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    SubjectRe: mmaping a kernel buffer to user space
    Hi Rolf,

    Thanks for your comments. Unfortunately, given some of the platforms we
    want to support, I cannot reserve memory at boot time, so I had to find
    some other way. Besides, it is anyway interesting to see how to present
    a buffer allocated using the DMA interfaces (pci_alloc or dma_alloc) to
    the user space.

    All the best,
    Guillermo

    Rolf Offermanns wrote:
    > Guillermo Marcus wrote:
    >> I recently run with the following situation while developing a PCI
    >> driver. The driver allocates memory for a PCI device using
    >> pci_alloc_consistent as this memory is going to be used to perform DMA
    >> transfers. To pass the data from/to the user application, I mmap the
    >> buffer into userspace. However, if I try to use remap_pfn_range
    >> (>=2.6.10) or the older remap_page_range(<=2.6.9) for mmaping, it ends
    >> up creating a new buffer, because they do not support RAM mapping, then
    >> pagefaulting to the VMA and by default allocating new pages. Therefore,
    >> I had to implement the nopage method and mmap one page at a time as they
    >> fault.
    >>
    >> However, to my point of view, this is unnecessary. The memory is already
    >> allocated, the memory is locked because it is consistent, and it may be
    >> a (very small) performance and stability issue to do them one-by-one.
    >> Why can't I simply mmap it all at once? am I missing some function? More
    >> important, why can't remap_{pfn/page}_range handle it?
    >>
    > Here is what I did some time ago:
    >
    > -> Reserve mem at boot time (mem=realmem-size_of_mem_you_need) / bigphysmem
    > -> I used the highmem allocator from the LDD2/3 examples to get a pointer
    > the this reserved memory at runtime.
    > -> Use ioremap() to remap the memory to kernelspace
    > -> do some magic (I don't remember the background, sorry) with the vma_flags
    > in your mmap() function:
    >
    > vma->vm_flags |= VM_RESERVED;
    > vma->vm_page_prot = pgprot_noncached(vma->vm_page_prot);
    >
    > and then do a remap_pfn_range() as usualy.
    >
    > HTH,
    > Rolf
    >
    >
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