[PATCH v2 3/4] scatterlist: add sgl_compare_sgl() function

[Douglas Gilbert <dgilbert@interlog.com>]

After enabling copies between scatter gather lists (sgl_s),
another storage related operation is to compare two sgl_s.
This new function is modelled on NVMe's Compare command and
the SCSI VERIFY(BYTCHK=1) command. Like memcmp() this function
returns false on the first miscompare and stop comparing.

Signed-off-by: Douglas Gilbert <dgilbert@interlog.com>
---
 include/linux/scatterlist.h |  4 +++
 lib/scatterlist.c           | 60 +++++++++++++++++++++++++++++++++++++
 2 files changed, 64 insertions(+)

diff --git a/include/linux/scatterlist.h b/include/linux/scatterlist.h
index 6649414c0749..ae260dc5fedb 100644
--- a/include/linux/scatterlist.h
+++ b/include/linux/scatterlist.h
@@ -325,6 +325,10 @@ size_t sgl_copy_sgl(struct scatterlist *d_sgl, unsigned int d_nents, off_t d_ski
 		    struct scatterlist *s_sgl, unsigned int s_nents, off_t s_skip,
 		    size_t n_bytes);
 
+bool sgl_compare_sgl(struct scatterlist *x_sgl, unsigned int x_nents, off_t x_skip,
+		     struct scatterlist *y_sgl, unsigned int y_nents, off_t y_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 a0a86059c10e..d910776a4c96 100644
--- a/lib/scatterlist.c
+++ b/lib/scatterlist.c
@@ -1048,3 +1048,63 @@ size_t sgl_copy_sgl(struct scatterlist *d_sgl, unsigned int d_nents, off_t d_ski
 }
 EXPORT_SYMBOL(sgl_copy_sgl);
 
+/**
+ * sgl_compare_sgl - Compare x and y (both sgl_s)
+ * @x_sgl:		 x (left) sgl
+ * @x_nents:		 Number of SG entries in x (left) sgl
+ * @x_skip:		 Number of bytes to skip in x (left) before starting
+ * @y_sgl:		 y (right) sgl
+ * @y_nents:		 Number of SG entries in y (right) 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).
+ *
+ * Notes:
+ *   x and y are symmetrical: they can be swapped and the result is the same.
+ *
+ *   Implementation is based on memcmp(). x and y segments may overlap.
+ *
+ *   The notes in sgl_copy_sgl() about large sgl_s _applies here as well.
+ *
+ **/
+bool sgl_compare_sgl(struct scatterlist *x_sgl, unsigned int x_nents, off_t x_skip,
+		     struct scatterlist *y_sgl, unsigned int y_nents, off_t y_skip,
+		     size_t n_bytes)
+{
+	bool equ = true;
+	size_t len;
+	size_t offset = 0;
+	struct sg_mapping_iter x_iter, y_iter;
+
+	if (n_bytes == 0)
+		return true;
+	sg_miter_start(&x_iter, x_sgl, x_nents, SG_MITER_ATOMIC | SG_MITER_FROM_SG);
+	sg_miter_start(&y_iter, y_sgl, y_nents, SG_MITER_ATOMIC | SG_MITER_FROM_SG);
+	if (!sg_miter_skip(&x_iter, x_skip))
+		goto fini;
+	if (!sg_miter_skip(&y_iter, y_skip))
+		goto fini;
+
+	while (equ && offset < n_bytes) {
+		if (!sg_miter_next(&x_iter))
+			break;
+		if (!sg_miter_next(&y_iter))
+			break;
+		len = min3(x_iter.length, y_iter.length, n_bytes - offset);
+
+		equ = !memcmp(x_iter.addr, y_iter.addr, len);
+		offset += len;
+		/* LIFO order is important when SG_MITER_ATOMIC is used */
+		y_iter.consumed = len;
+		sg_miter_stop(&y_iter);
+		x_iter.consumed = len;
+		sg_miter_stop(&x_iter);
+	}
+fini:
+	sg_miter_stop(&y_iter);
+	sg_miter_stop(&x_iter);
+	return equ;
+}
+EXPORT_SYMBOL(sgl_compare_sgl);
-- 
2.25.1