libata: convert to chained sg

libata used private sg iterator to handle padding sg.  Now that sg can
be chained, padding can be handled using standard sg ops.  Convert to
chained sg.

* s/qc->__sg/qc->sg/

* s/qc->pad_sgent/qc->extra_sg[]/.  Because chaining consumes one sg
  entry.  There need to be two extra sg entries.  The renaming is also
  for future addition of other extra sg entries.

* Padding setup is moved into ata_sg_setup_extra() which is organized
  in a way that future addition of other extra sg entries is easy.

* qc->orig_n_elem is unused and removed.

* qc->n_elem now contains the number of sg entries that LLDs should
  map.  qc->mapped_n_elem is added to carry the original number of
  mapped sgs for unmapping.

* The last sg of the original sg list is used to chain to extra sg
  list.  The original last sg is pointed to by qc->last_sg and the
  content is stored in qc->saved_last_sg.  It's restored during
  ata_sg_clean().

* All sg walking code has been updated.  Unnecessary assertions and
  checks for conditions the core layer already guarantees are removed.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Cc: Jens Axboe <jens.axboe@oracle.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>

1 files changed, 8 insertions, 34 deletions

diff --git a/include/linux/libata.h b/include/linux/libata.h
index acd90ad78417..162f8b5509ac 100644
--- a/include/linux/libata.h
+++ b/include/linux/libata.h
@@ -458,7 +458,7 @@ struct ata_queued_cmd {
         unsigned int            tag;
         unsigned int            n_elem;
         unsigned int            n_iter;
-        unsigned int            orig_n_elem;
+        unsigned int            mapped_n_elem;
 
         int                     dma_dir;
 
@@ -471,11 +471,12 @@ struct ata_queued_cmd {
         struct scatterlist      *cursg;
         unsigned int            cursg_ofs;
 
+        struct scatterlist      *last_sg;
+        struct scatterlist      saved_last_sg;
         struct scatterlist      sgent;
-        struct scatterlist      pad_sgent;
+        struct scatterlist      extra_sg[2];
 
-        /* DO NOT iterate over __sg manually, use ata_for_each_sg() */
-        struct scatterlist      *__sg;
+        struct scatterlist      *sg;
 
         unsigned int            err_mask;
         struct ata_taskfile     result_tf;
@@ -1123,35 +1124,6 @@ extern void ata_port_pbar_desc(struct ata_port *ap, int bar, ssize_t offset,
                                const char *name);
 #endif
 
-/*
- * qc helpers
- */
-static inline struct scatterlist *
-ata_qc_first_sg(struct ata_queued_cmd *qc)
-{
-        qc->n_iter = 0;
-        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 (++qc->n_iter < qc->n_elem)
-                return sg_next(sg);
-        if (qc->pad_len)
-                return &qc->pad_sgent;
-        return NULL;
-}
-
-#define ata_for_each_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)
 {
         return (tag < ATA_MAX_QUEUE) ? 1 : 0;
@@ -1386,15 +1358,17 @@ static inline void ata_tf_init(struct ata_device *dev, struct ata_taskfile *tf)
 static inline void ata_qc_reinit(struct ata_queued_cmd *qc)
 {
         qc->dma_dir = DMA_NONE;
-        qc->__sg = NULL;
+        qc->sg = NULL;
         qc->flags = 0;
         qc->cursg = NULL;
         qc->cursg_ofs = 0;
         qc->nbytes = qc->curbytes = 0;
         qc->n_elem = 0;
+        qc->mapped_n_elem = 0;
         qc->n_iter = 0;
         qc->err_mask = 0;
         qc->pad_len = 0;
+        qc->last_sg = NULL;
         qc->sect_size = ATA_SECT_SIZE;
 
         ata_tf_init(qc->dev, &qc->tf);