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

[Robin Murphy]

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?

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
> >
> > _______________________________________________
> > Freedreno mailing list
> > Freedreno@xxxxxxxxxxxxxxxxxxxxx
> > https://lists.freedesktop.org/mailman/listinfo/freedreno