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

[Robin Murphy]

On 16/08/2019 19:12, Rob Clark wrote:
> On Fri, Aug 16, 2019 at 9:58 AM Robin Murphy <robin.murphy@xxxxxxx> 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.

Robin.