[PATCH v2 4/4] scatterlist: add sgl_memset()

[Douglas Gilbert <dgilbert@interlog.com>]

The existing sg_zero_buffer() function is a bit restrictive.
For example protection information (PI) blocks are usually
initialized to 0xff bytes. As its name suggests sgl_memset()
is modelled on memset(). One difference is the type of the
val argument which is u8 rather than int. Plus it returns
the number of bytes (over)written.

Signed-off-by: Douglas Gilbert <dgilbert@interlog.com>
---
 include/linux/scatterlist.h |  3 +++
 lib/scatterlist.c           | 54 ++++++++++++++++++++++++++++++++++---
 2 files changed, 54 insertions(+), 3 deletions(-)

diff --git a/include/linux/scatterlist.h b/include/linux/scatterlist.h
index ae260dc5fedb..a40012c8a4e6 100644
--- a/include/linux/scatterlist.h
+++ b/include/linux/scatterlist.h
@@ -329,6 +329,9 @@ bool sgl_compare_sgl(struct scatterlist *x_sgl, unsigned int x_nents, off_t x_sk
 		     struct scatterlist *y_sgl, unsigned int y_nents, off_t y_skip,
 		     size_t n_bytes);
 
+size_t sgl_memset(struct scatterlist *sgl, unsigned int nents, off_t skip,
+		  u8 val, size_t n_bytes);
+
 /*
  * 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 d910776a4c96..a704039ab54d 100644
--- a/lib/scatterlist.c
+++ b/lib/scatterlist.c
@@ -985,7 +985,8 @@ EXPORT_SYMBOL(sg_zero_buffer);
  * @s_skip:		 Number of bytes to skip in source before starting
  * @n_bytes:		 The (maximum) number of bytes to copy
  *
- * Returns the number of copied bytes.
+ * Returns:
+ *   The number of copied bytes.
  *
  * Notes:
  *   Destination arguments appear before the source arguments, as with memcpy().
@@ -1058,8 +1059,9 @@ EXPORT_SYMBOL(sgl_copy_sgl);
  * @y_skip:		 Number of bytes to skip in y (right) before starting
  * @n_bytes:		 The (maximum) number of bytes to compare
  *
- * Returns true if x and y compare equal before x, y or n_bytes is exhausted.
- * Otherwise on a miscompare, returns false (and stops comparing).
+ * Returns:
+ *   true if x and y compare equal before x, y or n_bytes is exhausted.
+ *   Otherwise on a miscompare, returns false (and stops comparing).
  *
  * Notes:
  *   x and y are symmetrical: they can be swapped and the result is the same.
@@ -1108,3 +1110,49 @@ bool sgl_compare_sgl(struct scatterlist *x_sgl, unsigned int x_nents, off_t x_sk
 	return equ;
 }
 EXPORT_SYMBOL(sgl_compare_sgl);
+
+/**
+ * sgl_memset - set byte 'val' up to n_bytes times on SG list
+ * @sgl:		 The SG list
+ * @nents:		 Number of SG entries in sgl
+ * @skip:		 Number of bytes to skip before starting
+ * @val:		 byte value to write to sgl
+ * @n_bytes:		 The (maximum) number of bytes to modify
+ *
+ * Returns:
+ *   The number of bytes written.
+ *
+ * Notes:
+ *   Stops writing if either sgl or n_bytes is exhausted. If n_bytes is
+ *   set SIZE_MAX then val will be written to each byte until the end
+ *   of sgl.
+ *
+ *   The notes in sgl_copy_sgl() about large sgl_s _applies here as well.
+ *
+ **/
+size_t sgl_memset(struct scatterlist *sgl, unsigned int nents, off_t skip,
+		  u8 val, size_t n_bytes)
+{
+	size_t offset = 0;
+	size_t len;
+	struct sg_mapping_iter miter;
+
+	if (n_bytes == 0)
+		return 0;
+	sg_miter_start(&miter, sgl, nents, SG_MITER_ATOMIC | SG_MITER_TO_SG);
+	if (!sg_miter_skip(&miter, skip))
+		goto fini;
+
+	while ((offset < n_bytes) && sg_miter_next(&miter)) {
+		len = min(miter.length, n_bytes - offset);
+		memset(miter.addr, val, len);
+		offset += len;
+		miter.consumed = len;
+		sg_miter_stop(&miter);
+	}
+fini:
+	sg_miter_stop(&miter);
+	return offset;
+}
+EXPORT_SYMBOL(sgl_memset);
+
-- 
2.25.1