Question about dma_alloc_coherent and iommu

Question about dma_alloc_coherent and iommu

[Robert Walsh]

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Hi there,

Our driver (the ipath driver in drivers/infiniband) uses
dma_alloc_coherent to allocate buffers that our device will DMA into.
We use a 64-bit DMA mask.  Although we don't set a consistent DMA mask,
it doesn't seem to matter whether we do or not for the following
problem.

Everything works fine on 2.6.16: when the device tells us it's DMA'd
into a buffer, the data is there and everyone is happy.  It also works
fine on kernels from 2.6.9 through 2.6.15, but only as long as there is
4GB or less of memory on the machine.  On our machines with > 4GB
memory, the buffers do not appear to have had anything DMA'd into them.

If we use "iommu=off" on the kernel boot line, the problem goes away and
the data starts appearing.

Has anyone seen this behaviour on other devices?  Are we missing
something in the device setup, maybe?  As I mentioned, setting a
consistent DMA mask doesn't help, no matter whether we set it to 64-bits
or 32-bits.  Setting the regular DMA mask to 32-bits doesn't help
either: the problem still crops up.

Regards,
 Robert.

-- 
Robert Walsh
Amalgamated Durables, Inc.  -  "We don't make the things you buy."
Email: rjwalsh@durables.org

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