lib/scatterlist: add simple page iterator

Add an iterator to walk through a scatter list a page at a time starting
at a specific page offset.  As opposed to the mapping iterator this is
meant to be small, performing well even in simple loops like collecting
all pages on the scatterlist into an array or setting up an iommu table
based on the pages' DMA address.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Cc: Maxim Levitsky <maximlevitsky@gmail.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Tested-by: Stephen Warren <swarren@wwwdotorg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 38 insertions, 0 deletions

diff --git a/lib/scatterlist.c b/lib/scatterlist.c
index 7874b01e816e..a1d15647d7db 100644
--- a/lib/scatterlist.c
+++ b/lib/scatterlist.c
@@ -394,6 +394,44 @@ int sg_alloc_table_from_pages(struct sg_table *sgt,
 }
 EXPORT_SYMBOL(sg_alloc_table_from_pages);
 
+void __sg_page_iter_start(struct sg_page_iter *piter,
+                          struct scatterlist *sglist, unsigned int nents,
+                          unsigned long pgoffset)
+{
+        piter->__pg_advance = 0;
+        piter->__nents = nents;
+
+        piter->page = NULL;
+        piter->sg = sglist;
+        piter->sg_pgoffset = pgoffset;
+}
+EXPORT_SYMBOL(__sg_page_iter_start);
+
+static int sg_page_count(struct scatterlist *sg)
+{
+        return PAGE_ALIGN(sg->offset + sg->length) >> PAGE_SHIFT;
+}
+
+bool __sg_page_iter_next(struct sg_page_iter *piter)
+{
+        if (!piter->__nents || !piter->sg)
+                return false;
+
+        piter->sg_pgoffset += piter->__pg_advance;
+        piter->__pg_advance = 1;
+
+        while (piter->sg_pgoffset >= sg_page_count(piter->sg)) {
+                piter->sg_pgoffset -= sg_page_count(piter->sg);
+                piter->sg = sg_next(piter->sg);
+                if (!--piter->__nents || !piter->sg)
+                        return false;
+        }
+        piter->page = nth_page(sg_page(piter->sg), piter->sg_pgoffset);
+
+        return true;
+}
+EXPORT_SYMBOL(__sg_page_iter_next);
+
 /**
  * sg_miter_start - start mapping iteration over a sg list
  * @miter: sg mapping iter to be started