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    SubjectRe: [Nouveau] [PATCH v2 2/3] drm/ttm: introduce dma cache sync helpers
    op 24-06-14 14:23, Alexandre Courbot schreef:
    > On Tue, Jun 24, 2014 at 7:55 PM, Alexandre Courbot <acourbot@nvidia.com> wrote:
    >> On 06/24/2014 07:33 PM, Alexandre Courbot wrote:
    >>> On 06/24/2014 07:02 PM, Russell King - ARM Linux wrote:
    >>>> On Tue, Jun 24, 2014 at 06:54:26PM +0900, Alexandre Courbot wrote:
    >>>>> From: Lucas Stach <dev@lynxeye.de>
    >>>>>
    >>>>> On architectures for which access to GPU memory is non-coherent,
    >>>>> caches need to be flushed and invalidated explicitly at the
    >>>>> appropriate places. Introduce two small helpers to make things
    >>>>> easy for TTM-based drivers.
    >>>>
    >>>> Have you run this with DMA API debugging enabled? I suspect you haven't,
    >>>> and I recommend that you do.
    >>>
    >>> # cat /sys/kernel/debug/dma-api/error_count
    >>> 162621
    >>>
    >>> (╯°□°)╯︵ ┻━┻)
    >>
    >> *puts table back on its feet*
    >>
    >> So, yeah - TTM memory is not allocated using the DMA API, hence we cannot
    >> use the DMA API to sync it. Thanks Russell for pointing it out.
    >>
    >> The only alternative I see here is to flush the CPU caches when syncing for
    >> the device, and invalidate them for the other direction. Of course if the
    >> device has caches on its side as well the opposite operation must also be
    >> done for it. Guess the only way is to handle it all by ourselves here. :/
    > ... and it really sucks. Basically if we cannot use the DMA API here
    > we will lose the convenience of having a portable API that does just
    > the right thing for the underlying platform. Without it we would have
    > to duplicate arm_iommu_sync_single_for_cpu/device() and we would only
    > have support for ARM.
    >
    > The usage of the DMA API that we are doing might be illegal, but in
    > essence it does exactly what we need - at least for ARM. What are the
    > alternatives?
    Convert TTM to use the dma api? :-)

    ~Maarten
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