[openib-general] [RFC/BUG] DMA vs. CQ race

[Roland Dreier]

 >  > On our cell blade + PCI-e Mellanox.
 > 
 > I don't see anything in arch/powerpc that looks like
 > dma_alloc_coherent() will do anything other than allocate some memory
 > and map it with DMA_BIDIRECTIONAL.  So how does this altix fix help in
 > your situation?  Am I misreading the Cell IOMMU code?

Shirley, can you clarify why doing dma_alloc_coherent() in the kernel
helps on your Cell blade?  It really seems that dma_alloc_coherent()
just allocates some memory and then does dma_map(DMA_BIDIRECTIONAL),
which would be exactly the same as allocating the CQ buffer in
userspace and using ib_umem_get() to map it into the kernel.

I'm looking at a possibly cleaner solution to the Altix issue, so I
would like to make sure it fixes whatever the bug on Cell is as well.
So any details you can provide about the problem you see on Cell would
help a lot.

Thanks...