linux-kernel - Re: [v4,1/3] drm/prime: use dma length macro when mapping sg

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Message-ID: <4aef60ff-d9e4-d3d0-1a28-8c2dc3b94271@samsung.com>
Date:   Fri, 27 Mar 2020 08:54:53 +0100
From:   Marek Szyprowski <m.szyprowski@...sung.com>
To:     Shane Francis <bigbeeshane@...il.com>,
        dri-devel@...ts.freedesktop.org
Cc:     airlied@...ux.ie, linux-kernel@...r.kernel.org,
        amd-gfx-request@...ts.freedesktop.org, alexander.deucher@....com,
        christian.koenig@....com
Subject: Re: [v4,1/3] drm/prime: use dma length macro when mapping sg

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@...il.com>
> Reviewed-by: Michael J. Ruhl <michael.j.ruhl@...el.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