Re: [Freedreno] [PATCH 0/3] iommu/drm/msm: Allow non-coherent masters to use system cache

[Will Deacon <will@kernel.org>]

On Mon, Aug 09, 2021 at 10:18:21AM -0700, Rob Clark wrote:
> On Mon, Aug 9, 2021 at 10:05 AM Will Deacon <will@kernel.org> wrote:
> >
> > On Mon, Aug 09, 2021 at 09:57:08AM -0700, Rob Clark wrote:
> > > On Mon, Aug 9, 2021 at 7:56 AM Will Deacon <will@kernel.org> wrote:
> > > > On Mon, Aug 02, 2021 at 06:36:04PM -0700, Rob Clark wrote:
> > > > > On Mon, Aug 2, 2021 at 8:14 AM Will Deacon <will@kernel.org> wrote:
> > > > > > On Mon, Aug 02, 2021 at 08:08:07AM -0700, Rob Clark wrote:
> > > > > > > On Mon, Aug 2, 2021 at 3:55 AM Will Deacon <will@kernel.org> wrote:
> > > > > > > > On Thu, Jul 29, 2021 at 10:08:22AM +0530, Sai Prakash Ranjan wrote:
> > > > > > > > > On 2021-07-28 19:30, Georgi Djakov wrote:
> > > > > > > > > > On Mon, Jan 11, 2021 at 07:45:02PM +0530, Sai Prakash Ranjan wrote:
> > > > > > > > > > > commit ecd7274fb4cd ("iommu: Remove unused IOMMU_SYS_CACHE_ONLY flag")
> > > > > > > > > > > removed unused IOMMU_SYS_CACHE_ONLY prot flag and along with it went
> > > > > > > > > > > the memory type setting required for the non-coherent masters to use
> > > > > > > > > > > system cache. Now that system cache support for GPU is added, we will
> > > > > > > > > > > need to set the right PTE attribute for GPU buffers to be sys cached.
> > > > > > > > > > > Without this, the system cache lines are not allocated for GPU.
> > > > > > > > > > >
> > > > > > > > > > > So the patches in this series introduces a new prot flag IOMMU_LLC,
> > > > > > > > > > > renames IO_PGTABLE_QUIRK_ARM_OUTER_WBWA to IO_PGTABLE_QUIRK_PTW_LLC
> > > > > > > > > > > and makes GPU the user of this protection flag.
> > > > > > > > > >
> > > > > > > > > > Thank you for the patchset! Are you planning to refresh it, as it does
> > > > > > > > > > not apply anymore?
> > > > > > > > > >
> > > > > > > > >
> > > > > > > > > I was waiting on Will's reply [1]. If there are no changes needed, then
> > > > > > > > > I can repost the patch.
> > > > > > > >
> > > > > > > > I still think you need to handle the mismatched alias, no? You're adding
> > > > > > > > a new memory type to the SMMU which doesn't exist on the CPU side. That
> > > > > > > > can't be right.
> > > > > > > >
> > > > > > >
> > > > > > > Just curious, and maybe this is a dumb question, but what is your
> > > > > > > concern about mismatched aliases?  I mean the cache hierarchy on the
> > > > > > > GPU device side (anything beyond the LLC) is pretty different and
> > > > > > > doesn't really care about the smmu pgtable attributes..
> > > > > >
> > > > > > If the CPU accesses a shared buffer with different attributes to those which
> > > > > > the device is using then you fall into the "mismatched memory attributes"
> > > > > > part of the Arm architecture. It's reasonably unforgiving (you should go and
> > > > > > read it) and in some cases can apply to speculative accesses as well, but
> > > > > > the end result is typically loss of coherency.
> > > > >
> > > > > Ok, I might have a few other sections to read first to decipher the
> > > > > terminology..
> > > > >
> > > > > But my understanding of LLC is that it looks just like system memory
> > > > > to the CPU and GPU (I think that would make it "the point of
> > > > > coherence" between the GPU and CPU?)  If that is true, shouldn't it be
> > > > > invisible from the point of view of different CPU mapping options?
> > > >
> > > > You could certainly build a system where mismatched attributes don't cause
> > > > loss of coherence, but as it's not guaranteed by the architecture and the
> > > > changes proposed here affect APIs which are exposed across SoCs, then I
> > > > don't think it helps much.
> > > >
> > >
> > > Hmm, the description of the new mapping flag is that it applies only
> > > to transparent outer level cache:
> > >
> > > +/*
> > > + * Non-coherent masters can use this page protection flag to set cacheable
> > > + * memory attributes for only a transparent outer level of cache, also known as
> > > + * the last-level or system cache.
> > > + */
> > > +#define IOMMU_LLC      (1 << 6)
> > >
> > > But I suppose we could call it instead IOMMU_QCOM_LLC or something
> > > like that to make it more clear that it is not necessarily something
> > > that would work with a different outer level cache implementation?
> >
> > ... or we could just deal with the problem so that other people can reuse
> > the code. I haven't really understood the reluctance to solve this properly.
> >
> > Am I missing some reason this isn't solvable?
> 
> Oh, was there another way to solve it (other than foregoing setting
> INC_OCACHE in the pgtables)?  Maybe I misunderstood, is there a
> corresponding setting on the MMU pgtables side of things?

Right -- we just need to program the CPU's MMU with the matching memory
attributes! It's a bit more fiddly if you're just using ioremap_wc()
though, as it's usually the DMA API which handles the attributes under the
hood.

Anyway, sorry, I should've said that explicitly earlier on. We've done this
sort of thing in the Android tree so I assumed Sai knew what needed to be
done and then I didn't think to explain to you :(

Will