Re: [PATCH] iommu/io-pgtable-arm: Add support for contiguous hint bit

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

On 15/07/2026 1:50 pm, Jason Gunthorpe wrote:
> On Wed, Jul 15, 2026 at 01:14:18PM +0100, Robin Murphy wrote:
> 
>> Conversely though, I really wouldn't want to have to maintain arm-smmu
>> supporting two completely different pagetable APIs simultaneously either :/
> 
> Does arm-smmu era HW even support CONT?

Yes, it goes all the way back to ARMv7 LPAE, predating even SMMUv1, and 
certainly at least Arm's implementations (MMU-400/401/500) can all make 
meaningful use of it.

> AFAICR the biggest issue with arm-smmu was it using IO_PGTABLE_ARMV7S
> as well. I think it would be fine to implement all the unique LPAE
> features it needs in iommupt, I did most of them already.
> 
> I do have an ARMV7S implementation for iommupt, but I did not solve
> the sub page problem. So while it is functionally working it is not
> usable since it wastes so much memory. That's a tricky problem to
> solve since the algorithms depend on the gather->freelist.
> 
> I didn't spend any time trying to do anything about this as I was not
> intending to touch arm-smmu

Indeed between the irregular table sizes (before you even get to the 
Mediatek shenanigans...), and the awkward GFP_DMA limitations and/or 
reality that many of the systems using it really don't have memory to 
waste, I'd have considered v7s pretty much terminally incompatible with 
iommu_pages and the abstraction that iommupt is trying to be... :/

Thanks,
Robin.