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

On Mon, Aug 9, 2021 at 11:11 AM Sai Prakash Ranjan
[off-list ref] wrote:

On 2021-08-09 23:37, Rob Clark wrote:

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On Mon, Aug 9, 2021 at 10:47 AM Sai Prakash Ranjan
[off-list ref] wrote:

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On 2021-08-09 23:10, Will Deacon wrote:

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On Mon, Aug 09, 2021 at 10:18:21AM -0700, Rob Clark wrote:

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On Mon, Aug 9, 2021 at 10:05 AM Will Deacon [off-list ref] wrote:

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On Mon, Aug 09, 2021 at 09:57:08AM -0700, Rob Clark wrote:

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On Mon, Aug 9, 2021 at 7:56 AM Will Deacon [off-list ref] wrote:

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On Mon, Aug 02, 2021 at 06:36:04PM -0700, Rob Clark wrote:

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On Mon, Aug 2, 2021 at 8:14 AM Will Deacon [off-list ref] wrote:

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On Mon, Aug 02, 2021 at 08:08:07AM -0700, Rob Clark wrote:

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On Mon, Aug 2, 2021 at 3:55 AM Will Deacon [off-list ref] wrote:

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On Thu, Jul 29, 2021 at 10:08:22AM +0530, Sai Prakash Ranjan wrote:

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On 2021-07-28 19:30, Georgi Djakov wrote:

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On Mon, Jan 11, 2021 at 07:45:02PM +0530, Sai Prakash Ranjan wrote:

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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 :(

Right I was aware of that but even in the android tree there is no
user
:)
I think we can't have a new memory type without any user right in
upstream
like android tree?

@Rob, I think you  already tried adding a new MT and used
pgprot_syscached()
in GPU driver but it was crashing?

Correct, but IIRC there were some differences in the code for memory
types compared to the android tree.. I couldn't figure out the
necessary patches to cherry-pick to get the android patch to apply
cleanly, so I tried re-implementing it without having much of a clue
about how that code works (which was probably the issue) ;-)

Hehe no, even I get the same crash after porting/modifying the required
patches from android ;) and I think crashes would be seen in android as
well, its just that they don't have any user exercising that code.

Thing is I can't make head and tail of the GPU crash logs, maybe you
know
how to decode those errors, if not I can start a thread with QC GPU team
and ask them to decode?

If you have a gpu devcore dump, I can take a look at it with
crashdec.. otherwise I can try to find the branch where I had that
patch backported.

I'm more familiar with using crashdec to figure out mesa bugs, but
maybe I could spot something where what the GPU is seeing disagrees
with what the CPU expects it to be seeing.

BR,
-R

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