[PATCH 2/5] drm/i915/selftests: Always flush before unpining after writing

[Chris Wilson]

Quoting Mika Kuoppala (2020-05-11 14:45:23)
> Chris Wilson <chris at chris-wilson.co.uk> writes:
> 
> > Be consistent, and even when we know we had used a WC, flush the mapped
> > object after writing into it. The flush understands the mapping type and
> > will only flush the WCB if I915_MAP_WC.
> 
> on subject:
> s/unpining/unpinning.
> 
> which flush understands the mapping type?

__i915_gem_object_flush_map() looks the obj->mm.mapping type.

> i915_gem_object_flush_map does, but that check is after
> early return for bo_write_coherence. If the mapping is write
> combined, then we should explicitly wmb early before
> any coherence checks. But if the flush map is for cpu side
> memory coherence only, then I guess the wmb can be omitted.

I was assuming there was a wmb() in there anyway.

> But then the wcb is irrelevant regardless.

We can have one for paranoia.

> So the commit message and the current i915_gem_object_flush_map
> are in disagreement.
> 
> (chipset_flush does explicit wmb tho)

I'm still not sure which direction the HW flushes in its chipset_flush.
I keep thinking we have it entirely wrong and it is flushing residual
writes from the GPU to memory.
-Chris