Messages in this thread | | | From | Rolf Offermanns <> | Subject | Re: mmaping a kernel buffer to user space | Date | Tue, 31 Oct 2006 17:10:47 +0100 |
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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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