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

[Robin Murphy]

On 08/11/16 14:53, Marek Szyprowski wrote:
> Hi Robin,
> 
> 
> On 2016-11-08 15:44, Robin Murphy wrote:
>> On 08/11/16 13:41, Marek Szyprowski wrote:
>>> 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).
> 
> That would indeed look a bit simpler, but I've already tried such approach
> and the firmware crashes when its base in real DMA address space is set to
> zero.

Ah, sadly I'm not too surprised... ;)

Having pondered it a little more, what if the firmware base is instead
set equal to the max offset, then you can have a power-of-two DMA mask
at twice that size (e.g. 512M) such that the top-down IOVA allocations
start in the right place. That would appear to achieve pretty much the
same result as this patch, but more robustly.

Incidentally, how do the init size and DMA mask end up different in the
first place? Is this another case of the "dma-ranges" info from
of_dma_configure() getting clobbered by a driver calling dma_set_mask()
later?

Robin.

>>>>> 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).
> 
> Okay.
> 
> Best regards