[PATCH 1/2] drm/etnaviv: Fix misunderstanding about the scatterlist structure

[Sui Jingfeng]

Hi, Dear reviewers

On 2024/10/29 00:05, Sui Jingfeng wrote:
> The 'offset' data member of the 'struct scatterlist' denotes the offset
> into a SG entry in bytes. The sg_dma_len() macro could be used to get
> lengths of SG entries, those lengths are expected to be CPU page size
> aligned. Since, at least for now, we call drm_prime_pages_to_sg() to
> convert our various page array into an SG list. We pass the number of
> CPU page as the third argoument, to tell the size of the backing memory
> of GEM buffer object.
>
> drm_prime_pages_to_sg() call sg_alloc_table_from_pages_segment() do the
> work, sg_alloc_table_from_pages_segment() always hardcode the Offset to
> ZERO. The sizes of *all* SG enties will be a multiple of CPU page size,
> that is multiple of PAGE_SIZE.
>
> If the GPU want to map/unmap a bigger page partially, we should use
> 'sg_dma_address(sg) + sg->offset' to calculate the destination DMA
> address, and the size to be map/unmap is 'sg_dma_len(sg) - sg->offset'.
>
> While the current implement is wrong, but since the 'sg->offset' is
> alway equal to 0, drm/etnaviv works in practice by good luck. Fix this,
> to make it looks right at least from the perspective of concept.
>
> while at it, always fix the absue types:


'absue' -> 'abuse'


By the way, sorry I'm just receive your message from my Thunderbird client.


I sent those two patch first, then I run my Thunderbird client.

Not seeing that you have already merged part of my patch, then there will be
merge conflict I guess.

I think I could wait the next round and respin my patch.


> - sg_dma_address returns DMA address, the type is dma_addr_t, not
>    the phys_addr_t, for VRAM there may have another translation between
>    the bus address and the final physical address of VRAM or carved out
>    RAM.
>
> - The type of sg_dma_len(sg) return is unsigned int, not the size_t.
>    Avoid hint the compiler to do unnecessary integer promotion.
>
> Signed-off-by: Sui Jingfeng <sui.jingfeng at linux.dev>
> ---
>   drivers/gpu/drm/etnaviv/etnaviv_mmu.c | 10 +++++-----
>   1 file changed, 5 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/gpu/drm/etnaviv/etnaviv_mmu.c b/drivers/gpu/drm/etnaviv/etnaviv_mmu.c
> index 1661d589bf3e..4ee9ed96b1d8 100644
> --- a/drivers/gpu/drm/etnaviv/etnaviv_mmu.c
> +++ b/drivers/gpu/drm/etnaviv/etnaviv_mmu.c
> @@ -80,10 +80,10 @@ static int etnaviv_iommu_map(struct etnaviv_iommu_context *context, u32 iova,
>   		return -EINVAL;
>   
>   	for_each_sgtable_dma_sg(sgt, sg, i) {
> -		phys_addr_t pa = sg_dma_address(sg) - sg->offset;
> -		size_t bytes = sg_dma_len(sg) + sg->offset;
> +		dma_addr_t pa = sg_dma_address(sg) + sg->offset;
> +		unsigned int bytes = sg_dma_len(sg) - sg->offset;
>   
> -		VERB("map[%d]: %08x %pap(%zx)", i, iova, &pa, bytes);
> +		VERB("map[%d]: %08x %pap(%x)", i, iova, &pa, bytes);
>   
>   		ret = etnaviv_context_map(context, da, pa, bytes, prot);
>   		if (ret)
> @@ -109,11 +109,11 @@ static void etnaviv_iommu_unmap(struct etnaviv_iommu_context *context, u32 iova,
>   	int i;
>   
>   	for_each_sgtable_dma_sg(sgt, sg, i) {
> -		size_t bytes = sg_dma_len(sg) + sg->offset;
> +		unsigned int bytes = sg_dma_len(sg) - sg->offset;
>   
>   		etnaviv_context_unmap(context, da, bytes);
>   
> -		VERB("unmap[%d]: %08x(%zx)", i, iova, bytes);
> +		VERB("unmap[%d]: %08x(%x)", i, iova, bytes);
>   
>   		BUG_ON(!PAGE_ALIGNED(bytes));
>   

-- 
Best regards,
Sui