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    SubjectRe: [RFC PATCH] arm64: mm: Add invalidate back in arch_sync_dma_for_device when FROM_DEVICE
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    On 2022/11/17 18:23, Robin Murphy wrote:

    > On 2022-11-17 08:24, Ard Biesheuvel wrote:
    >> On Thu, 17 Nov 2022 at 07:33, Nanyong Sun <sunnanyong@huawei.com> wrote:
    >>>
    >>> The commit c50f11c6196f ("arm64: mm: Don't invalidate FROM_DEVICE
    >>> buffers at start of DMA transfer") replaces invalidate with clean
    >>> when DMA_FROM_DEVICE, this changes the behavior of functions like
    >>> dma_map_single() and dma_sync_single_for_device(*, *, *,
    >>> DMA_FROM_DEVICE),
    >>> then it may make some drivers works unwell because the implementation
    >>> of these DMA APIs lose the original cache invalidation.
    >>>
    >>> Situation 1:
    >> ...
    >>> Situation 2:
    >>> After backporting the above commit, we find a network card driver go
    >>> wrong with cache inconsistency when doing DMA transfer: CPU got the
    >>> stale data in cache when reading DMA data received from device.
    >>
    >> I suppose this means those drivers may lack dma_sync_single_for_cpu()
    >> calls after the inbound transfers complete, and are instead relying on
    >> the cache invalidation performed before the transfer to make the DMA'd
    >> data visible to the CPU.
    >
    > It also commonly goes wrong the other way round when the drivers are
    > correct but DT/ACPI failed to indicate a coherent device as such. If
    > writes from the device actually snoop, they hit the still-present
    > cache line, which then gets invalidated by unmap/sync_for_cpu and the
    > new data is lost.
    >
    > Robin.
    >
    > .
    Is it acceptable?  Does the coherent device MUST run with coherent
    identifier? At least they can run with no wrong before.

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