3 files changed, 24 insertions, 9 deletions

diff --git a/drivers/scsi/libata-core.c b/drivers/scsi/libata-core.c
index 62340f4ef004..70efde99f652 100644
--- a/drivers/scsi/libata-core.c
+++ b/drivers/scsi/libata-core.c
@@ -2590,7 +2590,7 @@ static void ata_sg_clean(struct ata_queued_cmd *qc)
         WARN_ON(sg == NULL);
 
         if (qc->flags & ATA_QCFLAG_SINGLE)
-                WARN_ON(qc->n_elem != 1);
+                WARN_ON(qc->n_elem > 1);
 
         VPRINTK("unmapping %u sg elements\n", qc->n_elem);
 
@@ -2613,7 +2613,7 @@ static void ata_sg_clean(struct ata_queued_cmd *qc)
                         kunmap_atomic(addr, KM_IRQ0);
                 }
         } else {
-                if (sg_dma_len(&sg[0]) > 0)
+                if (qc->n_elem)
                         dma_unmap_single(ap->host_set->dev,
                                 sg_dma_address(&sg[0]), sg_dma_len(&sg[0]),
                                 dir);
@@ -2646,7 +2646,7 @@ static void ata_fill_sg(struct ata_queued_cmd *qc)
         unsigned int idx;
 
         WARN_ON(qc->__sg == NULL);
-        WARN_ON(qc->n_elem == 0);
+        WARN_ON(qc->n_elem == 0 && qc->pad_len == 0);
 
         idx = 0;
         ata_for_each_sg(sg, qc) {
@@ -2791,6 +2791,7 @@ static int ata_sg_setup_one(struct ata_queued_cmd *qc)
         int dir = qc->dma_dir;
         struct scatterlist *sg = qc->__sg;
         dma_addr_t dma_address;
+        int trim_sg = 0;
 
         /* we must lengthen transfers to end on a 32-bit boundary */
         qc->pad_len = sg->length & 3;
@@ -2810,13 +2811,15 @@ static int ata_sg_setup_one(struct ata_queued_cmd *qc)
                 sg_dma_len(psg) = ATA_DMA_PAD_SZ;
                 /* trim sg */
                 sg->length -= qc->pad_len;
+                if (sg->length == 0)
+                        trim_sg = 1;
 
                 DPRINTK("padding done, sg->length=%u pad_len=%u\n",
                         sg->length, qc->pad_len);
         }
 
-        if (!sg->length) {
-                sg_dma_address(sg) = 0;
+        if (trim_sg) {
+                qc->n_elem--;
                 goto skip_map;
         }
 
@@ -2829,9 +2832,9 @@ 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;
 
+skip_map:
         DPRINTK("mapped buffer of %d bytes for %s\n", sg_dma_len(sg),
                 qc->tf.flags & ATA_TFLAG_WRITE ? "write" : "read");
 
diff --git a/drivers/scsi/sata_qstor.c b/drivers/scsi/sata_qstor.c
index 286482630be3..9602f43a298e 100644
--- a/drivers/scsi/sata_qstor.c
+++ b/drivers/scsi/sata_qstor.c
@@ -277,7 +277,7 @@ static unsigned int qs_fill_sg(struct ata_queued_cmd *qc)
         u8 *prd = pp->pkt + QS_CPB_BYTES;
 
         WARN_ON(qc->__sg == NULL);
-        WARN_ON(qc->n_elem == 0);
+        WARN_ON(qc->n_elem == 0 && qc->pad_len == 0);
 
         nelem = 0;
         ata_for_each_sg(sg, qc) {
diff --git a/include/linux/libata.h b/include/linux/libata.h
index 0d6bf50ad029..66b6847225df 100644
--- a/include/linux/libata.h
+++ b/include/linux/libata.h
@@ -616,17 +616,29 @@ ata_sg_is_last(struct scatterlist *sg, struct ata_queued_cmd *qc)
 }
 
 static inline struct scatterlist *
+ata_qc_first_sg(struct ata_queued_cmd *qc)
+{
+        if (qc->n_elem)
+                return qc->__sg;
+        if (qc->pad_len)
+                return &qc->pad_sgent;
+        return NULL;
+}
+
+static inline struct scatterlist *
 ata_qc_next_sg(struct scatterlist *sg, struct ata_queued_cmd *qc)
 {
         if (sg == &qc->pad_sgent)
                 return NULL;
         if (++sg - qc->__sg < qc->n_elem)
                 return sg;
-        return qc->pad_len ? &qc->pad_sgent : NULL;
+        if (qc->pad_len)
+                return &qc->pad_sgent;
+        return NULL;
 }
 
 #define ata_for_each_sg(sg, qc) \
-        for (sg = qc->__sg; sg; sg = ata_qc_next_sg(sg, qc))
+        for (sg = ata_qc_first_sg(qc); sg; sg = ata_qc_next_sg(sg, qc))
 
 static inline unsigned int ata_tag_valid(unsigned int tag)
 {