Re: dma_alloc_coherent - cma - and IOMMU question

[Joerg Roedel <joro-zLv9SwRftAIdnm+yROfE0A@public.gmane.org>]

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