[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.