On Thu, Aug 09, 2018 at 10:01:16AM +1000, Benjamin Herrenschmidt wrote:

On Tue, 2018-07-31 at 14:16 +0200, Christoph Hellwig wrote:

quoted

It turns out cxl actually uses it.  So for now skip this patch,
although random code in drivers messing with dma ops will need to
be sorted out sooner or later.

CXL devices are "special", they bypass the classic iommu in favor of
allowing the device to operate using the main processor page tables
using an MMU context (so basically the device can use userspace
addresses directly), akin to ATS.

I think the code currently uses the nommu ops as a way to do a simple
kernel mapping for kernel drivers using CXL (not userspace stuff)
though.

Its still a horrible idea to have this in drivers/, we need some
core API to mediate this behavior.  Also if the device supports
using virtual addresses dma_nommu_ops seems wrong as it won't do
the right thing for e.g. vmalloc addresses not mapped into the
kernel linear mapping (which I guess can't currently happen on
powerpc, but still..)