[ofa-general] iommu dma mapping alignment requirements

[Steve Wise]

Steve Wise wrote:
> Roland Dreier wrote:
>>  > It appears that my problem boils down to a single host page of memory
>>  > that is mapped for dma, and the dma address returned by dma_map_sg()
>>  > is _not_ 64KB aligned.  Here is an example:
>>
>>  > My first question is: Is there an assumption or requirement in linux
>>  > that dma_addressess should have the same alignment as the host address
>>  > they are mapped to?  IE the rdma core is mapping the entire 64KB page,
>>  > but the mapping doesn't begin on a 64KB page boundary.
>>
>> I don't think this is explicitly documented anywhere, but it certainly
>> seems that we want the bus address to be page-aligned in this case.
>> For mthca/mlx4 at least, we tell the adapter what the host page size
>> is (so that it knows how to align doorbell pages etc) and I think this
>> sort of thing would confuse the HW.
>>
>>  - R.
> 
> 
> In arch/powerpc/kernel/iommu.c:iommu_map_sg() I see that it calls 
> iommu_range_alloc() with a alignment_order of 0:
> 
>>                 vaddr = (unsigned long)page_address(s->page) + s->offset;
>>                 npages = iommu_num_pages(vaddr, slen);
>>                 entry = iommu_range_alloc(tbl, npages, &handle, mask 
>> >> IOMMU_PAGE_SHIFT, 0);
> 
> But perhaps the alignment order needs to be based on the host page size?
> 

Or based on the alignment of vaddr actually...