linux-kernel - Re: [PATCH V2 2/3] powerpc: Add support for swiotlb on 32-bit

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Message-ID: <4A1592CF.8000208@goop.org>
Date:	Thu, 21 May 2009 10:43:43 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Becky Bruce <beckyb@...nel.crashing.org>
CC:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>,
	linuxppc-dev@...abs.org, linux-kernel@...r.kernel.org,
	Ian Campbell <Ian.Campbell@...rix.com>
Subject: Re: [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 
powerpc?

    J
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