Re: [PATCH v2 06/11] ARM: dma-mapping: Handle foreign buffers in arm_iommu_get_sgtable

[Robin Murphy <robin.murphy@arm.com>]

On 16/10/2026 7:46 am, Mikko Perttunen wrote:
> arm_iommu_get_sgtable can only describe buffers which came from
> arm_iommu_alloc_attrs, and returns -ENXIO for anything else. This is
> stricter than iommu_dma_get_sgtable(), which falls back to describing
> the buffer as a single chunk.
> 
> The difference shows up when a buffer is allocated for one device and
> mapped for another, as TegraDRM does. Buffers are allocated on the
> host1x logical device, which has no dma_configure and therefore uses
> direct DMA ops, and are then mapped for a display controller behind the
> SMMU.
> 
> Add the same fallback as iommu_dma_get_sgtable().

Hmm, that's not actually what it's for... :(

The "fallback" is only for the case when iommu_dma_alloc() has hit the 
dma_alloc_from_pool() path, see a98d9ae937d2 ("arm64/iommu: handle 
non-remapped addresses in ->mmap and ->get_sgtable"). It was never meant 
for dipping into *other* DMA ops because the caller has erroneously 
passed the wrong device. Just like dma_mmap_attrs() and 
dma_free_attrs(), dma_get_sgtable_attrs() must match the original 
dma_alloc_attrs() call - that's a bug in TegraDRM.

Thanks,
Robin.

> Signed-off-by: Mikko Perttunen <mperttunen@nvidia.com>
> ---
>   arch/arm/mm/dma-mapping.c | 25 +++++++++++++++++++++----
>   1 file changed, 21 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/arm/mm/dma-mapping.c b/arch/arm/mm/dma-mapping.c
> index d6315baeddb9..c2ec4f107808 100644
> --- a/arch/arm/mm/dma-mapping.c
> +++ b/arch/arm/mm/dma-mapping.c
> @@ -1169,12 +1169,29 @@ static int arm_iommu_get_sgtable(struct device *dev, struct sg_table *sgt,
>   {
>   	unsigned int count = PAGE_ALIGN(size) >> PAGE_SHIFT;
>   	struct page **pages = __iommu_get_pages(cpu_addr, attrs);
> +	struct page *page;
> +	int ret;
>   
> -	if (!pages)
> -		return -ENXIO;
> +	if (pages)
> +		return sg_alloc_table_from_pages(sgt, pages, count, 0, size,
> +						 GFP_KERNEL);
>   
> -	return sg_alloc_table_from_pages(sgt, pages, count, 0, size,
> -					 GFP_KERNEL);
> +	/*
> +	 * The buffer was not allocated through these DMA ops. It may belong to
> +	 * a device which is not behind an IOMMU at all, which can happen when a
> +	 * buffer is allocated on one device and mapped for another. Describe it
> +	 * as a single chunk, as iommu_dma_get_sgtable() does.
> +	 */
> +	if (is_vmalloc_addr(cpu_addr))
> +		page = vmalloc_to_page(cpu_addr);
> +	else
> +		page = virt_to_page(cpu_addr);
> +
> +	ret = sg_alloc_table(sgt, 1, GFP_KERNEL);
> +	if (!ret)
> +		sg_set_page(sgt->sgl, page, PAGE_ALIGN(size), 0);
> +
> +	return ret;
>   }
>   
>   /*
>