scatterlist: add sg_alloc_table_from_pages function

This patch adds a new constructor for an sg table. The table is constructed from an array of struct pages. All contiguous chunks of the pages are merged into a single sg nodes. A user may provide an offset and a size of a buffer if the buffer is not page-aligned.

The function is dedicated for DMABUF exporters which often perform conversion from an page array to a scatterlist. Moreover the scatterlist should be squashed in order to save memory and to speed-up the process of DMA mapping using dma_map_sg.

The code is based on the patch 'v4l: vb2-dma-contig: add support for scatterlist in userptr mode' and hints from Laurent Pinchart.

---
v4:
- fix typos
- add Changelog

v3:
- use PFNs instead of page pointers to check contiguity

v2:
- rename sg_table_alloc_by_pages to sg_table_alloc_from_pages
- add some comments about error value

v1:
- initial version

---
 include/linux/scatterlist.h |    4 +++
 lib/scatterlist.c           |   64 +++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 68 insertions(+)

index ac9586d..7b600da 100644
@@ -214,6 +214,10 @@ void sg_free_table(struct sg_table *);
 int __sg_alloc_table(struct sg_table *, unsigned int, unsigned int, gfp_t,
 		     sg_alloc_fn *);
 int sg_alloc_table(struct sg_table *, unsigned int, gfp_t);
+int sg_alloc_table_from_pages(struct sg_table *sgt,
+	struct page **pages, unsigned int n_pages,
+	unsigned long offset, unsigned long size,
+	gfp_t gfp_mask);
 
 size_t sg_copy_from_buffer(struct scatterlist *sgl, unsigned int nents,
 			   void *buf, size_t buflen);

1.7.9.5