKIP_CPU_SYNC` doesn't directly affect `add_dma_entry()`. Rather, by skipping the CPU cache maintenance, the overlapping mappings become harmless, and the debug infrastructure's concern about cacheline sharing is moot. The warning is still technically possible if both mappings exist simultaneously, but in practice the flag prevents the cache ops that make the overlap dangerous. The actual suppression mechanism may be more nuanced, but the end result is correct — the warning goes away and the behavior is safe.

**Verdict:** Reviewed-by worthy. Clean two-line fix, well-motivated, consistent with how other dma-buf exporters handle this.

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Generated by Claude Code Patch Reviewer