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Subject[PATCH] lib/scatterlist: introduce sg_nents_for_dma() helper
Date
From: Andy Shevchenko <andy.shevchenko@gmail.com>

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.

Signed-off-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
---

Patch was sent in 2016 initially to the DMA engine sub-system.
Link:
https://patchwork.kernel.org/patch/9389821/
This was part of a larger series:
https://patchwork.kernel.org/project/linux-dmaengine/list/?q=sg_nents_for_dma&archive=both&series=&submitter=&delegate=&state=*

I'm not sure if this is supposed to go into DMAEngine or lib/scatterlist.
It doesn't look like lib/scatterlist is managed by DMAEngine, so (by using
the `get_maintainers.pl` script) I'm sending this patch to this group of
parties.

Thanks
Alex

include/linux/scatterlist.h | 1 +
lib/scatterlist.c | 26 ++++++++++++++++++++++++++
2 files changed, 27 insertions(+)

diff --git a/include/linux/scatterlist.h b/include/linux/scatterlist.h
index b96f0d0b5b8f..4f40455c40e2 100644
--- a/include/linux/scatterlist.h
+++ b/include/linux/scatterlist.h
@@ -253,6 +253,7 @@ 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_next(struct scatterlist *);
struct scatterlist *sg_last(struct scatterlist *s, unsigned int);
void sg_init_table(struct scatterlist *, unsigned int);
diff --git a/lib/scatterlist.c b/lib/scatterlist.c
index 9ba349e775ef..5a6fc32f485d 100644
--- a/lib/scatterlist.c
+++ b/lib/scatterlist.c
@@ -86,6 +86,32 @@ int sg_nents_for_len(struct scatterlist *sg, u64 len)
}
EXPORT_SYMBOL(sg_nents_for_len);

+/**
+ * sg_nents_for_dma - return 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
--
2.17.1
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