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SubjectRe: [v4,1/3] drm/prime: use dma length macro when mapping sg
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Hi All,

On 2020-03-25 10:07, Shane Francis wrote:
> As dma_map_sg can reorganize scatter-gather lists in a
> way that can cause some later segments to be empty we should
> always use the sg_dma_len macro to fetch the actual length.
>
> This could now be 0 and not need to be mapped to a page or
> address array
>
> Signed-off-by: Shane Francis <bigbeeshane@gmail.com>
> Reviewed-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
This patch landed in linux-next 20200326 and it causes a kernel panic on
various Exynos SoC based boards.
> ---
> drivers/gpu/drm/drm_prime.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/drm_prime.c b/drivers/gpu/drm/drm_prime.c
> index 86d9b0e45c8c..1de2cde2277c 100644
> --- a/drivers/gpu/drm/drm_prime.c
> +++ b/drivers/gpu/drm/drm_prime.c
> @@ -967,7 +967,7 @@ int drm_prime_sg_to_page_addr_arrays(struct sg_table *sgt, struct page **pages,
>
> index = 0;
> for_each_sg(sgt->sgl, sg, sgt->nents, count) {
> - len = sg->length;
> + len = sg_dma_len(sg);
> page = sg_page(sg);
> addr = sg_dma_address(sg);
>

Sorry, but this code is wrong :(

The scatterlist elements (sg) describes memory chunks in physical memory
and in the DMA (IO virtual) space. However in general, you cannot assume
1:1 mapping between them. If you access sg_page(sg) (basically
sg->page), you must match it with sg->length. When you access
sg_dma_address(sg) (again, in most cases it is sg->dma_address), then
you must match it with sg_dma_len(sg). The sg->dma_address might not be
the dma address of the sg->page.

In some cases (when IOMMU is available, it performs aggregation of the
scatterlist chunks and a few other, minor requirements), the whole
scatterlist might be mapped into contiguous DMA address space and filled
only to the first sg element.

The proper way to iterate over a scatterlists to get both the pages and
the DMA addresses assigned to them is:

int drm_prime_sg_to_page_addr_arrays(struct sg_table *sgt, struct page
**pages,
                                     dma_addr_t *addrs, int max_entries)
{
        unsigned count;
        struct scatterlist *sg;
        struct page *page;
        u32 page_len, page_index;
        dma_addr_t addr;
        u32 dma_len, dma_index;

        page_index = 0;
        dma_index = 0;
        for_each_sg(sgt->sgl, sg, sgt->nents, count) {
                page_len = sg->length;
                page = sg_page(sg);
                dma_len = sg_dma_len(sg);
                addr = sg_dma_address(sg);

                while (pages && page_len > 0) {
                        if (WARN_ON(page_index >= max_entries))
                                return -1;
                        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;
                        dma_len -= PAGE_SIZE;
                        dma_index++;
                }
        }

        return 0;
}

I will send a patch in a few minutes with the above fixed code.

Best regards
--
Marek Szyprowski, PhD
Samsung R&D Institute Poland

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