Re: [RFC PATCH 1/3] dma-buf: keep the DMA flags in the DMABUF_DEBUG copy

[Christian König]

On 10/5/26 08:41, Karl Mehltretter wrote:
> With DMABUF_DEBUG, dma_buf_map_attachment() hands the importer a copy
> of the sg_table that keeps only sg_dma_address() and sg_dma_len(). The
> dma_flags (SG_DMA_BUS_ADDRESS, SG_DMA_SWIOTLB) are dropped.
> 
> The flags describe the DMA side of an entry, which is the side
> importers may use. Copy them as well.
> 
> This is not a bug fix. No importer reads the flags today. Their only
> readers are dma-iommu, dma-direct and iommu_map_sg(), which are not
> supposed to see an importer's copy at all. The warn mode added in the
> next patch keeps its marker in dma_flags and lets importers that do
> get there continue. They should then see the same flags as without
> DMABUF_DEBUG.
> 
> Assisted-by: LLM
> Signed-off-by: Karl Mehltretter <kmehltretter@xxxxxxxxx>
> ---
> 
> Notes:
>     Built alone on x86_64 with NEED_SG_DMA_FLAGS=y (dma-buf.o, W=1). The
>     KUnit test in patch 3 checks the flags in the copy. It passes on x86_64
>     under QEMU in strict and in warn mode.
> 
>  drivers/dma-buf/dma-buf.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> index 4c9add51f9ef..b3d311acb883 100644
> --- a/drivers/dma-buf/dma-buf.c
> +++ b/drivers/dma-buf/dma-buf.c
> @@ -904,6 +904,10 @@ static int dma_buf_wrap_sg_table(struct sg_table **sg_table)
>  		sg_assign_page(to_sg, NULL);
>  		sg_dma_address(to_sg) = sg_dma_address(from_sg);
>  		sg_dma_len(to_sg) = sg_dma_len(from_sg);
> +#ifdef CONFIG_NEED_SG_DMA_FLAGS
> +		/* the flags describe the DMA side, e.g. SG_DMA_BUS_ADDRESS */
> +		to_sg->dma_flags = from_sg->dma_flags;

We should probably have a WARN_ON_ONCE(sg_dma_is_swiotlb(from_sg)) here as well since some importers/exporters doesn't realize that this combination will usually not work.

Apart from that looks good to me.

Regards,
Christian.

> +#endif
>  		to_sg = sg_next(to_sg);
>  	}
>