[PATCH v2 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           | 74 +++++++++++++++++++++++++++++++++++++
 2 files changed, 78 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..a0a86059c10e 100644
--- a/lib/scatterlist.c
+++ b/lib/scatterlist.c
@@ -974,3 +974,77 @@ 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 starting
+ * @s_sgl:		 Source sgl
+ * @s_nents:		 Number of SG entries in source sgl
+ * @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.
+ *
+ * Notes:
+ *   Destination arguments appear before the source arguments, as with memcpy().
+ *
+ *   Stops copying if either d_sgl, s_sgl or n_bytes is exhausted.
+ *
+ *   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.
+ *
+ *   Large copies are broken into copy segments whose sizes may vary. Those
+ *   copy segment sizes are chosen by the min3() statement in the code below.
+ *   Since SG_MITER_ATOMIC is used for both sides, each copy segment is started
+ *   with kmap_atomic() [in sg_miter_next()] and completed with kunmap_atomic()
+ *   [in sg_miter_stop()]. This means pre-emption is inhibited for relatively
+ *   short periods even in very large copies.
+ *
+ *   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 len;
+	size_t offset = 0;
+	struct sg_mapping_iter d_iter, s_iter;
+
+	if (n_bytes == 0)
+		return 0;
+	sg_miter_start(&s_iter, s_sgl, s_nents, SG_MITER_ATOMIC | SG_MITER_FROM_SG);
+	sg_miter_start(&d_iter, d_sgl, d_nents, SG_MITER_ATOMIC | SG_MITER_TO_SG);
+	if (!sg_miter_skip(&s_iter, s_skip))
+		goto fini;
+	if (!sg_miter_skip(&d_iter, d_skip))
+		goto fini;
+
+	while (offset < n_bytes) {
+		if (!sg_miter_next(&s_iter))
+			break;
+		if (!sg_miter_next(&d_iter))
+			break;
+		len = min3(d_iter.length, s_iter.length, n_bytes - offset);
+
+		memcpy(d_iter.addr, s_iter.addr, len);
+		offset += len;
+		/* LIFO order (stop d_iter before s_iter) needed with SG_MITER_ATOMIC */
+		d_iter.consumed = len;
+		sg_miter_stop(&d_iter);
+		s_iter.consumed = len;
+		sg_miter_stop(&s_iter);
+	}
+fini:
+	sg_miter_stop(&d_iter);
+	sg_miter_stop(&s_iter);
+	return offset;
+}
+EXPORT_SYMBOL(sgl_copy_sgl);
+
-- 
2.25.1