[PATCH v2 1/3] drm/prime: Iterate SG DMA addresses separately

[Christian König]

Am 17.04.2018 um 17:58 schrieb Robin Murphy:
> For dma_map_sg(), DMA API implementations are free to merge consecutive
> segments into a single DMA mapping if conditions are suitable, thus the
> resulting DMA addresses which drm_prime_sg_to_page_addr_arrays()
> iterates may be packed into fewer entries than ttm->sg->nents implies.
>
> The current implementation does not account for this, meaning that its
> callers either have to reject the 0 < count < nents case or risk getting
> bogus DMA addresses beyond the first segment. Fortunately this is quite
> easy to handle without having to rejig structures to also store the
> mapped count, since the total DMA length should still be equal to the
> total buffer length. All we need is a second scatterlist cursor to
> iterate through the DMA addresses independently of the page addresses.
>
> Signed-off-by: Robin Murphy <robin.murphy at arm.com>

Reviewed-by: Christian König <christian.koenig at amd.com> for the whole 
series.

> ---
>
> v2: Remember to iterate dma_len correctly as well.
>
>   drivers/gpu/drm/drm_prime.c | 15 ++++++++++++---
>   1 file changed, 12 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_prime.c b/drivers/gpu/drm/drm_prime.c
> index 7856a9b3f8a8..b8ca06ea7d80 100644
> --- a/drivers/gpu/drm/drm_prime.c
> +++ b/drivers/gpu/drm/drm_prime.c
> @@ -933,16 +933,18 @@ 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 scatterlist *sg, *dma_sg;
>   	struct page *page;
> -	u32 len, index;
> +	u32 len, dma_len, index;
>   	dma_addr_t addr;
>   
>   	index = 0;
> +	dma_sg = sgt->sgl;
> +	dma_len = sg_dma_len(dma_sg);
> +	addr = sg_dma_address(dma_sg);
>   	for_each_sg(sgt->sgl, sg, sgt->nents, count) {
>   		len = sg->length;
>   		page = sg_page(sg);
> -		addr = sg_dma_address(sg);
>   
>   		while (len > 0) {
>   			if (WARN_ON(index >= max_entries))
> @@ -955,8 +957,15 @@ int drm_prime_sg_to_page_addr_arrays(struct sg_table *sgt, struct page **pages,
>   			page++;
>   			addr += PAGE_SIZE;
>   			len -= PAGE_SIZE;
> +			dma_len -= PAGE_SIZE;
>   			index++;
>   		}
> +
> +		if (dma_len == 0) {
> +			dma_sg = sg_next(dma_sg);
> +			dma_len = sg_dma_len(dma_sg);
> +			addr = sg_dma_address(dma_sg);
> +		}
>   	}
>   	return 0;
>   }