noveau vs arm dma ops

[hch@infradead.org (Christoph Hellwig)]

[discussion about this patch, which should have been cced to the iommu
 and linux-arm-kernel lists, but wasn't:
 https://www.spinics.net/lists/dri-devel/msg173630.html]

On Wed, Apr 25, 2018 at 09:41:51AM +0200, Thierry Reding wrote:
> > API from the iommu/dma-mapping code.  Drivers have no business poking
> > into these details.
> 
> The interfaces that the above patch uses are all EXPORT_SYMBOL_GPL,
> which is rather misleading if they are not meant to be used by drivers
> directly.

The only reason the DMA ops are exported is because get_arch_dma_ops
references (or in case of the coherent ones used to reference).  We
don't let drivers assign random dma ops.

> 
> > Thierry, please resend this with at least the iommu list and
> > linux-arm-kernel in Cc to have a proper discussion on the right API.
> 
> I'm certainly open to help with finding a correct solution, but the
> patch above was purposefully terse because this is something that I
> hope we can get backported to v4.16 to unbreak Nouveau. Coordinating
> such a backport between ARM and DRM trees does not sound like something
> that would help getting this fixed in v4.16.

Coordinating the backport of a trivial helper in the arm tree is not
the end of the world.  Really, this cowboy attitude is a good reason
why graphics folks have such a bad rep.  You keep poking into random
kernel internals, don't talk to anoyone and then complain if people
are upset.  This shouldn't be surprising.

> Granted, this issue could've been caught with a little more testing, but
> in retrospect I think it would've been a lot better if ARM_DMA_USE_IOMMU
> was just enabled unconditionally if it has side-effects that platforms
> don't opt in to but have to explicitly opt out of.

Agreed on that count.  Please send a patch.