[PATCH 2/3] iommu/io-pgtable-arm: Add IOMMU_LLC page protection flag

[Sai Prakash Ranjan]

Hi Rob,

On 2021-03-16 22:46, Rob Clark wrote:

<snip>...

>> > >
>> > > When the GPU has a buffer mapped with IOMMU_LLC, is the buffer also mapped
>> > > into the CPU and with what attributes? Rob said "writecombine for
>> > > everything" -- does that mean ioremap_wc() / MEMREMAP_WC?
>> >
>> > Currently userspace asks for everything WC, so pgprot_writecombine()
>> >
>> > The kernel doesn't enforce this, but so far provides no UAPI to do
>> > anything useful with non-coherent cached mappings (although there is
>> > interest to support this)
>> >
>> 
>> btw, I'm looking at a benchmark (gl_driver2_off) where (after some
>> other in-flight optimizations land) we end up bottlenecked on writing
>> to WC cmdstream buffers.  I assume in the current state, WC goes all
>> the way to main memory rather than just to system cache?
>> 
> 
> oh, I guess this (mentioned earlier in thread) is what I really want
> for this benchmark:
> 
> https://android-review.googlesource.com/c/kernel/common/+/1549097/3
> 

You can also check if the system cache lines are allocated for GPU
or not with patch in https://crrev.com/c/2766723

With the above patch applied,
cat /sys/kernel/debug/llcc_stats/llcc_scid_status

The SCIDs for GPU are listed in include/linux/soc/qcom/llcc-qcom.h

Thanks,
Sai

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