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    SubjectRe: [PATCH] MIPS: Fixups of ALSA memory maps
    On Tue, Nov 17, 2009 at 10:29:10AM +0100, Takashi Iwai wrote:

    > > > Actually, this has been a looong-standing problem.
    > > > I have a series of patches to fix these issues, but it's more
    > > > intensively involved with dma_*() functions.
    > > >
    > > > The patches can be found in test/dma-fix branch of sound GIT tree.
    > > > git://git.kernel.org/pub/scm/linux/kernel/git/tiwai/sound-2.6.git test/dma-fix
    > > >
    > > > This basically adds dma_mmap_coherent() function to feasible
    > > > architectures, which is already implemented for ARM, so far.
    > >
    > > Cool - but needs a little further tweaking to work right. That's a
    > > solution which will use uncached accesses on all MIPS systems.
    > >
    > > IP27/IP35-family machines will explode when you try that. Eventually the
    > > cache coherency logic will notice that cache, directory caches and memory
    > > have become inconsistent and bombard the CPU with a bunch of nasty
    > > exceptions.
    >
    > OK, that's really bad.

    Hardware designers do such things to you. Often even for a reason.

    > > For cache-coherent machines otoh it's a big waste of performance.
    > >
    > > int dma_mmap_coherent(struct device *dev, struct vm_area_struct *vma,
    > > void *cpu_addr, dma_addr_t handle, size_t size)
    > > {
    > > struct page *pg;
    > >
    > > if (!plat_device_is_coherent(dev))
    > > vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
    > > cpu_addr = (void *)dma_addr_to_virt(handle);
    > > pg = virt_to_page(cpu_addr);
    > >
    > > return remap_pfn_range(vma, vma->vm_start,
    > > page_to_pfn(pg) + vma->vm_pgoff,
    > > size, vma->vm_page_prot);
    > > }
    > > EXPORT_SYMBOL(dma_mmap_coherent);
    > >
    > > Thomas - you're the IP28 specialist. Would the plat_device_is_coherent()
    > > above have to become a cpu_is_noncoherent_r10000() call? Any further
    > > nasties?
    >
    > Thanks for checking!

    You're welcome!

    So basically I'd not mind putting this into the Linux/MIPS tree; we still
    can iron out the kinks from there on and probably much better than by
    having arch stuff in the ALSA tree.

    I recall this new API having been posted for discussion to linux-arch.
    What was the outcome? I'd only like to add a new API if the other arch
    maintainers see it fit their needs also.

    Ralf


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