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    SubjectRe: [Linux-c6x-dev] [PATCH 3/9] c6x: Provide dma_mmap_coherent() and dma_get_sgtable()
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    On Tue, Jan 15, 2013 at 5:56 PM, James Bottomley
    <James.Bottomley@hansenpartnership.com> wrote:
    > On Tue, 2013-01-15 at 15:07 +0100, Marek Szyprowski wrote:
    >> On 1/15/2013 10:13 AM, Geert Uytterhoeven wrote:
    >> > Marek?
    >> >
    >> > On Tue, Jan 15, 2013 at 5:16 AM, Vineet Gupta
    >> > <Vineet.Gupta1@synopsys.com> wrote:
    >> > > On Monday 14 January 2013 09:07 PM, Mark Salter wrote:
    >> > >> On Sun, 2013-01-13 at 11:44 +0100, Geert Uytterhoeven wrote:
    >> > >>> c6x/allmodconfig (assumed):
    >> > >>>
    >> > >>> drivers/media/v4l2-core/videobuf2-dma-contig.c: In function ‘vb2_dc_mmap’:
    >> > >>> drivers/media/v4l2-core/videobuf2-dma-contig.c:204: error: implicit declaration of function ‘dma_mmap_coherent’
    >> > >>> drivers/media/v4l2-core/videobuf2-dma-contig.c: In function ‘vb2_dc_get_base_sgt’:
    >> > >>> drivers/media/v4l2-core/videobuf2-dma-contig.c:387: error: implicit declaration of function ‘dma_get_sgtable’
    >> > >>>
    >> > >>> For architectures using dma_map_ops, dma_mmap_coherent() and
    >> > >>> dma_get_sgtable() are provided in <asm-generic/dma-mapping-common.h>.
    >> > >>>
    >> > >>> C6x does not use dma_map_ops, hence it should implement them as inline
    >> > >>> stubs using dma_common_mmap() and dma_common_get_sgtable().
    >> > >>>
    >> > >> So are dma_mmap_coherent() and dma_get_sgtable() part of the DMA API
    >> > >> now? I don't them in Documentation/DMA*.txt anywhere.
    >> > >>
    >> > >> Why does the default dma_common_mmap() for !CONFIG_MMU return an
    >> > >> error?
    >> > >>
    >> > >> Wouldn't it be better to provide default implementations that an arch
    >> > >> could override rather than having to patch all "no dma_map_ops"
    >> > >> architectures?
    >> > >>
    >> > > Speaking for the still-reviewed ARC Port, I completely agree with Mark.
    >>
    >> dma_mmap_coherent() was partially in the DMA mapping API for some time, but
    >> it was available only on a few architectures (afair ARM, powerpc and avr32).
    >> This caused significant problems for writing unified device drivers or some
    >> device helper modules which were aimed to work on more than one
    >> architecture.
    >>
    >> dma_get_sgtable() is an extension discussed during the Linaro meetings. It
    >> is required to correctly implement buffer sharing between device driver
    >> without hacks or any assumptions about memory layout in the device drivers.
    >>
    >> I have implemented some generic code for both of those two functions,
    >> keeping
    >> in mind that on some hardware architectures (like already mentioned VIVT)
    >> it might be not possible to provide coherent mapping to userspace. It is
    >> perfectly fine for those functions to return an error in such case.
    >
    > It's not possible on VIPT either. This means that the API is unusable
    > on quite a large number of architectures. Surely, if we're starting to
    > write drivers using this, we need to fix the API before more people try
    > to use it.
    >
    > For PA-RISC (and all other VIPT, I assume) I need an API which allows me
    > to remap the virtual address of the kernel component (probably using the
    > kmap area) so the user space and kernel space addresses are congruent.

    So what are we gonna do for 3.8? I'd like to get my allmodconfig build
    green again ;-)

    Change the API?

    Keep the API but do a best-effort fix to unbreak the builds?
    - Apply my patches that got acks (avr32/blackfin/cris/m68k),
    - Use static inlines that return -EINVAL for the rest
    (c6x/frv/mn10300/parisc/xtensa).
    I still have a few m68k fixes queued for 3.8, for which I've been postponing the
    pull request to get this sorted out, so I could include the above.

    Any other solution?

    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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