lib/scatterlist: Provide a DMA page iterator

Commit 2db76d7c3c6d ("lib/scatterlist: sg_page_iter: support sg lists w/o
backing pages") introduced the sg_page_iter_dma_address() function without
providing a way to use it in the general case. If the sg_dma_len() is not
equal to the sg length callers cannot safely use the
for_each_sg_page/sg_page_iter_dma_address combination.

Resolve this API mistake by providing a DMA specific iterator,
for_each_sg_dma_page(), that uses the right length so
sg_page_iter_dma_address() works as expected with all sglists.

A new iterator type is introduced to provide compile-time safety against
wrongly mixing accessors and iterators.

Acked-by: Christoph Hellwig <hch@lst.de> (for scatterlist)
Acked-by: Thomas Hellstrom <thellstrom@vmware.com>
Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com> (ipu3-cio2)
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

1 files changed, 40 insertions, 9 deletions

diff --git a/include/linux/scatterlist.h b/include/linux/scatterlist.h
index b96f0d0b5b8f..b4be960c7e5d 100644
--- a/include/linux/scatterlist.h
+++ b/include/linux/scatterlist.h
@@ -339,12 +339,12 @@ int sg_alloc_table_chained(struct sg_table *table, int nents,
 /*
  * sg page iterator
  *
- * Iterates over sg entries page-by-page.  On each successful iteration,
- * you can call sg_page_iter_page(@piter) and sg_page_iter_dma_address(@piter)
- * to get the current page and its dma address. @piter->sg will point to the
- * sg holding this page and @piter->sg_pgoffset to the page's page offset
- * within the sg. The iteration will stop either when a maximum number of sg
- * entries was reached or a terminating sg (sg_last(sg) == true) was reached.
+ * Iterates over sg entries page-by-page.  On each successful iteration, you
+ * can call sg_page_iter_page(@piter) to get the current page and its dma
+ * address. @piter->sg will point to the sg holding this page and
+ * @piter->sg_pgoffset to the page's page offset within the sg. The iteration
+ * will stop either when a maximum number of sg entries was reached or a
+ * terminating sg (sg_last(sg) == true) was reached.
  */
 struct sg_page_iter {
         struct scatterlist      *sg;            /* sg holding the page */
@@ -356,7 +356,19 @@ struct sg_page_iter {
                                                  * next step */
 };
 
+/*
+ * sg page iterator for DMA addresses
+ *
+ * This is the same as sg_page_iter however you can call
+ * sg_page_iter_dma_address(@dma_iter) to get the page's DMA
+ * address. sg_page_iter_page() cannot be called on this iterator.
+ */
+struct sg_dma_page_iter {
+        struct sg_page_iter base;
+};
+
 bool __sg_page_iter_next(struct sg_page_iter *piter);
+bool __sg_page_iter_dma_next(struct sg_dma_page_iter *dma_iter);
 void __sg_page_iter_start(struct sg_page_iter *piter,
                           struct scatterlist *sglist, unsigned int nents,
                           unsigned long pgoffset);
@@ -372,11 +384,13 @@ static inline struct page *sg_page_iter_page(struct sg_page_iter *piter)
 /**
  * sg_page_iter_dma_address - get the dma address of the current page held by
  * the page iterator.
- * @piter:      page iterator holding the page
+ * @dma_iter:   page iterator holding the page
  */
-static inline dma_addr_t sg_page_iter_dma_address(struct sg_page_iter *piter)
+static inline dma_addr_t
+sg_page_iter_dma_address(struct sg_dma_page_iter *dma_iter)
 {
-        return sg_dma_address(piter->sg) + (piter->sg_pgoffset << PAGE_SHIFT);
+        return sg_dma_address(dma_iter->base.sg) +
+               (dma_iter->base.sg_pgoffset << PAGE_SHIFT);
 }
 
 /**
@@ -385,11 +399,28 @@ static inline dma_addr_t sg_page_iter_dma_address(struct sg_page_iter *piter)
  * @piter:      page iterator to hold current page, sg, sg_pgoffset
  * @nents:      maximum number of sg entries to iterate over
  * @pgoffset:   starting page offset
+ *
+ * Callers may use sg_page_iter_page() to get each page pointer.
  */
 #define for_each_sg_page(sglist, piter, nents, pgoffset)                   \
         for (__sg_page_iter_start((piter), (sglist), (nents), (pgoffset)); \
              __sg_page_iter_next(piter);)
 
+/**
+ * for_each_sg_dma_page - iterate over the pages of the given sg list
+ * @sglist:     sglist to iterate over
+ * @dma_iter:   page iterator to hold current page
+ * @dma_nents:  maximum number of sg entries to iterate over, this is the value
+ *              returned from dma_map_sg
+ * @pgoffset:   starting page offset
+ *
+ * Callers may use sg_page_iter_dma_address() to get each page's DMA address.
+ */
+#define for_each_sg_dma_page(sglist, dma_iter, dma_nents, pgoffset)            \
+        for (__sg_page_iter_start(&(dma_iter)->base, sglist, dma_nents,        \
+                                  pgoffset);                                   \
+             __sg_page_iter_dma_next(dma_iter);)
+
 /*
  * Mapping sg iterator
  *