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