On 02/04/2015 09:15 AM, Joerg Roedel wrote:
On Mon, Feb 02, 2015 at 10:35:36AM -0700, Alex Williamson wrote:
[cc +joerg]
On Mon, 2015-02-02 at 11:01 -0500, Mark Hounschell wrote:
From what I understand of CMA, and it seems provable to me, is that
dma_alloc_coherent allocates my 8MB buffer from CMA defined on the
cmdline. Without CMA specified on the cmdline, dma_alloc_coherent
definitely fails to allocate an 8MB contiguous buffer. From what I've
read about it, it is supposed to transparently "just work" when
dma_alloc_coherent is used?
Yes, if you're running with the software iotlb (aka. bounce buffers),
then dma_ops->alloc is x86_swiotlb_alloc_coherent(), which calls
dma_generic_alloc_coherent(), which attempts to use CMA via
dma_alloc_from_contiguous().
If you look at the same path with AMD-Vi, dma_ops->alloc is
amd_iommu.c:alloc_coherent(), which simply uses __get_free_pages() to
allocate the buffer. I don't see any CMA integration along that path.
If you were using Intel VT-d, then the buffer is again allocated with
dma_alloc_from_contiguous() and should use CMA. This was added in
kernel v3.16, but no corresponding AMD-Vi change was added. Joerg, this
might be an easily fixed oversight.
Right, CMA is not yet integrated in the AMD IOMMU driver, nobody asked
fot it until now :) I will add that fall-back there for big allocations.
Joerg
Hi Joerg,
Is device to device (crossing pci busses) or even pci-e to pci-e support
there or in the future?
Thanks and Regards
Mark
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