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

From: Jordan Crouse
Date: Mon Jun 03 2019 - 10:52:45 EST

> 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).



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