[PATCH] of/device: add blacklist for iommu dma_ops

[Jordan Crouse]

> It shouldn't be a problem to hook something else up to the IOMMU
> subsystem. Hopefully it's something that people are going to standardize
> on.
> 
> > 3) The automatic attach of DMA domain is also causing a different
> >    problem for us on the GPU side, preventing us from supporting per-
> >    context pagetables (since we end up with a disagreement about
> >    which context bank is used between arm-smmu and the firmware).
> 
> I'm not sure I understand this issue. Is the context bank hard-coded in
> the firmware somehow? Or is it possible to rewrite which one is going to
> be used at runtime? Do you switch out the actual page tables rather than
> the IOMMU domains for context switching?
 
We have a rather long history on this but the tl;dr is that the GPU microcode
switches the pagetables by rewriting TTBR0 on the fly (since this is
arm-smmu-v2 we have no better option) and yes, unfortunately it is hard coded
to use context bank 0. [1] is the current patchset to support all this,
including my own take on avoiding the dma-domain (all the cool kids have one).

Jordan

[1] https://patchwork.freedesktop.org/series/57441/

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