[PATCH] udmabuf: respect the device's maximum segment size

[Karl Mehltretter <kmehltretter@gmail.com>]

get_sg_table() merges physically contiguous pages without accounting
for the mapping device's maximum segment size. This affects both
importer mappings and the udmabuf misc device mapping used for CPU
access.

With DMA_API_DEBUG enabled, DMA_BUF_IOCTL_SYNC on a 64 MiB udmabuf
reports:

  DMA-API: misc udmabuf: mapping sg segment longer than device claims to support [len=65884160] [max=65536]

Use sg_alloc_table_from_pages_segment() with the mapping device's
maximum segment size. Keep a PAGE_SIZE minimum because the allocator
warns and returns -EINVAL for smaller limits.

Before commit 5bf888673e0d ("udmabuf: Do not create malformed
scatterlists"), each entry covered one page.

Fixes: 5bf888673e0d ("udmabuf: Do not create malformed scatterlists")
Assisted-by: LLM
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
---

Notes:
    Tested on v7.3-rc4-70-gfe2ec83746e5 in QEMU (x86_64, TCG) with
    DMA_API_DEBUG (all_errors=1) and DMABUF_DEBUG, A/B against the same
    base:
    
                                            before        after
      DMA_BUF_IOCTL_SYNC, 64 MiB udmabuf    1 report      0
      vivid import, 4 MiB udmabuf           2 reports     0
      vivid import, 2 MiB hugetlb udmabuf   2 reports     0
        frames captured                     5/5           5/5
    
    vb2-dma-contig rejected the non-contiguous import in both runs.

 drivers/dma-buf/udmabuf.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/drivers/dma-buf/udmabuf.c b/drivers/dma-buf/udmabuf.c
index df6dd00462423..09f1eb8432f19 100644
--- a/drivers/dma-buf/udmabuf.c
+++ b/drivers/dma-buf/udmabuf.c
@@ -139,9 +139,13 @@ static struct sg_table *get_sg_table(struct device *dev, struct dma_buf *buf,
 	if (!sg)
 		return ERR_PTR(-ENOMEM);
 
-	ret = sg_alloc_table_from_pages(sg, ubuf->pages, ubuf->pagecount, 0,
-					ubuf->pagecount << PAGE_SHIFT,
-					GFP_KERNEL);
+	/* The SG allocator requires a segment limit of at least PAGE_SIZE. */
+	ret = sg_alloc_table_from_pages_segment(sg, ubuf->pages, ubuf->pagecount,
+						0, ubuf->pagecount << PAGE_SHIFT,
+						max_t(unsigned int,
+						      dma_get_max_seg_size(dev),
+						      PAGE_SIZE),
+						GFP_KERNEL);
 	if (ret < 0)
 		goto err_alloc;
 
-- 
2.53.0