scatterlist: add generic wrappers for iterating over sgtable objects

struct sg_table is a common structure used for describing a memory
buffer. It consists of a scatterlist with memory pages and DMA addresses
(sgl entry), as well as the number of scatterlist entries: CPU pages
(orig_nents entry) and DMA mapped pages (nents entry).

It turned out that it was a common mistake to misuse nents and orig_nents
entries, calling the scatterlist iterating functions with a wrong number
of the entries.

To avoid such issues, lets introduce a common wrappers operating directly
on the struct sg_table objects, which take care of the proper use of
the nents and orig_nents entries.

While touching this, lets clarify some ambiguities in the comments for
the existing for_each helpers.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>

include/linux/scatterlist.h

@@ -151,6 +151,20 @@ static inline void sg_set_buf(struct scatterlist *sg, const void *buf,
 #define for_each_sg(sglist, sg, nr, __i)	\
 	for (__i = 0, sg = (sglist); __i < (nr); __i++, sg = sg_next(sg))
 
+/*
+ * Loop over each sg element in the given sg_table object.
+ */
+#define for_each_sgtable_sg(sgt, sg, i)		\
+	for_each_sg(sgt->sgl, sg, sgt->orig_nents, i)
+
+/*
+ * Loop over each sg element in the given *DMA mapped* sg_table object.
+ * Please use sg_dma_address(sg) and sg_dma_len(sg) to extract DMA addresses
+ * of the each element.
+ */
+#define for_each_sgtable_dma_sg(sgt, sg, i)	\
+	for_each_sg(sgt->sgl, sg, sgt->nents, i)
+
 /**
  * sg_chain - Chain two sglists together
  * @prv:	First scatterlist
@@ -401,9 +415,10 @@ sg_page_iter_dma_address(struct sg_dma_page_iter *dma_iter)
  * @sglist:	sglist to iterate over
  * @piter:	page iterator to hold current page, sg, sg_pgoffset
  * @nents:	maximum number of sg entries to iterate over
- * @pgoffset:	starting page offset
+ * @pgoffset:	starting page offset (in pages)
  *
  * Callers may use sg_page_iter_page() to get each page pointer.
+ * In each loop it operates on PAGE_SIZE unit.
  */
 #define for_each_sg_page(sglist, piter, nents, pgoffset)		   \
 	for (__sg_page_iter_start((piter), (sglist), (nents), (pgoffset)); \
@@ -412,18 +427,47 @@ sg_page_iter_dma_address(struct sg_dma_page_iter *dma_iter)
 /**
  * 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_iter:	DMA 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
+ * @pgoffset:	starting page offset (in pages)
  *
  * Callers may use sg_page_iter_dma_address() to get each page's DMA address.
+ * In each loop it operates on PAGE_SIZE unit.
  */
 #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);)
 
+/**
+ * for_each_sgtable_page - iterate over all pages in the sg_table object
+ * @sgt:	sg_table object to iterate over
+ * @piter:	page iterator to hold current page
+ * @pgoffset:	starting page offset (in pages)
+ *
+ * Iterates over the all memory pages in the buffer described by
+ * a scatterlist stored in the given sg_table object.
+ * See also for_each_sg_page(). In each loop it operates on PAGE_SIZE unit.
+ */
+#define for_each_sgtable_page(sgt, piter, pgoffset)	\
+	for_each_sg_page(sgt->sgl, piter, sgt->orig_nents, pgoffset)
+
+/**
+ * for_each_sgtable_dma_page - iterate over the DMA mapped sg_table object
+ * @sgt:	sg_table object to iterate over
+ * @dma_iter:	DMA page iterator to hold current page
+ * @pgoffset:	starting page offset (in pages)
+ *
+ * Iterates over the all DMA mapped pages in the buffer described by
+ * a scatterlist stored in the given sg_table object.
+ * See also for_each_sg_dma_page(). In each loop it operates on PAGE_SIZE
+ * unit.
+ */
+#define for_each_sgtable_dma_page(sgt, dma_iter, pgoffset)	\
+	for_each_sg_dma_page(sgt->sgl, dma_iter, sgt->nents, pgoffset)
+
+
 /*
  * Mapping sg iterator
  *