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    SubjectRE: [PATCH 00/14] DMA-mapping framework redesign preparation

    On Friday, December 23, 2011 5:35 PM Matthew Wilcox wrote:

    > On Fri, Dec 23, 2011 at 01:27:19PM +0100, Marek Szyprowski wrote:
    > > The first issue we identified is the fact that on some platform (again,
    > > mainly ARM) there are several functions for allocating DMA buffers:
    > > dma_alloc_coherent, dma_alloc_writecombine and dma_alloc_noncoherent
    > Is this write-combining from the point of view of the device (ie iommu),
    > or from the point of view of the CPU, or both?

    It is about write-combining from the CPU point of view. Right now there are
    no devices with such advanced memory interface to do write combining on the
    DMA side, but I believe that they might appear at some point in the future
    as well.

    > > The next step in dma mapping framework update is the introduction of
    > > dma_mmap/dma_mmap_attrs() function. There are a number of drivers
    > > (mainly V4L2 and ALSA) that only exports the DMA buffers to user space.
    > > Creating a userspace mapping with correct page attributes is not an easy
    > > task for the driver. Also the DMA-mapping framework is the only place
    > > where the complete information about the allocated pages is available,
    > > especially if the implementation uses IOMMU controller to provide a
    > > contiguous buffer in DMA address space which is scattered in physical
    > > memory space.
    > Surely we only need a helper which drivrs can call from their mmap routine
    > to solve this?

    On ARM architecture it is already implemented this way and a bunch of drivers
    use dma_mmap_coherent/dma_mmap_writecombine calls. We would like to standardize
    these calls across all architectures.

    > > Usually these drivers don't touch the buffer data at all, so the mapping
    > > in kernel virtual address space is not needed. We can introduce
    > > DMA_ATTRIB_NO_KERNEL_MAPPING attribute which lets kernel to skip/ignore
    > > creation of kernel virtual mapping. This way we can save previous
    > > vmalloc area and simply some mapping operation on a few architectures.
    > I really think this wants to be a separate function. dma_alloc_coherent
    > is for allocating memory to be shared between the kernel and a driver;
    > we already have dma_map_sg for mapping userspace I/O as an alternative
    > interface. This feels like it's something different again rather than
    > an option to dma_alloc_coherent.

    That is just a starting point for the discussion.

    I thought about this API a bit and came to conclusion that there is no much
    difference between a dma_alloc_coherent which creates a mapping in kernel
    virtual space and the one that does not. It is just a hint from the driver
    that it will not use that mapping at all. Of course this attribute makes sense
    only together with adding a dma_mmap_attrs() call, because otherwise drivers
    won't be able to get access to the buffer data.

    On coherent architectures where dma_alloc_coherent is just a simple wrapper
    around alloc_pages_exact() such attribute can be simply ignored without any
    impact on the drivers (that's the main idea behind dma attributes!).
    However such hint will help a lot on non-coherent architectures where
    additional work need to be done to provide a cohenent mapping in kernel
    address space. It also saves some precious kernel resources like vmalloc
    address range.

    Best regards
    Marek Szyprowski
    Samsung Poland R&D Center

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