Re: [PATCH RFC 1/2] iommu/dma: skip swiotlb bounce for DMA_ATTR_MMIO in iommu_dma_map_phys

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

On 16/09/2026 4:23 pm, Peng Fan (OSS) wrote:
> From: Peng Fan <peng.fan@nxp.com>
> 
> MMIO resources mapped via dma_map_resource() carry DMA_ATTR_MMIO. These
> never need swiotlb bounce buffering, but the current code enters the
> swiotlb path when the physical address is not page-aligned, then rejects
> the mapping because MMIO cannot be bounced.
> 
> Move the DMA_ATTR_MMIO check before the swiotlb block so the entire bounce
> path is skipped for MMIO, falling through directly to __iommu_dma_map()
> which creates the IOVA mapping.

Acked-by: Robin Murphy <robin.murphy@arm.comn>

I'd agree that we shouldn't need patch #2 - I don't see any good reason 
to ever try to use dma_iova_link() instead of dma_map_resource() for 
mapping actual device data registers, and for the link API use-cases 
where the "MMIO" is peer device memory and IOMMU isolation is more 
relevant, it seems sensible to keep the alignment constraint unless and 
until there is a definite compelling need to do otherwise.

Thanks,
Robin.

> Fixes: f9374de14c0e8 ("iommu/dma: implement DMA_ATTR_MMIO for iommu_dma_(un)map_phys()")
> Assisted-by: Claude:claude-opus-4-6
> Signed-off-by: Peng Fan <peng.fan@nxp.com>
> ---
>   drivers/iommu/dma-iommu.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
> index 65d692ec7fced..26b9c2d71cd30 100644
> --- a/drivers/iommu/dma-iommu.c
> +++ b/drivers/iommu/dma-iommu.c
> @@ -1228,9 +1228,9 @@ dma_addr_t iommu_dma_map_phys(struct device *dev, phys_addr_t phys, size_t size,
>   	 * If both the physical buffer start address and size are page aligned,
>   	 * we don't need to use a bounce page.
>   	 */
> -	if (dev_use_swiotlb(dev, size, dir) &&
> +	if (!(attrs & DMA_ATTR_MMIO) && dev_use_swiotlb(dev, size, dir) &&
>   	    iova_unaligned(iovad, phys, size)) {
> -		if (attrs & (DMA_ATTR_MMIO | DMA_ATTR_REQUIRE_COHERENT))
> +		if (attrs & DMA_ATTR_REQUIRE_COHERENT)
>   			return DMA_MAPPING_ERROR;
>   
>   		phys = iommu_dma_map_swiotlb(dev, phys, size, dir, attrs);
>