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.
Signed-off-by: Tomasz Stanislawski <t.stanislaws@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
CC: Andrew Morton <akpm@linux-foundation.org>

include/linux/scatterlist.h

@@ -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);

lib/scatterlist.c

@@ -318,6 +318,70 @@ int sg_alloc_table(struct sg_table *table, unsigned int nents, gfp_t gfp_mask)
 }
 EXPORT_SYMBOL(sg_alloc_table);
 
+/**
+ * sg_alloc_table_from_pages - Allocate and initialize an sg table from
+ *			       an array of pages
+ * @sgt:	The sg table header to use
+ * @pages:	Pointer to an array of page pointers
+ * @n_pages:	Number of pages in the pages array
+ * @offset:     Offset from start of the first page to the start of a buffer
+ * @size:       Number of valid bytes in the buffer (after offset)
+ * @gfp_mask:	GFP allocation mask
+ *
+ *  Description:
+ *    Allocate and initialize an sg table from a list of pages. Contiguous
+ *    ranges of the pages are squashed into a single scatterlist node. A user
+ *    may provide an offset at a start and a size of valid data in a buffer
+ *    specified by the page array. The returned sg table is released by
+ *    sg_free_table.
+ *
+ * Returns:
+ *   0 on success, negative error on failure
+ */
+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)
+{
+	unsigned int chunks;
+	unsigned int i;
+	unsigned int cur_page;
+	int ret;
+	struct scatterlist *s;
+
+	/* compute number of contiguous chunks */
+	chunks = 1;
+	for (i = 1; i < n_pages; ++i)
+		if (page_to_pfn(pages[i]) != page_to_pfn(pages[i - 1]) + 1)
+			++chunks;
+
+	ret = sg_alloc_table(sgt, chunks, gfp_mask);
+	if (unlikely(ret))
+		return ret;
+
+	/* merging chunks and putting them into the scatterlist */
+	cur_page = 0;
+	for_each_sg(sgt->sgl, s, sgt->orig_nents, i) {
+		unsigned long chunk_size;
+		unsigned int j;
+
+		/* look for the end of the current chunk */
+		for (j = cur_page + 1; j < n_pages; ++j)
+			if (page_to_pfn(pages[j]) !=
+			    page_to_pfn(pages[j - 1]) + 1)
+				break;
+
+		chunk_size = ((j - cur_page) << PAGE_SHIFT) - offset;
+		sg_set_page(s, pages[cur_page], min(size, chunk_size), offset);
+		size -= chunk_size;
+		offset = 0;
+		cur_page = j;
+	}
+
+	return 0;
+}
+EXPORT_SYMBOL(sg_alloc_table_from_pages);
+
 /**
  * sg_miter_start - start mapping iteration over a sg list
  * @miter: sg mapping iter to be started