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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