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SubjectRe: [PATCH V2 2/3] powerpc: Add support for swiotlb on 32-bit
Becky Bruce wrote:
>> If we have something like in arch/{x86|ia64|powerpc}/dma-mapping.h:
>> static inline int is_buffer_dma_capable(struct device *dev,
>> dma_addr_t addr, size_t size)
>> then we don't need two checking functions, address_needs_mapping and
>> range_needs_mapping.
> It's never been clear to me *why* we had both in the first place - if
> you can explain this, I'd be grateful :)

I was about to ask the same thing. It seems that range_needs_mapping
should be able to do both jobs.

I think range_needs_mapping came from the Xen swiotlb changes, and
address_needs_mapping came from your powerpc changes. Many of the
changes were exact overlaps; I think this was one of the few instances
where there was a difference.

We need a range check in Xen (rather than iterating over individual
pages) because we want to check that the underlying pages are machine
contiguous, but I think that's also sufficient to do whatever checks you
need to do.

The other difference is that is_buffer_dma_capable operates on a
dma_addr_t, which presumes that you can generate a dma address and then
test for its validity. For Xen, it doesn't make much sense to talk
about the dma_addr_t for memory which isn't actually dma-capable; we
need the test to be in terms of phys_addr_t. Given that the two
functions are always called from the same place, that doesn't seem to
pose a problem.

So I think the unified function would be something like:

int range_needs_mapping(struct device *hwdev, phys_addr_t addr,
size_t size);

which would be defined somewhere under asm/*.h. Would that work for


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