linux-kernel - Re: [PATCH 4/7] swiotlb: Allow arch override of address_needs

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Date:	Wed, 8 Apr 2009 23:43:27 -0500
From:	Kumar Gala <galak@...nel.crashing.org>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>, hch@...radead.org,
	linux-kernel@...r.kernel.org, mingo@...e.hu,
	ian.campbell@...rix.com, beckyb@...nel.crashing.org
Subject: Re: [PATCH 4/7] swiotlb: Allow arch override of address_needs_mapping


On Apr 8, 2009, at 7:09 PM, Jeremy Fitzhardinge wrote:

> FUJITA Tomonori wrote:
>> On Wed, 08 Apr 2009 16:16:17 -0700
>> Jeremy Fitzhardinge <jeremy@...p.org> wrote:
>>
>>
>>> FUJITA Tomonori wrote:
>>>
>>>>> Becky's patches of last week also added __weak annotations to  
>>>>> swiotlb_bus_to_virt, virt_to_bus and bus_to_phys; added the  
>>>>> hwdev parameter to swiotlb_bus_to_phys; and added a weak  
>>>>> swiotlb_arch_address_needs_mapping.  I assume that was needed  
>>>>> because powerpc needs non-trivial implementations for those  
>>>>> functions.
>>>>>
>>>> Hmm, what she added are wrappers of virt_to_bus and bus_to_virt. We
>>>> can remove these and directly use virt_to_bus and bus_to_virt.
>>>>
>>> In general those interfaces are deprecated.  Are we un-deprecating  
>>> them?  Or do you mean adding virt<->bus to dma_ops?
>>>
>>
>> Hmm, these interfaces are wrong for drivers surely because they can't
>> handle dma mapping properly. However, they are exactly what swiotlb
>> needs (swiotlb doesn't need to care about dma mapping).
>
> It needs to care about the mapping from phys to bus.  On x86 they're  
> identical, but on powerpc there can be at least an offset between  
> them.
>
>> Until 2.6.28,
>> swiotlb has used them. They are with IA64, X86_64 and PPC_32, I  
>> think.
>>
>
> Well, Becky's patches also added the hwdev argument to them, so  
> presumably the powerpc implementation needs that (different devices/ 
> buses have differing views of physical memory, I guess).

On powerpc we need the hwdev because things vary based on bus.  For  
our SoC chips we don't need any mapping between phys & bus.  However  
something like PCI does have a mapping (a simple offset).

- k
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