On Tue, 08 May 2012 11:50:33 +0200
Tomasz Stanislawski <t.stanislaws@xxxxxxxxxxx> 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);
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