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On Wed, 2019-10-23 at 16:42 +1100, Michael Ellerman wrote:
> 
> Right, it seems of_dma_is_coherent() has baked in the assumption that
> devices are non-coherent unless explicitly marked as coherent.
> 
> Which is wrong on all or at least most existing powerpc systems
> according to Ben.

This is probably broken on sparc(64) as well and whatever else uses
DT and is an intrinsicly coherent architecture (did we ever have
DT enabled x86s ? Wasn't OLPC such a beast ?).

I think this should have been done the other way around and default to
coherent since most traditional OF platforms are coherent, and you
can't just require those DTs to change.

> > Any ideas from the PPC maintainers?
> 
> Fixing it at the source seems like the best option to prevent future
> breakage.
> 
> So I guess that would mean making of_dma_is_coherent() return true/false
> based on CONFIG_NOT_COHERENT_CACHE on powerpc.
> 
> We could do it like below, which would still allow the dma-coherent
> property to work if it ever makes sense on a future powerpc platform.
> 
> I don't really know any of this embedded stuff well, so happy to take
> other suggestions on how to handle this mess.