[PATCH 2/4] scatterlist: add sgl_copy_sgl() function

[Douglas Gilbert <dgilbert@interlog.com>]

Both the SCSI and NVMe subsystems receive user data from the block
layer in scatterlist_s (aka scatter gather lists (sgl) which are
often arrays). If drivers in those subsystems represent storage
(e.g. a ramdisk) or cache "hot" user data then they may also
choose to use scatterlist_s. Currently there are no sgl to sgl
operations in the kernel. Start with a copy.

Signed-off-by: Douglas Gilbert <dgilbert@interlog.com>
---
 include/linux/scatterlist.h |  4 ++
 lib/scatterlist.c           | 86 +++++++++++++++++++++++++++++++++++++
 2 files changed, 90 insertions(+)

diff --git a/include/linux/scatterlist.h b/include/linux/scatterlist.h
index 80178afc2a4a..6649414c0749 100644
--- a/include/linux/scatterlist.h
+++ b/include/linux/scatterlist.h
@@ -321,6 +321,10 @@ size_t sg_pcopy_to_buffer(struct scatterlist *sgl, unsigned int nents,
 size_t sg_zero_buffer(struct scatterlist *sgl, unsigned int nents,
 		       size_t buflen, off_t skip);
 
+size_t sgl_copy_sgl(struct scatterlist *d_sgl, unsigned int d_nents, off_t d_skip,
+		    struct scatterlist *s_sgl, unsigned int s_nents, off_t s_skip,
+		    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 d5770e7f1030..1ec2c909c8d4 100644
--- a/lib/scatterlist.c
+++ b/lib/scatterlist.c
@@ -974,3 +974,89 @@ size_t sg_zero_buffer(struct scatterlist *sgl, unsigned int nents,
 	return offset;
 }
 EXPORT_SYMBOL(sg_zero_buffer);
+
+/**
+ * sgl_copy_sgl - Copy over a destination sgl from a source sgl
+ * @d_sgl:		 Destination sgl
+ * @d_nents:		 Number of SG entries in destination sgl
+ * @d_skip:		 Number of bytes to skip in destination before copying
+ * @s_sgl:		 Source sgl
+ * @s_nents:		 Number of SG entries in source sgl
+ * @s_skip:		 Number of bytes to skip in source before copying
+ * @n_bytes:		 The number of bytes to copy
+ *
+ * Returns the number of copied bytes.
+ *
+ * Notes:
+ *   Destination arguments appear before the source arguments, as with memcpy().
+ *
+ *   Stops copying if the end of d_sgl or s_sgl is reached.
+ *
+ *   Since memcpy() is used, overlapping copies (where d_sgl and s_sgl belong
+ *   to the same sgl and the copy regions overlap) are not supported.
+ *
+ *   If d_skip is large, potentially spanning multiple d_nents then some
+ *   integer arithmetic to adjust d_sgl may improve performance. For example
+ *   if d_sgl is built using sgl_alloc_order(chainable=false) then the sgl
+ *   will be an array with equally sized segments facilitating that
+ *   arithmetic. The suggestion applies to s_skip, s_sgl and s_nents as well.
+ *
+ **/
+size_t sgl_copy_sgl(struct scatterlist *d_sgl, unsigned int d_nents, off_t d_skip,
+		    struct scatterlist *s_sgl, unsigned int s_nents, off_t s_skip,
+		    size_t n_bytes)
+{
+	size_t d_off, s_off, len, d_len, s_len;
+	size_t offset = 0;
+	struct sg_mapping_iter d_iter;
+	struct sg_mapping_iter s_iter;
+
+	if (n_bytes == 0)
+		return 0;
+	sg_miter_start(&d_iter, d_sgl, d_nents, SG_MITER_ATOMIC | SG_MITER_TO_SG);
+	sg_miter_start(&s_iter, s_sgl, s_nents, SG_MITER_ATOMIC | SG_MITER_FROM_SG);
+	if (!sg_miter_skip(&d_iter, d_skip))
+		goto fini;
+	if (!sg_miter_skip(&s_iter, s_skip))
+		goto fini;
+
+	for (d_off = 0, s_off = 0; true ; ) {
+		/* Assume d_iter.length and s_iter.length can never be 0 */
+		if (d_off == 0) {
+			if (!sg_miter_next(&d_iter))
+				break;
+			d_len = d_iter.length;
+		} else {
+			d_len = d_iter.length - d_off;
+		}
+		if (s_off == 0) {
+			if (!sg_miter_next(&s_iter))
+				break;
+			s_len = s_iter.length;
+		} else {
+			s_len = s_iter.length - s_off;
+		}
+		len = min3(d_len, s_len, n_bytes - offset);
+
+		memcpy(d_iter.addr + d_off, s_iter.addr + s_off, len);
+		offset += len;
+		if (offset >= n_bytes)
+			break;
+		if (d_len == s_len) {
+			d_off = 0;
+			s_off = 0;
+		} else if (d_len < s_len) {
+			d_off = 0;
+			s_off += len;
+		} else {
+			d_off += len;
+			s_off = 0;
+		}
+	}
+fini:
+	sg_miter_stop(&d_iter);
+	sg_miter_stop(&s_iter);
+	return offset;
+}
+EXPORT_SYMBOL(sgl_copy_sgl);
+
-- 
2.25.1