Claude review: udmabuf: emit one sg entry per pinned folio

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

Patch Review

This patch generalizes udmabuf's sg table construction to coalesce contiguous folio pages into single sg entries.

**Observations:**

1. **Tree divergence:** The current drm-next tree uses `ubuf->pages[]` and `sg_alloc_table_from_pages()`, while this patch operates on `ubuf->folios[]` and `ubuf->offsets[]`. This is a dependency on an upstream udmabuf refactoring that isn't in drm-next yet. Not a bug in the patch, but the base must merge first.

2. **Double traversal of folio array:**

```c
nents = udmabuf_sg_nents(ubuf);
/* ... allocate ... */
for (i = 0; i < ubuf->pagecount; i += run) {
    run = udmabuf_folio_nr_pages(ubuf, i);
```

`udmabuf_folio_nr_pages` is called once in `udmabuf_sg_nents()` and again in the sg construction loop, so the folio array is traversed twice. This is fine for setup-time code, but if a reviewer is concerned about large page counts (NUM_PAGES is 16000), a single-pass approach could be considered.

3. **`sg_next()` after last entry:**

```c
sgl = sg->sgl;
for (i = 0; i < ubuf->pagecount; i += run) {
    run = udmabuf_folio_nr_pages(ubuf, i);
    sg_set_folio(sgl, ubuf->folios[i], run << PAGE_SHIFT,
                 ubuf->offsets[i]);
    sgl = sg_next(sgl);
}
```

After the last iteration, `sg_next(sgl)` returns NULL, and `sgl` is never used again. Harmless, but a more defensive approach would be to only call `sg_next` when there's a next iteration.

4. **The comment block style** uses `/* ... */` spanning multiple lines but without leading `*` on intermediate lines, which is inconsistent with typical kernel style (though acceptable per the coding guidelines as long as it's consistent within the file):

```c
+/* Return the number of contiguous pages backed by the folio at @i.
+ * A udmabuf may map only part of a folio, or reference the same folio
+ * in multiple non-contiguous runs, so folio_nr_pages() can't be used.
+ */
```

This is actually fine -- it does use `*` prefixes. No issue.

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Generated by Claude Code Patch Reviewer