Re: [Linaro-mm-sig] [RFC] ARM DMA mapping TODO, v1

[Alex Deucher]

On Wed, Apr 27, 2011 at 7:02 AM, Arnd Bergmann <arnd@xxxxxxxx> wrote:
> On Wednesday 27 April 2011, Russell King - ARM Linux wrote:
>> On Wed, Apr 27, 2011 at 10:56:49AM +0200, Arnd Bergmann wrote:
>> > We probably still need to handle both the coherent and noncoherent case
>> > in each dma_map_ops implementation, at least for those combinations where
>> > they matter (definitely the linear mapping). However, I think that using
>> > dma_mapping_common.h would let us use an architecture-independent dma_map_ops
>> > for the generic iommu code that Marek wants to introduce now.
>>
>> The 'do we have an iommu or not' question and the 'do we need to do cache
>> coherency' question are two independent questions which are unrelated to
>> each other.  There are four unique but equally valid combinations.
>>
>> Pushing the cache coherency question down into the iommu stuff will mean
>> that we'll constantly be fighting against the 'but this iommu works on x86'
>> shite that we've fought with over block device crap for years.  I have
>> no desire to go there.
>
> Ok, I see. I believe we could avoid having to fight with the people that
> only care about coherent architectures if we just have two separate
> implementations of dma_map_ops in the iommu code, one for coherent
> and one for noncoherent DMA. Any architecture that only needs one
> of them would then only enable the Kconfig options for that implementation
> and not care about the other one.
>
>> What we need is a proper abstraction where the DMA ops can say whether
>> they can avoid DMA cache handling (eg, swiotlb or dmabounce stuff) but
>> default to DMA cache handling being the norm - and the DMA cache handling
>> performed in the level above the DMA ops indirection.
>
> Yes, that sounds definitely possible. I guess it could be as simple
> as having a flag somewhere in struct device if we want to make it
> architecture independent.
>
> As for making the default being to do cache handling, I'm not completely
> sure how that would work on architectures where most devices are coherent.
> If I understood the DRM people correctly, some x86 machine have noncoherent
> DMA in their GPUs while everything else is coherent.

On radeon hardware at least the on chip gart mechanism supports both
snooped cache coherent pages and uncached, non-snooped pages.

Alex

>
> Maybe we can default to arch_is_coherent() and allow a device to override
> that when it knows better.
>
>        Arnd
>
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