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    SubjectRe: [PATCH 1/2] arm64: dma_mapping: allow PCI host driver to limit DMA mask
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    Date
    > commit 9a57d58d116800a535510053136c6dd7a9c26e25
    > Author: Arnd Bergmann <arnd@arndb.de>
    > Date: Tue Nov 17 14:06:55 2015 +0100
    >
    > [EXPERIMENTAL] ARM64: check implement dma_set_mask
    >
    > Needs work for coherent mask
    >
    > Signed-off-by: Arnd Bergmann <arnd@arndb.de>

    Unfortunately this is far incomplete

    > @@ -957,6 +983,18 @@ void arch_setup_dma_ops(struct device *dev, u64 dma_base, u64 size,
    > if (!dev->archdata.dma_ops)
    > dev->archdata.dma_ops = &swiotlb_dma_ops;
    >
    > + /*
    > + * we don't yet support buses that have a non-zero mapping.
    > + * Let's hope we won't need it
    > + */
    > + WARN_ON(dma_base != 0);
    > +
    > + /*
    > + * Whatever the parent bus can set. A device must not set
    > + * a DMA mask larger than this.
    > + */
    > + dev->archdata.parent_dma_mask = size;
    > +

    ... because size/mask passed here for PCI devices are meaningless.

    For OF platforms, this is called via of_dma_configure(), that checks
    dma-ranges of node that is *parent* for host bridge. Host bridge
    currently does not control this at all.

    In current device trees no dma-ranges is defined for nodes that are
    parents to pci host bridges. This will make of_dma_configure() to fall
    back to 32-bit size for all devices on all current platforms. Thus
    applying this patch will immediately break 64-bit dma masks on all
    hardware that supports it.


    Also related: dma-ranges property used by several pci host bridges is
    *not* compatible with "legacy" dma-ranges parsed by of_get_dma_range() -
    former uses additional flags word at beginning.

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