linux-kernel - [PATCH v2 01/13] scatterlist: introduce sg_nents_for

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Message-ID: <20251110103805.3562136-2-andriy.shevchenko@linux.intel.com>
Date: Mon, 10 Nov 2025 11:23:28 +0100
From: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
	Stefan Wahren <wahrenst@....net>,
	Vinod Koul <vkoul@...nel.org>,
	Thomas Andreatta <thomasandreatta2000@...il.com>,
	Caleb Sander Mateos <csander@...estorage.com>,
	dmaengine@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	linux-rpi-kernel@...ts.infradead.org,
	linux-arm-kernel@...ts.infradead.org,
	linux-arm-msm@...r.kernel.org
Cc: Olivier Dautricourt <olivierdautricourt@...il.com>,
	Stefan Roese <sr@...x.de>,
	Florian Fainelli <florian.fainelli@...adcom.com>,
	Broadcom internal kernel review list <bcm-kernel-feedback-list@...adcom.com>,
	Ray Jui <rjui@...adcom.com>,
	Scott Branden <sbranden@...adcom.com>,
	Lars-Peter Clausen <lars@...afoo.de>,
	Eugeniy Paltsev <Eugeniy.Paltsev@...opsys.com>,
	Daniel Mack <daniel@...que.org>,
	Haojian Zhuang <haojian.zhuang@...il.com>,
	Robert Jarzmik <robert.jarzmik@...e.fr>,
	Lizhi Hou <lizhi.hou@....com>,
	Brian Xu <brian.xu@....com>,
	Raj Kumar Rampelli <raj.kumar.rampelli@....com>,
	Michal Simek <michal.simek@....com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: [PATCH v2 01/13] 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.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
---
 include/linux/scatterlist.h |  2 ++
 lib/scatterlist.c           | 25 +++++++++++++++++++++++++
 2 files changed, 27 insertions(+)

diff --git a/include/linux/scatterlist.h b/include/linux/scatterlist.h
index 29f6ceb98d74..6de1a2434299 100644
--- a/include/linux/scatterlist.h
+++ b/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);
diff --git a/lib/scatterlist.c b/lib/scatterlist.c
index 4af1c8b0775a..4d1a010f863c 100644
--- a/lib/scatterlist.c
+++ b/lib/scatterlist.c
@@ -64,6 +64,31 @@ 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;
+}
+
 /**
  * sg_last - return the last scatterlist entry in a list
  * @sgl:	First entry in the scatterlist
-- 
2.50.1