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    SubjectRe: [RFC PATCH] drm: disable WC optimization for cache coherent devices on non-x86
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    Am 24.01.19 um 10:28 schrieb Ard Biesheuvel:
    > On Thu, 24 Jan 2019 at 10:25, Koenig, Christian
    > <Christian.Koenig@amd.com> wrote:
    >> Am 24.01.19 um 10:13 schrieb Christoph Hellwig:
    >>> On Wed, Jan 23, 2019 at 05:52:50PM +0100, Ard Biesheuvel wrote:
    >>>> But my concern is that it seems likely that non-cache coherent
    >>>> implementations are relying on this hack as well. There must be a
    >>>> reason that this hack is only disabled for PowerPC platforms if they
    >>>> are cache coherent, for instance, and I suspect that that reason is
    >>>> that the hack is the only thing ensuring that the CPU mapping
    >>>> attributes match the device ones used for these buffers (the vmap()ed
    >>>> ones), whereas the rings and other consistent data structures are
    >>>> using the DMA API as intended, and thus getting uncached attributes in
    >>>> the correct way.
    >>> Dave, who added that commit is on Cc together with just about everyone
    >>> involved in the review chain. Based on the previous explanation
    >>> that idea what we might want an uncached mapping for some non-coherent
    >>> architectures for this to work at all makes sense, but then again
    >>> the way to create those mappings is entirely architecture specific,
    >>> and also need a cache flushing before creating the mapping to work
    >>> properly. So my working theory is that this code never properly
    >>> worked on architectures without DMA coherent for PCIe at all, but
    >>> I'd love to be corrected by concrete examples including an explanation
    >>> of how it actually ends up working.
    >> Cache coherency is mandatory for modern GPU operation.
    >>
    >> Otherwise you can't implement a bunch of the requirements of the
    >> userspace APIs.
    >>
    >> In other words the applications doesn't inform the driver that the GPU
    >> or the CPU is accessing data, it just does it and assumes that it works.
    >>
    > Wonderful!
    >
    > In that case, do you have any objections to the patch proposed by
    > Christoph above?

    Yeah, the patch of Christoph actually goes way to far cause we have
    reports that this works on a bunch of other architectures.

    E.g. X86 64bit, PowerPC (under some conditions) and some MIPS.

    The only problematic here actually seems to be ARM, so you should
    probably just add an "#ifdef .._ARM return false;".

    Regards,
    Christian.
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