linux-kernel - [PATCH] lib/scatterlist: introduce sg_nents_for

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Date:   Wed, 27 Feb 2019 10:50:50 +0200
From:   Alexandru Ardelean <alexandru.ardelean@...log.com>
To:     <axboe@...nel.dk>, <khandual@...ux.vnet.ibm.com>,
        <bhole_prashant_q7@....ntt.co.jp>, <keescook@...omium.org>,
        <john.fastabend@...il.com>, <linux-kernel@...r.kernel.org>
CC:     Andy Shevchenko <andy.shevchenko@...il.com>,
        Alexandru Ardelean <alexandru.ardelean@...log.com>
Subject: [PATCH] lib/scatterlist: introduce sg_nents_for_dma() helper

From: Andy Shevchenko <andy.shevchenko@...il.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@...il.com>
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@...log.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