[PATCH] libata: make ata_sg_setup_one() trim zero length sg

This patch makes ata_sg_setup_one() trim sg entry (thus making
qc->n_elem zero) if padding results in zero length sg entry.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jgarzik@pobox.com>

1 files changed, 8 insertions, 5 deletions

diff --git a/drivers/scsi/libata-core.c b/drivers/scsi/libata-core.c
index bbac87a13d57..5f1d7580218d 100644
--- a/drivers/scsi/libata-core.c
+++ b/drivers/scsi/libata-core.c
@@ -2514,7 +2514,7 @@ static void ata_sg_clean(struct ata_queued_cmd *qc)
         assert(sg != NULL);
 
         if (qc->flags & ATA_QCFLAG_SINGLE)
-                assert(qc->n_elem == 1);
+                assert(qc->n_elem <= 1);
 
         VPRINTK("unmapping %u sg elements\n", qc->n_elem);
 
@@ -2537,7 +2537,7 @@ 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);
@@ -2715,6 +2715,7 @@ 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;
@@ -2734,13 +2735,15 @@ 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;
         }
 
@@ -2753,9 +2756,9 @@ 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");