Re: [PATCH v1 2/6] arm/smmu: Add auxiliary domain support for arm-smmuv2

[Will Deacon]

On Mon, May 18, 2020 at 08:50:27AM -0700, Rob Clark wrote:
> On Mon, May 18, 2020 at 8:18 AM Will Deacon <will@xxxxxxxxxx> wrote:
> > On Wed, Mar 18, 2020 at 04:43:07PM -0700, Rob Clark wrote:
> > > We do in fact need live domain switching, that is really the whole
> > > point.  The GPU CP (command processor/parser) is directly updating
> > > TTBR0 and triggering TLB flush, asynchronously from the CPU.
> > >
> > > And I think the answer about ASID is easy (on current hw).. it must be zero[*].
> >
> > Using ASID zero is really bad, because it means that you will end up sharing
> > TLB entries with whichever device is using context bank 0.
> >
> > Is the SMMU only used by the GPU in your SoC?
> >
> 
> yes, the snapdragon SoCs have two SMMU instances, one used by the GPU,
> where ASID0/cb0 is the gpu itself, and another cb is the GMU
> (basically power control for the gpu), and the second SMMU is
> everything else.

Right, in which case I'm starting to think that we should treat this GPU
SMMU instance specially. Give it its own compatible string (looks like you
need this for HUPCFG anyway) and hook in via arm_smmu_impl_init(). You can
then set IO_PGTABLE_QUIRK_ARM_TTBR1 when talking to the io-pgtable code
without having to add a domain attribute.

With that. you'll need to find a way to allow the GPU driver to call into
your own hooks for getting at the TTBR0 tables -- given that you're
programming these in the hardware, I don't think it makes sense to expose
that in the IOMMU API, since most devices won't be able to do anything with
that data. Perhaps you could install a couple of function pointers
(subdomain_alloc/subdomain_free) in the GPU device when you see it appear
from the SMMU driver? Alternatively, you could make an io_pgtable_cfg
available so that the GPU driver can interface with io-pgtable directly.

Yes, it's ugly, but I don't think it's worth trying to abstract this.

Thoughts? It's taken me a long time to figure out what's going on here,
so sorry if it feels like I'm leading you round the houses.

Will