On 2020-03-27 08:54, Marek Szyprowski wrote:
> 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@xxxxxxxxx>
>> Reviewed-by: Michael J. Ruhl <michael.j.ruhl@xxxxxxxxx>
> 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.
Here is the fix: https://patchwork.freedesktop.org/patch/359081/
Best regards
--
Marek Szyprowski, PhD
Samsung R&D Institute Poland
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