1 files changed, 26 insertions, 5 deletions

diff --git a/drivers/scsi/libata-core.c b/drivers/scsi/libata-core.c
index d81db3a3d4b9..1ccaf467d516 100644
--- a/drivers/scsi/libata-core.c
+++ b/drivers/scsi/libata-core.c
@@ -2409,7 +2409,8 @@ static void ata_sg_clean(struct ata_queued_cmd *qc)
                 pad_buf = ap->pad + (qc->tag * ATA_DMA_PAD_SZ);
 
         if (qc->flags & ATA_QCFLAG_SG) {
-                dma_unmap_sg(ap->host_set->dev, sg, qc->n_elem, dir);
+                if (qc->n_elem)
+                        dma_unmap_sg(ap->host_set->dev, sg, qc->n_elem, dir);
                 /* restore last sg */
                 sg[qc->orig_n_elem - 1].length += qc->pad_len;
                 if (pad_buf) {
@@ -2419,8 +2420,10 @@ static void ata_sg_clean(struct ata_queued_cmd *qc)
                         kunmap_atomic(psg->page, KM_IRQ0);
                 }
         } else {
-                dma_unmap_single(ap->host_set->dev, sg_dma_address(&sg[0]),
-                                 sg_dma_len(&sg[0]), dir);
+                if (sg_dma_len(&sg[0]) > 0)
+                        dma_unmap_single(ap->host_set->dev,
+                                sg_dma_address(&sg[0]), sg_dma_len(&sg[0]),
+                                dir);
                 /* restore sg */
                 sg->length += qc->pad_len;
                 if (pad_buf)
@@ -2619,6 +2622,11 @@ static int ata_sg_setup_one(struct ata_queued_cmd *qc)
                         sg->length, qc->pad_len);
         }
 
+        if (!sg->length) {
+                sg_dma_address(sg) = 0;
+                goto skip_map;
+        }
+
         dma_address = dma_map_single(ap->host_set->dev, qc->buf_virt,
                                      sg->length, dir);
         if (dma_mapping_error(dma_address)) {
@@ -2628,6 +2636,7 @@ static int ata_sg_setup_one(struct ata_queued_cmd *qc)
         }
 
         sg_dma_address(sg) = dma_address;
+skip_map:
         sg_dma_len(sg) = sg->length;
 
         DPRINTK("mapped buffer of %d bytes for %s\n", sg_dma_len(sg),
@@ -2655,7 +2664,7 @@ static int ata_sg_setup(struct ata_queued_cmd *qc)
         struct ata_port *ap = qc->ap;
         struct scatterlist *sg = qc->__sg;
         struct scatterlist *lsg = &sg[qc->n_elem - 1];
-        int n_elem, dir;
+        int n_elem, pre_n_elem, dir, trim_sg = 0;
 
         VPRINTK("ENTER, ata%u\n", ap->id);
         assert(qc->flags & ATA_QCFLAG_SG);
@@ -2689,13 +2698,24 @@ static int ata_sg_setup(struct ata_queued_cmd *qc)
                 sg_dma_len(psg) = ATA_DMA_PAD_SZ;
                 /* trim last sg */
                 lsg->length -= qc->pad_len;
+                if (lsg->length == 0)
+                        trim_sg = 1;
 
                 DPRINTK("padding done, sg[%d].length=%u pad_len=%u\n",
                         qc->n_elem - 1, lsg->length, qc->pad_len);
         }
 
+        pre_n_elem = qc->n_elem;
+        if (trim_sg && pre_n_elem)
+                pre_n_elem--;
+
+        if (!pre_n_elem) {
+                n_elem = 0;
+                goto skip_map;
+        }
+
         dir = qc->dma_dir;
-        n_elem = dma_map_sg(ap->host_set->dev, sg, qc->n_elem, dir);
+        n_elem = dma_map_sg(ap->host_set->dev, sg, pre_n_elem, dir);
         if (n_elem < 1) {
                 /* restore last sg */
                 lsg->length += qc->pad_len;
@@ -2704,6 +2724,7 @@ static int ata_sg_setup(struct ata_queued_cmd *qc)
 
         DPRINTK("%d sg elements mapped\n", n_elem);
 
+skip_map:
         qc->n_elem = n_elem;
 
         return 0;