[PATCH 7/9] scsi_debug: add sdeb_sgl_copy_sgl and friends

[Douglas Gilbert <dgilbert@interlog.com>]

The scatter-gather list helper functions in this patch have been
proposed for inclusion in lib/scatterlist.c where they would be
exported. Before that happens they need to be accepted via another
kernel subsystem maintainer. In the meantime they are placed in
this driver with a "sdeb_" prefix and with static scope. The next
patch in this set makes extensive use of these functions as
the backing store(s) of the scsi_debug driver is changed from
vmalloc() based to sgl based.

These functions where proposed for the patchset whose cover was
titled: "[PATCH v4 0/4] scatterlist: add new capabilities". That
patchset was sent to the linux-block, linux-scsi and linux-kernel
lists on 20201016. Since all patches in that set were reviewed by
Bodo Stroesser his review is carried over to this patch.

Reviewed-by: Bodo Stroesser <bostroesser@gmail.com>
Signed-off-by: Douglas Gilbert <dgilbert@interlog.com>
---
 drivers/scsi/scsi_debug.c | 127 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 127 insertions(+)

diff --git a/drivers/scsi/scsi_debug.c b/drivers/scsi/scsi_debug.c
index 44c74cf6b498..464467fbbf1a 100644
--- a/drivers/scsi/scsi_debug.c
+++ b/drivers/scsi/scsi_debug.c
@@ -1206,6 +1206,133 @@ static int p_fill_from_dev_buffer(struct scsi_cmnd *scp, const void *arr,
 	return 0;
 }
 
+/*
+ * The following 4 functions are proposed for lib/scatterlist.c (without the
+ * "sdeb_" prefix) and will be removed from this driver if the proposal is accepted.
+ */
+static size_t sdeb_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;
+}
+
+static bool sdeb_sgl_compare_sgl_idx(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, size_t *miscompare_idx)
+{
+	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 (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);
+		if (!equ)
+			goto fini;
+		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:
+	if (miscompare_idx && !equ) {
+		u8 *xp = x_iter.addr;
+		u8 *yp = y_iter.addr;
+		u8 *x_endp;
+
+		for (x_endp = xp + len ; xp < x_endp; ++xp, ++yp) {
+			if (*xp != *yp)
+				break;
+		}
+		*miscompare_idx = offset + len - (x_endp - xp);
+	}
+	sg_miter_stop(&y_iter);
+	sg_miter_stop(&x_iter);
+	return equ;
+}
+
+static bool sdeb_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)
+{
+	return sdeb_sgl_compare_sgl_idx(x_sgl, x_nents, x_skip, y_sgl, y_nents, y_skip, n_bytes,
+					NULL);
+}
+
+static size_t sdeb_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;
+	}
+fini:
+	sg_miter_stop(&miter);
+	return offset;
+}
+
+/* end of functions proposed for lib/scatterlist.c */
+
 /* Fetches from SCSI "data-out" buffer. Returns number of bytes fetched into
  * 'arr' or -1 if error.
  */
-- 
2.25.1