3 files changed, 78 insertions, 1 deletions

diff --git a/block/elevator.c b/block/elevator.c
index f9736fbdab..8cd5775acd 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -741,7 +741,21 @@ struct request *elv_next_request(struct request_queue *q)
                         q->boundary_rq = NULL;
                 }
 
-                if ((rq->cmd_flags & REQ_DONTPREP) || !q->prep_rq_fn)
+                if (rq->cmd_flags & REQ_DONTPREP)
+                        break;
+
+                if (q->dma_drain_size && rq->data_len) {
+                        /*
+                         * make sure space for the drain appears we
+                         * know we can do this because max_hw_segments
+                         * has been adjusted to be one fewer than the
+                         * device can handle
+                         */
+                        rq->nr_phys_segments++;
+                        rq->nr_hw_segments++;
+                }
+
+                if (!q->prep_rq_fn)
                         break;
 
                 ret = q->prep_rq_fn(q, rq);
@@ -754,6 +768,16 @@ struct request *elv_next_request(struct request_queue *q)
                          * avoid resource deadlock.  REQ_STARTED will
                          * prevent other fs requests from passing this one.
                          */
+                        if (q->dma_drain_size && rq->data_len &&
+                            !(rq->cmd_flags & REQ_DONTPREP)) {
+                                /*
+                                 * remove the space for the drain we added
+                                 * so that we don't add it again
+                                 */
+                                --rq->nr_phys_segments;
+                                --rq->nr_hw_segments;
+                        }
+
                         rq = NULL;
                         break;
                 } else if (ret == BLKPREP_KILL) {
diff --git a/block/ll_rw_blk.c b/block/ll_rw_blk.c
index 3d0422f484..768987dc26 100644
--- a/block/ll_rw_blk.c
+++ b/block/ll_rw_blk.c
@@ -726,6 +726,45 @@ void blk_queue_stack_limits(struct request_queue *t, struct request_queue *b)
 EXPORT_SYMBOL(blk_queue_stack_limits);
 
 /**
+ * blk_queue_dma_drain - Set up a drain buffer for excess dma.
+ *
+ * @q:  the request queue for the device
+ * @buf:        physically contiguous buffer
+ * @size:       size of the buffer in bytes
+ *
+ * Some devices have excess DMA problems and can't simply discard (or
+ * zero fill) the unwanted piece of the transfer.  They have to have a
+ * real area of memory to transfer it into.  The use case for this is
+ * ATAPI devices in DMA mode.  If the packet command causes a transfer
+ * bigger than the transfer size some HBAs will lock up if there
+ * aren't DMA elements to contain the excess transfer.  What this API
+ * does is adjust the queue so that the buf is always appended
+ * silently to the scatterlist.
+ *
+ * Note: This routine adjusts max_hw_segments to make room for
+ * appending the drain buffer.  If you call
+ * blk_queue_max_hw_segments() or blk_queue_max_phys_segments() after
+ * calling this routine, you must set the limit to one fewer than your
+ * device can support otherwise there won't be room for the drain
+ * buffer.
+ */
+int blk_queue_dma_drain(struct request_queue *q, void *buf,
+                                unsigned int size)
+{
+        if (q->max_hw_segments < 2 || q->max_phys_segments < 2)
+                return -EINVAL;
+        /* make room for appending the drain */
+        --q->max_hw_segments;
+        --q->max_phys_segments;
+        q->dma_drain_buffer = buf;
+        q->dma_drain_size = size;
+
+        return 0;
+}
+
+EXPORT_SYMBOL_GPL(blk_queue_dma_drain);
+
+/**
  * blk_queue_segment_boundary - set boundary rules for segment merging
  * @q:  the request queue for the device
  * @mask:  the memory boundary mask
@@ -1379,6 +1418,16 @@ new_segment:
                 bvprv = bvec;
         } /* segments in rq */
 
+        if (q->dma_drain_size) {
+                sg->page_link &= ~0x02;
+                sg = sg_next(sg);
+                sg_set_page(sg, virt_to_page(q->dma_drain_buffer),
+                            q->dma_drain_size,
+                            ((unsigned long)q->dma_drain_buffer) &
+                            (PAGE_SIZE - 1));
+                nsegs++;
+        }
+
         if (sg)
                 sg_mark_end(sg);
 
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index c7a3ab575c..e542c8fd92 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -429,6 +429,8 @@ struct request_queue
         unsigned int            max_segment_size;
 
         unsigned long           seg_boundary_mask;
+        void                    *dma_drain_buffer;
+        unsigned int            dma_drain_size;
         unsigned int            dma_alignment;
 
         struct blk_queue_tag    *queue_tags;
@@ -760,6 +762,8 @@ extern void blk_queue_max_hw_segments(struct request_queue *, unsigned short);
 extern void blk_queue_max_segment_size(struct request_queue *, unsigned int);
 extern void blk_queue_hardsect_size(struct request_queue *, unsigned short);
 extern void blk_queue_stack_limits(struct request_queue *t, struct request_queue *b);
+extern int blk_queue_dma_drain(struct request_queue *q, void *buf,
+                               unsigned int size);
 extern void blk_queue_segment_boundary(struct request_queue *, unsigned long);
 extern void blk_queue_prep_rq(struct request_queue *, prep_rq_fn *pfn);
 extern void blk_queue_merge_bvec(struct request_queue *, merge_bvec_fn *);