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    SubjectRe: [PATCH v3 7/8] arm64: mm: Set ZONE_DMA size based on early IORT scan
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    On Thu, 2020-10-15 at 22:26 +0800, Hanjun Guo wrote:
    > On 2020/10/15 3:12, Nicolas Saenz Julienne wrote:
    > > From: Ard Biesheuvel <ardb@kernel.org>
    > >
    > > We recently introduced a 1 GB sized ZONE_DMA to cater for platforms
    > > incorporating masters that can address less than 32 bits of DMA, in
    > > particular the Raspberry Pi 4, which has 4 or 8 GB of DRAM, but has
    > > peripherals that can only address up to 1 GB (and its PCIe host
    > > bridge can only access the bottom 3 GB)
    > >
    > > Instructing the DMA layer about these limitations is straight-forward,
    > > even though we had to fix some issues regarding memory limits set in
    > > the IORT for named components, and regarding the handling of ACPI _DMA
    > > methods. However, the DMA layer also needs to be able to allocate
    > > memory that is guaranteed to meet those DMA constraints, for bounce
    > > buffering as well as allocating the backing for consistent mappings.
    > >
    > > This is why the 1 GB ZONE_DMA was introduced recently. Unfortunately,
    > > it turns out the having a 1 GB ZONE_DMA as well as a ZONE_DMA32 causes
    > > problems with kdump, and potentially in other places where allocations
    > > cannot cross zone boundaries. Therefore, we should avoid having two
    > > separate DMA zones when possible.
    > >
    > > So let's do an early scan of the IORT, and only create the ZONE_DMA
    > > if we encounter any devices that need it. This puts the burden on
    > > the firmware to describe such limitations in the IORT, which may be
    > > redundant (and less precise) if _DMA methods are also being provided.
    > > However, it should be noted that this situation is highly unusual for
    > > arm64 ACPI machines. Also, the DMA subsystem still gives precedence to
    > > the _DMA method if implemented, and so we will not lose the ability to
    > > perform streaming DMA outside the ZONE_DMA if the _DMA method permits
    > > it.
    >
    > Sorry, I'm still a little bit confused. With this patch, if we have
    > a device which set the right _DMA method (DMA size >= 32), but with the
    > wrong DMA size in IORT, we still have the ZONE_DMA created which
    > is actually not needed?

    Yes, that would be the case.

    Regards,
    Nicolas

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