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SubjectRe: [Linaro-mm-sig] [RFC] ARM DMA mapping TODO, v1
On 04/29/2011 01:26 PM, Arnd Bergmann wrote:
> On Thursday 28 April 2011, Benjamin Herrenschmidt wrote:
>
>>>> For PCI you can have the flag propagate from the PHB down, for busses
>>>> without a bus type (platform) then whoever instanciate them (the
>>>> platform code) can set that appropriately.
>>>>
>>> How can you do that when it changes mid-bus heirarchy? I'm thinking
>>> of the situation where the DRM stuff is on a child bus below the
>>> root bus, and the root bus has DMA coherent devices on it but the DRM
>>> stuff doesn't.
>>>
>> But that's not PCI right ? IE. with PCI, coherency is a property of the
>> PHB...
>>
> That is my understanding at least, but I'd like to have a confirmation
> from the DRM folks.
>
> I believe that the PC graphics cards that have noncoherent DMA mappings
> are all of the unified memory (integrated into the northbridge) kind,
> so they are not on the same host bridge as all regular PCI devices,
> even if they appear as a PCI device.
>

I think Jerome has mentioned at one point that the Radeon graphics cards
support
non-coherent mappings.

Fwiw, the PowerVR SGX MMU also supports this mode of operation, although
it being functional I guess depends on the system implementation.

/Thomas



> Arnd
>
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