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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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    >



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