Re: [RFC 0/2] iommu: arm-smmu: Add support for early direct mappings

[Will Deacon <will-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>]

On Wed, Jul 01, 2020 at 12:40:50AM -0700, Bjorn Andersson wrote:
> On Wed 03 Jun 04:00 PDT 2020, Robin Murphy wrote:
> > at that point I'm inclined to suggest we give up and stop trying to
> > drive these things with arm-smmu. The XZR thing was bad enough, but if
> > they're not even going to pretend to implement the architecture correctly
> > then I'm not massively keen to continue tying the architectural driver in
> > further knots if innocent things like CONFIG_IOMMU_DEFAULT_PASSTHROUGH are
> > going to unexpectedly and catastrophically fail. We have qcom-iommu for
> > hypervisor-mediated SMMUs, and this new hypervisor behaviour sounds to me
> > more like "qcom-iommu++" with reassignable stream-to-context mappings,
> > rather than a proper Arm SMMU emulation.
> > 
> 
> I've been going through over and over, hoping to perhaps be able to
> evolve qcom_iommu into a qcom-iommu++, but afaict the new hypervisor is
> different enough that this isn't feasible. In particular, the platforms
> using qcom_iommu relies entirely on the hypervisor to configure stream
> mapping etc - and we can't even read most of the registers.
> 
> On the other hand I agree with you that we're messing around quite a bit
> with the arm-smmu driver, and I'm uncertain where we are on supporting
> the various GPU features, so I'm adding Jordan to the thread.
> 
> So, afaict we have the options of either shoehorning this too into the
> arm-smmu driver or we essentially fork arm-smmu.c to create a
> qcom-smmu.c.
> 
> While I don't fancy the code duplication, it would allow us to revert
> the Qualcomm quirks from arm-smmu and would unblock a number of
> activities that we have depending on getting the SMMU enabled on various
> platforms.

We added the impl hooks to cater for implementation differences, so I'd
still prefer to see this done as part of arm-smmu than introduce another
almost-the-same-but-not-quite IOMMU driver that has the lifetime of
a single SoC.

Will