[SCSI] libosd: osd_req_read_sg, optimize the single entry case

Since sg-read is a bidi operation, it is a gain to convert
a single sg entry into a regular read. Better do this in the
generic layer then force each caller to do so.

Signed-off-by: Boaz Harrosh <bharrosh@panasas.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>

1 files changed, 16 insertions, 4 deletions

diff --git a/drivers/scsi/osd/osd_initiator.c b/drivers/scsi/osd/osd_initiator.c
index b37c8a3c1bb0..86afb13f1e79 100644
--- a/drivers/scsi/osd/osd_initiator.c
+++ b/drivers/scsi/osd/osd_initiator.c
@@ -1005,11 +1005,23 @@ int osd_req_read_sg(struct osd_request *or,
         const struct osd_sg_entry *sglist, unsigned numentries)
 {
         u64 len;
-        int ret = _add_sg_continuation_descriptor(or, sglist, numentries, &len);
+        u64 off;
+        int ret;
 
-        if (ret)
-                return ret;
-        osd_req_read(or, obj, 0, bio, len);
+        if (numentries > 1) {
+                off = 0;
+                ret = _add_sg_continuation_descriptor(or, sglist, numentries,
+                                                      &len);
+                if (ret)
+                        return ret;
+        } else {
+                /* Optimize the case of single segment, read_sg is a
+                 * bidi operation.
+                 */
+                len = sglist->len;
+                off = sglist->offset;
+        }
+        osd_req_read(or, obj, off, bio, len);
 
         return 0;
 }