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    SubjectRe: mmaping a kernel buffer to user space
    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.


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      Last update: 2006-10-31 17:17    [W:0.021 / U:34.392 seconds]
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