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

[Mikko Perttunen <mperttunen@nvidia.com>]

On Tuesday, October 20, 2026 12:47 AM Robin Murphy wrote:
> 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.

Ah, indeed. That's easy to fix in TegraDRM. I think this happened 
because TegraDRM has this (a bit unsavoury) use of DMA API on both the 
drm device and the actual platform devices side by side.

Thanks,
Mikko

> 
> 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;
> >   }
> >   
> >   /*
> > 
> 
>