scatterlist: introduce sg_nents_for_dma() helper

Sometimes the user needs to split each entry on the mapped scatter list
due to DMA length constrains. This helper returns a number of entities
assuming that each of them is not bigger than supplied maximum length.

Reviewed-by: Bjorn Andersson <andersson@kernel.org>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://patch.msgid.link/20260108105619.3513561-2-andriy.shevchenko@linux.intel.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>

Author: andy-shev

include/linux/scatterlist.h

@@ -441,6 +441,8 @@ static inline void sg_init_marker(struct scatterlist *sgl,
 
 int sg_nents(struct scatterlist *sg);
 int sg_nents_for_len(struct scatterlist *sg, u64 len);
+int sg_nents_for_dma(struct scatterlist *sgl, unsigned int sglen, size_t len);
+
 struct scatterlist *sg_last(struct scatterlist *s, unsigned int);
 void sg_init_table(struct scatterlist *, unsigned int);
 void sg_init_one(struct scatterlist *, const void *, unsigned int);

lib/scatterlist.c

@@ -64,6 +64,32 @@ int sg_nents_for_len(struct scatterlist *sg, u64 len)
 }
 EXPORT_SYMBOL(sg_nents_for_len);
 
+/**
+ * sg_nents_for_dma - return the count of DMA-capable entries in scatterlist
+ * @sgl:	The scatterlist
+ * @sglen:	The current number of entries
+ * @len:	The maximum length of DMA-capable block
+ *
+ * Description:
+ * Determines the number of entries in @sgl which would be permitted in
+ * DMA-capable transfer if list had been split accordingly, taking into
+ * account chaining as well.
+ *
+ * Returns:
+ *   the number of sgl entries needed
+ *
+ **/
+int sg_nents_for_dma(struct scatterlist *sgl, unsigned int sglen, size_t len)
+{
+	struct scatterlist *sg;
+	int i, nents = 0;
+
+	for_each_sg(sgl, sg, sglen, i)
+		nents += DIV_ROUND_UP(sg_dma_len(sg), len);
+	return nents;
+}
+EXPORT_SYMBOL(sg_nents_for_dma);
+
 /**
  * sg_last - return the last scatterlist entry in a list
  * @sgl:	First entry in the scatterlist