Re: [PATCH v3] scatterlist: add sg_alloc_table_from_pages function

On Tue, 08 May 2012 11:50:33 +0200 Tomasz Stanislawski t.stanislaws@samsung.com wrote:

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.

...

/**

• • 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. Continuous

s/Continuous/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
• **/

nit: Use */, not **/ here.

+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)

I guess a 32-bit n_pages is OK. A 16TB IO seems enough ;)

+{

• unsigned int chunks;
• unsigned int i;

erk, please choose a different name for this. When a C programmer sees "i", he very much assumes it has type "int". Making it unsigned causes surprise.

And don't rename it to "u"! Let's give it a nice meaningful name. pageno?

• 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)
  

This assumes that if two pages have contiguous pfn's then they are physically contiguous. Is that true for all architectures and memory models, including sparsemem? See sparse_encode_mem_map().

• 	++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;
  

"j" is an "int", too.

• /* looking for the end of the current chunk */
  

s/looking/look/

• 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);