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

[Peng Fan]

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?

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>
>> 
>> 
>
>