Re: [PATCH RFC 0/2] iommu/dma: Fix DMA_ATTR_MMIO swiotlb rejection

[Leon Romanovsky]

On Fri, Sep 18, 2026 at 10:12:24PM +0800, Peng Fan wrote:
> Hi Leon,
> 
> On Fri, Sep 18, 2026 at 03:37:36PM +0300, Leon Romanovsky wrote:
> >On Wed, Sep 16, 2026 at 11:23:32PM +0800, Peng Fan (OSS) wrote:
> >> Commit f9374de14c0e8 ("iommu/dma: implement DMA_ATTR_MMIO for
> >> iommu_dma_(un)map_phys()") and commit c288d657dd515 ("iommu/dma: implement
> >> DMA_ATTR_MMIO for dma_iova_link().") added DMA_ATTR_MMIO support to skip
> >> swiotlb bouncing and cache flushing for MMIO resources. However, both
> >> implementations placed the DMA_ATTR_MMIO check inside the swiotlb bounce block,
> >> causing non-page-aligned MMIO mappings to be rejected outright instead of
> >> skipping the bounce and proceeding to create the IOVA mapping.
> >> 
> >> This breaks dma_map_resource() for any non-page-aligned device register when
> >> behind an IOMMU: DMA controllers that map peripheral FIFO addresses
> >> (e.g. SPI controller TX/RX data registers) through an IOMMU for per-channel
> >> isolation.
> >> 
> >> Both patches move the DMA_ATTR_MMIO check before the swiotlb block so MMIO,
> >> resources skip the entire bounce path and fall through directly to the IOMMU
> >> mapping function.
> >
> >While I understand the rationale behind the first patch, why do we need the
> >second one? Do we want to allow unaligned addresses for the _link_ as well?
> >Current users don't need it.
> 
> My intention with the second patch was to make the DMA_ATTR_MMIO handling
> consistent with iommu_dma_map_phys(), since MMIO cannot be bounced by
> SWIOTLB in either case.
> 
> However, I don't have a dma_iova_link() user that requires an
> unaligned MMIO address, so no need to relax the existing restriction there.
>  
> I'll drop the second patch and keep this fix scoped to iommu_dma_map_phys()
> in v2. Does that sounds good to you?
> 
> BTW, does patch 1 look good to you?

Yes, for both questions.

Thanks

> 
> Thanks,
> Peng
> 
> >
> >Thanks
> >
> >> 
> >> Tested on NXP i.MX95 with ARM SMMUv3 where the fsl-edma DMA controller maps
> >> SPI peripheral FIFO registers (non-page-aligned) via dma_map_resource()
> >> through per-channel IOMMU domains. Only the iommu_dma_map_phys() path was
> >> exercised; the dma_iova_link() fix is by code inspection of the same pattern.
> >> 
> >> Signed-off-by: Peng Fan <peng.fan@xxxxxxx>
> >> ---
> >> Peng Fan (2):
> >>       iommu/dma: skip swiotlb bounce for DMA_ATTR_MMIO in iommu_dma_map_phys
> >>       iommu/dma: skip swiotlb bounce for DMA_ATTR_MMIO in dma_iova_link()
> >> 
> >>  drivers/iommu/dma-iommu.c | 8 ++++----
> >>  1 file changed, 4 insertions(+), 4 deletions(-)
> >> ---
> >> base-commit: e6e35979777d646fe3c7c94dca7dd32fb25d45f4
> >> change-id: 20260916-iommu-dma-fix-9f55af1beb46
> >> 
> >> Best regards,
> >> --  
> >> Peng Fan <peng.fan@xxxxxxx>
> >> 
> >> 
> >
> >
>