[PATCH 1/6] lib/scatterlist: Add contiguous DMA chunks helpers

[Thierry Reding]

From: Thierry Reding <treding at nvidia.com>

Add a few helpers to count the number of contiguous DMA chunks found in
an SG table. This is useful to determine whether or not a mapping can be
used by drivers whose devices need contiguous memory.

Signed-off-by: Thierry Reding <treding at nvidia.com>
---
 include/linux/scatterlist.h | 11 +++++++++++
 lib/scatterlist.c           | 26 ++++++++++++++++++++++++++
 2 files changed, 37 insertions(+)

diff --git a/include/linux/scatterlist.h b/include/linux/scatterlist.h
index 266754a55327..cca235ff2d0a 100644
--- a/include/linux/scatterlist.h
+++ b/include/linux/scatterlist.h
@@ -281,6 +281,7 @@ int sg_split(struct scatterlist *in, const int in_mapped_nents,
 	     const size_t *split_sizes,
 	     struct scatterlist **out, int *out_mapped_nents,
 	     gfp_t gfp_mask);
+unsigned int sg_dma_count_chunks(struct scatterlist *sgl, unsigned int nents);
 
 typedef struct scatterlist *(sg_alloc_fn)(unsigned int, gfp_t);
 typedef void (sg_free_fn)(struct scatterlist *, unsigned int);
@@ -358,6 +359,16 @@ size_t sg_pcopy_to_buffer(struct scatterlist *sgl, unsigned int nents,
 size_t sg_zero_buffer(struct scatterlist *sgl, unsigned int nents,
 		       size_t buflen, off_t skip);
 
+static inline unsigned int sgt_dma_count_chunks(struct sg_table *sgt)
+{
+	return sg_dma_count_chunks(sgt->sgl, sgt->nents);
+}
+
+static inline bool sgt_dma_contiguous(struct sg_table *sgt)
+{
+	return sgt_dma_count_chunks(sgt) == 1;
+}
+
 /*
  * Maximum number of entries that will be allocated in one piece, if
  * a list larger than this is required then chaining will be utilized.
diff --git a/lib/scatterlist.c b/lib/scatterlist.c
index abb3432ed744..fae2179a218a 100644
--- a/lib/scatterlist.c
+++ b/lib/scatterlist.c
@@ -142,6 +142,32 @@ void sg_init_one(struct scatterlist *sg, const void *buf, unsigned int buflen)
 }
 EXPORT_SYMBOL(sg_init_one);
 
+/**
+ * sg_dma_count_chunks - return number of contiguous DMA chunks in scatterlist
+ * @sgl: SG table
+ * @nents: number of entries in SG table
+ */
+unsigned int sg_dma_count_chunks(struct scatterlist *sgl, unsigned int nents)
+{
+	dma_addr_t next = ~(dma_addr_t)0;
+	unsigned int count = 0, i;
+	struct scatterlist *s;
+
+	for_each_sg(sgl, s, nents, i) {
+		/* sg_dma_address(s) is only valid for entries that have sg_dma_len(s) != 0. */
+		if (!sg_dma_len(s))
+			continue;
+
+		if (sg_dma_address(s) != next) {
+			next = sg_dma_address(s) + sg_dma_len(s);
+			count++;
+		}
+	}
+
+	return count;
+}
+EXPORT_SYMBOL(sg_dma_count_chunks);
+
 /*
  * The default behaviour of sg_alloc_table() is to use these kmalloc/kfree
  * helpers.
-- 
2.33.0