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    SubjectRe: [PATCH 0/2] Fix incorrect warning from dma-debug
    Hi Jon,

    On Sun, May 7, 2017 at 2:06 AM, Jon Masters <jcm@jonmasters.org> wrote:
    > On 05/09/2016 06:00 AM, Robin Murphy wrote:
    >> On 09/05/16 10:37, Robin Murphy wrote:
    >>> On 08/05/16 11:59, Niklas Söderlund wrote:
    >>>> While using CONFIG_DMA_API_DEBUG i came across this warning which I
    >>>> think is a false positive. As shown dma_sync_single_for_device() are
    >>>> called from the dma_map_single() call path. This triggers the warning
    >>>> since the dma-debug code have not yet been made aware of the mapping.
    >>>
    >>> Almost right ;) The thing being mapped (the SPI device's buffer) and the
    >>> thing being synced (the IOMMU's PTE) are entirely unrelated. Due to the
    >>> current of_iommu_init() setup, the IOMMU is probed long before
    >>> dma_debug_init() gets called, therefore DMA debug is missing entries for
    >>> some of the initial page table mappings and gets confused when we update
    >>> them later.
    >
    > What was the eventual followup/fix for this? We're seeing similar traces
    > to this during internal testing of 4.11 kernels.

    A quick hack is to change arch/arm64/mm/dma-mapping.c:dma_debug_do_init()
    from fs_initcall() to arch_initcall().

    However, then you loose the debugfs features. A proper fix would involve
    splitting dma_debug_init() in two phases: an early one doing the real work,
    and a late one registering the debugfs interface.

    Gr{oetje,eeting}s,

    Geert

    --
    Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

    In personal conversations with technical people, I call myself a hacker. But
    when I'm talking to journalists I just say "programmer" or something like that.
    -- Linus Torvalds

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