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

[Rob Clark]

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.
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...
>
> Robin.
>
> > If so, then is adding the changes to io-pgtable-arm.c possible for 5.4 and then
> > add the implementation specific code on top of Robin's stack later or do you
> > feel they should come as part of a package deal?
> >
> > Jordan
> >
> >> Jordan Crouse (2):
> >>    iommu/io-pgtable-arm: Add support for ARM_ADRENO_GPU_LPAE io-pgtable
> >>      format
> >>    iommu/arm-smmu: Add support for Adreno GPU pagetable formats
> >>
> >>   drivers/iommu/arm-smmu.c       |   8 +-
> >>   drivers/iommu/io-pgtable-arm.c | 214 ++++++++++++++++++++++++++++++++++++++---
> >>   drivers/iommu/io-pgtable.c     |   1 +
> >>   include/linux/io-pgtable.h     |   2 +
> >>   4 files changed, 209 insertions(+), 16 deletions(-)
> >>
> >> --
> >> 2.7.4
> >>
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> >
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