Messages in this thread | | | Date | Tue, 31 Oct 2006 17:33:32 +0100 | From | Guillermo Marcus <> | Subject | Re: mmaping a kernel buffer to user space |
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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 > > > - > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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