Re: [PATCH v2] drm/prime: fix extracting of the DMA addresses from a scatterlist

[Alex Deucher <alexdeucher@gmail.com>]

On Fri, Mar 27, 2020 at 12:23 PM Marek Szyprowski
<m.szyprowski@samsung.com> wrote:
>
> Scatterlist elements contains both pages and DMA addresses, but one
> should not assume 1:1 relation between them. The sg->length is the size
> of the physical memory chunk described by the sg->page, while
> sg_dma_len(sg) is the size of the DMA (IO virtual) chunk described by
> the sg_dma_address(sg).
>
> The proper way of extracting both: pages and DMA addresses of the whole
> buffer described by a scatterlist it to iterate independently over the
> sg->pages/sg->length and sg_dma_address(sg)/sg_dma_len(sg) entries.
>
> Fixes: 42e67b479eab ("drm/prime: use dma length macro when mapping sg")
> Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
> Reviewed-by: Alex Deucher <alexander.deucher@amd.com>

Applied.  Thanks and sorry for the breakage.

Alex

> ---
>  drivers/gpu/drm/drm_prime.c | 37 +++++++++++++++++++++++++------------
>  1 file changed, 25 insertions(+), 12 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_prime.c b/drivers/gpu/drm/drm_prime.c
> index 1de2cde2277c..282774e469ac 100644
> --- a/drivers/gpu/drm/drm_prime.c
> +++ b/drivers/gpu/drm/drm_prime.c
> @@ -962,27 +962,40 @@ int drm_prime_sg_to_page_addr_arrays(struct sg_table *sgt, struct page **pages,
>         unsigned count;
>         struct scatterlist *sg;
>         struct page *page;
> -       u32 len, index;
> +       u32 page_len, page_index;
>         dma_addr_t addr;
> +       u32 dma_len, dma_index;
>
> -       index = 0;
> +       /*
> +        * Scatterlist elements contains both pages and DMA addresses, but
> +        * one shoud not assume 1:1 relation between them. The sg->length is
> +        * the size of the physical memory chunk described by the sg->page,
> +        * while sg_dma_len(sg) is the size of the DMA (IO virtual) chunk
> +        * described by the sg_dma_address(sg).
> +        */
> +       page_index = 0;
> +       dma_index = 0;
>         for_each_sg(sgt->sgl, sg, sgt->nents, count) {
> -               len = sg_dma_len(sg);
> +               page_len = sg->length;
>                 page = sg_page(sg);
> +               dma_len = sg_dma_len(sg);
>                 addr = sg_dma_address(sg);
>
> -               while (len > 0) {
> -                       if (WARN_ON(index >= max_entries))
> +               while (pages && page_len > 0) {
> +                       if (WARN_ON(page_index >= max_entries))
>                                 return -1;
> -                       if (pages)
> -                               pages[index] = page;
> -                       if (addrs)
> -                               addrs[index] = addr;
> -
> +                       pages[page_index] = page;
>                         page++;
> +                       page_len -= PAGE_SIZE;
> +                       page_index++;
> +               }
> +               while (addrs && dma_len > 0) {
> +                       if (WARN_ON(dma_index >= max_entries))
> +                               return -1;
> +                       addrs[dma_index] = addr;
>                         addr += PAGE_SIZE;
> -                       len -= PAGE_SIZE;
> -                       index++;
> +                       dma_len -= PAGE_SIZE;
> +                       dma_index++;
>                 }
>         }
>         return 0;
> --
> 2.17.1
>
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