Re: [PATCH] iommu/dma-iommu: properly respect configured address space size

[Robin Murphy]

On 08/11/16 13:41, Marek Szyprowski wrote:
> Hi Robin,
> 
> 
> On 2016-11-08 12:37, Robin Murphy wrote:
>> On 07/11/16 13:06, Marek Szyprowski wrote:
>>> When one called iommu_dma_init_domain() with size smaller than device's
>>> DMA mask, the alloc_iova() will not respect it and always assume that
>>> all
>>> IOVA addresses will be allocated from the the (base ...
>>> dev->dma_mask) range.
>> Is that actually a problem for anything?
> 
> Yes, I found this issue while working on next version of ARM & ARM64
> DMA-mapping/IOMMU integration patchset and adapting Exynos drivers for the
> new IOMMU/DMA-mapping glue.
> 
> Some Exynos devices (codec and camera isp) operate only on the limited (and
> really small: 256M for example) DMA window. They use non-standard way of
> addressing memory: an offset from the firmware base. However they still
> have
> 32bit DMA mask, as the firmware can be located basically everywhere in the
> real DMA address space, but then they can access only next 256M from that
> firmware base.

OK, that's good to know, thanks. However, I think in this case it sounds
like it's really the DMA mask that's the underlying problem - if those
blocks themselves can only drive 28 address bits, then the struct
devices representing them should have 28-bit DMA masks, and the
"firmware base" of whoever's driving the upper bits modelled with a
dma_pfn_offset. Even if they do have full 32-bit interfaces themselves,
but are constrained to segment-offset addressing internally, I still
think it would be tidier to represent things that way.

At some point in dma-iommu development I did have support for DMA
offsets upstream of the IOMMU, and am happy to reinstate it if there's a
real use case (assuming you can't simply always set the firmware base to
0 when using the IOMMU).

>>> This patch fixes this issue by taking the configured address space size
>>> parameter into account (if it is smaller than the device's dma_mask).
>> TBH I've been pondering ripping the size stuff out of dma-iommu, as it
>> all stems from me originally failing to understand what dma_32bit_pfn is
>> actually for. The truth is that iova_domains just don't have a size or
>> upper limit; however if devices with both large and small DMA masks
>> share a domain, then the top-down nature of the allocator means that
>> allocating for the less-capable devices would involve walking through
>> every out-of-range entry in the tree every time. Having cached32_node
>> based on dma_32bit_pfn just provides an optimised starting point for
>> searching within the smaller mask.
> 
> Right, this dma_32bit_pfn was really misleading at the first glance,
> but then I found that this was something like end_pfn in case of dma-iommu
> code.

Yes, that was my incorrect assumption - at the time I interpreted it as
a de-facto upper limit which was still possible to allocate above in
special circumstances, which turns out to be almost entirely backwards.
I'd rather not bake that into the dma-iommu code any further if we can
avoid it (I'll try throwing an RFC together to clear up what's there
already).

Robin.

>> Would it hurt any of your use-cases to relax/rework the reinitialisation
>> checks in iommu_dma_init_domain()? Alternatively if we really do have a
>> case for wanting a hard upper limit, it might make more sense to add an
>> end_pfn to the iova_domain and handle it in the allocator itself.
> 
> The proper support for end_pfn would be a better approach probably,
> especially if we consider readability of the code.
> 
> Best regards