Messages in this thread | | | Subject | Re: [Freedreno] [PATCH v3 0/2] iommu/arm-smmu: Split pagetable support | From | Robin Murphy <> | Date | Fri, 16 Aug 2019 20:43:53 +0100 |
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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.
Robin.
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