[PATCH] arm64: avoid increasing DMA masks above what hardware supports

[Nikita Yushchenko]

11.01.2017 17:50, Robin Murphy пишет:
> On 11/01/17 13:41, Nikita Yushchenko wrote:
>>> Yes, I think that ought to work, although the __iommu_setup_dma_ops()
>>> call will still want a real size reflecting the default mask
>>
>> I see iommu_dma_ops do not define set_dma_mask.
>>
>> So what if setup was done for size reflecting one mask and then driver
>> changes mask?  Will things still operate correctly?
> 
> We've overridden dma_set_mask() at the function level, so it should
> always apply regardless. Besides, none of the arm64 ops implement
> .set_dma_mask anyway, so we could possibly drop the references to it
> altogether.
> 
> Conversely, I suppose we could just implement said callback for
> swiotlb_dma_ops and iommu_dma_ops with the parent_dma_mask-checking
> function and drop the HAVE_ARCH_DMA_SET_MASK override instead. I'm not
> sure which approach is preferable - the latter seems arguably cleaner in
> isolation, but would also be less consistent with how the coherent mask
> has to be handled. Ho hum.

I mean, before patch is applied.

In the current mainline codebase, arm64 iommu does setup dependent on
[default] dma_mask, but does not anyhow react on dma mask change.

I don't know much details about arm64 iommu, but from distant view this
combination looks incorrect:
- if behavior of this hardware should depend on dma_mask of device, then
it should handle mask change,
- if behavior of this hardware should not depend on dma_mask of device,
then what for to pass size to it's setup?