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

[Rob Clark]

On Wed, Mar 17, 2021 at 2:33 AM Sai Prakash Ranjan
<saiprakash.ranjan at codeaurora.org> wrote:
>
> 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
>

Actually for the benchmark I was referring to, it is the *CPU*
bottlenecked on writes to writecombine mappings.. so I think what I
want is for CPU mappings to be able to use systemcache..

BR,
-R