[Intel-gfx] [PATCH] drm/i915: Force pte cacheline to main memory

[Chris Wilson]

Quoting Mika Kuoppala (2020-05-11 17:08:03)
> We have problems of tgl not seeing a valid pte entry
> when iommu is enabled. Add heavy handed flushing
> of entry modification by flushing the cpu, cacheline
> and then wcb. This forces the pte out to main memory
> past this point regarless of promises of coherency.
> 
> This is an evolution of an experimental patch from
> Chris Wilson of adding wmb for coherent partners,
> by adding a clflush to force the cache->memory step.
> 
> Testcase: igt/gem_exec_fence/parallel
> Cc: Chris Wilson <chris at chris-wilson.co.uk>
> Signed-off-by: Mika Kuoppala <mika.kuoppala at linux.intel.com>

Not only does it help tgl, but it is also helping with a coherency
problem on Braswell. We see similar problems on gen9 and icl, and I have
a trybot run to see if it helps with those.

As it is helping with multiple platforms and diverse symptoms, even if
we can't explain why it helps, it is. That makes it prudent to apply to
improve the baseline and work from there.

Reviewed-by: Chris Wilson <chris at chris-wilson.co.uk>
-Chris