libata: change ATA_QCFLAG_DMAMAP semantics

ATA_QCFLAG_DMAMAP was a bit peculiar in that it got set during qc
initialization and cleared if DMA mapping wasn't necessary.  Make it
more straight forward by making the following changes.

* Don't set it during initialization.  Set it after DMA is actually
  mapped.

* Add BUG_ON() to guarantee that there is data to transfer if DMAMAP
  is set.  This always holds for the current code.  The BUG_ON() is
  for docummentation and sanity check.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>

1 files changed, 7 insertions, 8 deletions

diff --git a/drivers/ata/libata-core.c b/drivers/ata/libata-core.c
index 76360f0ca20d..865428a64de3 100644
--- a/drivers/ata/libata-core.c
+++ b/drivers/ata/libata-core.c
@@ -4475,7 +4475,6 @@ void ata_sg_clean(struct ata_queued_cmd *qc)
         int dir = qc->dma_dir;
         void *pad_buf = NULL;
 
-        WARN_ON(!(qc->flags & ATA_QCFLAG_DMAMAP));
         WARN_ON(sg == NULL);
 
         VPRINTK("unmapping %u sg elements\n", qc->n_elem);
@@ -4762,11 +4761,9 @@ void ata_noop_qc_prep(struct ata_queued_cmd *qc) { }
  *      LOCKING:
  *      spin_lock_irqsave(host lock)
  */
-
 void ata_sg_init(struct ata_queued_cmd *qc, struct scatterlist *sg,
                  unsigned int n_elem)
 {
-        qc->flags |= ATA_QCFLAG_DMAMAP;
         qc->__sg = sg;
         qc->n_elem = n_elem;
         qc->orig_n_elem = n_elem;
@@ -4795,7 +4792,6 @@ static int ata_sg_setup(struct ata_queued_cmd *qc)
         int n_elem, pre_n_elem, dir, trim_sg = 0;
 
         VPRINTK("ENTER, ata%u\n", ap->print_id);
-        WARN_ON(!(qc->flags & ATA_QCFLAG_DMAMAP));
 
         /* we must lengthen transfers to end on a 32-bit boundary */
         qc->pad_len = lsg->length & 3;
@@ -4855,6 +4851,7 @@ static int ata_sg_setup(struct ata_queued_cmd *qc)
 
 skip_map:
         qc->n_elem = n_elem;
+        qc->flags |= ATA_QCFLAG_DMAMAP;
 
         return 0;
 }
@@ -5912,12 +5909,15 @@ void ata_qc_issue(struct ata_queued_cmd *qc)
         qc->flags |= ATA_QCFLAG_ACTIVE;
         ap->qc_active |= 1 << qc->tag;
 
+        /* We guarantee to LLDs that they will have at least one
+         * non-zero sg if the command is a data command.
+         */
+        BUG_ON(ata_is_data(prot) && (!qc->__sg || !qc->n_elem || !qc->nbytes));
+
         if (ata_is_dma(prot) || (ata_is_pio(prot) &&
-                                 (ap->flags & ATA_FLAG_PIO_DMA))) {
+                                 (ap->flags & ATA_FLAG_PIO_DMA)))
                 if (ata_sg_setup(qc))
                         goto sg_err;
-        } else
-                qc->flags &= ATA_QCFLAG_DMAMAP;
 
         /* if device is sleeping, schedule softreset and abort the link */
         if (unlikely(qc->dev->flags & ATA_DFLAG_SLEEPING)) {
@@ -5935,7 +5935,6 @@ void ata_qc_issue(struct ata_queued_cmd *qc)
         return;
 
 sg_err:
-        qc->flags &= ~ATA_QCFLAG_DMAMAP;
         qc->err_mask |= AC_ERR_SYSTEM;
 err:
         ata_qc_complete(qc);