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SubjectRe: [PATCH v2] scatterlist: add sg_alloc_table_from_pages function
On Tue, Apr 24, 2012 at 02:58:56PM +0200, Tomasz Stanislawski 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.
>
> Signed-off-by: Tomasz Stanislawski <t.stanislaws@samsung.com>
> Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>

I like this and we could replace the drm helper function in drm prime with
this one. One comment below.
-Daniel

> ---
> include/linux/scatterlist.h | 4 +++
> lib/scatterlist.c | 63 +++++++++++++++++++++++++++++++++++++++++++
> 2 files changed, 67 insertions(+), 0 deletions(-)
>
> diff --git a/include/linux/scatterlist.h b/include/linux/scatterlist.h
> index ac9586d..7b600da 100644
> --- a/include/linux/scatterlist.h
> +++ b/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);
> diff --git a/lib/scatterlist.c b/lib/scatterlist.c
> index 6096e89..90f9265 100644
> --- a/lib/scatterlist.c
> +++ b/lib/scatterlist.c
> @@ -319,6 +319,69 @@ 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 a start of the first page to a 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
> + * 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 (pages[i] != pages[i - 1] + 1)
> + ++chunks;

I'm not aware of the details (and didn't bother to do a full audit), but
I guess this could blow up on funky discontig-mem setups. Imo would be
safer to compare the pfn or phys address. And also slightly more readable.
Imo sg table creation is not a fastpath, the exporter should cache these
anyway, so we can waste some cpu cycles here.

> + 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;
> +
> + /* looking for the end of the current chunk */
> + for (j = cur_page + 1; j < n_pages; ++j)
> + if (pages[j] != 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
> * @sgl: sg list to iterate over
> --
> 1.7.5.4
>
> _______________________________________________
> dri-devel mailing list
> dri-devel@lists.freedesktop.org
> http://lists.freedesktop.org/mailman/listinfo/dri-devel

--
Daniel Vetter
Mail: daniel@ffwll.ch
Mobile: +41 (0)79 365 57 48


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