Re: [Freedreno] [PATCH v3 0/2] iommu/arm-smmu: Split pagetable support

[Jordan Crouse <jcrouse@codeaurora.org>]

On Fri, Aug 16, 2019 at 08:43:53PM +0100, Robin Murphy wrote:
> On 16/08/2019 19:12, Rob Clark wrote:
> >On Fri, Aug 16, 2019 at 9:58 AM Robin Murphy <robin.murphy@arm.com> wrote:
> >>
> >>Hi Jordan,
> >>
> >>On 15/08/2019 16:33, Jordan Crouse wrote:
> >>>On Wed, Aug 07, 2019 at 04:21:38PM -0600, Jordan Crouse wrote:
> >>>>(Sigh, resend. I freaked out my SMTP server)
> >>>>
> >>>>This is part of an ongoing evolution for enabling split pagetable support for
> >>>>arm-smmu. Previous versions can be found [1].
> >>>>
> >>>>In the discussion for v2 Robin pointed out that this is a very Adreno specific
> >>>>use case and that is exactly true. Not only do we want to configure and use a
> >>>>pagetable in the TTBR1 space, we also want to configure the TTBR0 region but
> >>>>not allocate a pagetable for it or touch it until the GPU hardware does so. As
> >>>>much as I want it to be a generic concept it really isn't.
> >>>>
> >>>>This revision leans into that idea. Most of the same io-pgtable code is there
> >>>>but now it is wrapped as an Adreno GPU specific format that is selected by the
> >>>>compatible string in the arm-smmu device.
> >>>>
> >>>>Additionally, per Robin's suggestion we are skipping creating a TTBR0 pagetable
> >>>>to save on wasted memory.
> >>>>
> >>>>This isn't as clean as I would like it to be but I think that this is a better
> >>>>direction than trying to pretend that the generic format would work.
> >>>>
> >>>>I'm tempting fate by posting this and then taking some time off, but I wanted
> >>>>to try to kick off a conversation or at least get some flames so I can try to
> >>>>refine this again next week. Please take a look and give some advice on the
> >>>>direction.
> >>>
> >>>Will, Robin -
> >>>
> >>>Modulo the impl changes from Robin, do you think that using a dedicated
> >>>pagetable format is the right approach for supporting split pagetables for the
> >>>Adreno GPU?
> >>
> >>How many different Adreno drivers would benefit from sharing it?
> >
> >Hypothetically everything back to a3xx, so I *could* see usefulness of
> >this in qcom_iommu (or maybe even msm-iommu).  OTOH maybe with
> >"modularizing" arm-smmu we could re-combine qcom_iommu and arm-smmu.
> 
> Indeed, that's certainly something I'm planning to investigate as a future
> refactoring step.
> 
> >And as a practical matter, I'm not sure if anyone will get around to
> >backporting per-context pagetables as far back as a3xx.
> >
> >BR,
> >-R
> >
> >>The more I come back to this, the more I'm convinced that io-pgtable
> >>should focus on the heavy lifting of pagetable management - the code
> >>that nobody wants to have to write at all, let alone more than once -
> >>and any subtleties which aren't essential to that should be pushed back
> >>into whichever callers actually care. Consider that already, literally
> >>no caller actually uses an unmodified stage 1 TCR value as provided in
> >>the io_pgtable_cfg.
> >>
> >>I feel it would be most productive to elaborate further in the form of
> >>patches, so let me get right on that and try to bash something out
> >>before I go home tonight...
> 
> ...and now there's a rough WIP branch here:
> 
> http://linux-arm.org/git?p=linux-rm.git;a=shortlog;h=refs/heads/iommu/pgtable
> 
> I'll finish testing and polishing those patches at some point next week,
> probably, but hopefully they're sufficiently illustrative for the moment.

This looks great so far. I can see where the TTBR1 stuff would fit in and I like
it a lot. I'll try to have some patches ready when you are done polishing.

Jordan

> Robin.

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