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    SubjectRe: [PATCH 3/4] dma-buf: add support for mapping with dma mapping attributes
    On Mon, 21 Jan 2019, Christoph Hellwig wrote:

    > On Sat, Jan 19, 2019 at 08:50:41AM -0800, Laura Abbott wrote:
    > > > And who is going to decide which ones to pass? And who documents
    > > > which ones are safe?
    > > >
    > > > I'd much rather have explicit, well documented dma-buf flags that
    > > > might get translated to the DMA API flags, which are not error checked,
    > > > not very well documented and way to easy to get wrong.
    > > >
    > >
    > > I'm not sure having flags in dma-buf really solves anything
    > > given drivers can use the attributes directly with dma_map
    > > anyway, which is what we're looking to do. The intention
    > > is for the driver creating the dma_buf attachment to have
    > > the knowledge of which flags to use.
    >
    > Well, there are very few flags that you can simply use for all calls of
    > dma_map*. And given how badly these flags are defined I just don't want
    > people to add more places where they indirectly use these flags, as
    > it will be more than enough work to clean up the current mess.
    >
    > What flag(s) do you want to pass this way, btw? Maybe that is where
    > the problem is.
    >

    The main use case is for allowing clients to pass in
    DMA_ATTR_SKIP_CPU_SYNC in order to skip the default cache maintenance
    which happens in dma_buf_map_attachment and dma_buf_unmap_attachment. In
    ION the buffers aren't usually accessed from the CPU so this allows
    clients to often avoid doing unnecessary cache maintenance.


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