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

[Sai Prakash Ranjan]

On 2021-08-09 23:37, Rob Clark wrote:
> On Mon, Aug 9, 2021 at 10:47 AM Sai Prakash Ranjan
> <saiprakash.ranjan at codeaurora.org> wrote:
>> 
>> On 2021-08-09 23:10, Will Deacon wrote:
>> > 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 at 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 at 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 at 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 at 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 :(
>> >
>> 
>> 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?

Thanks,
Sai

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