[Bug 102577] [BAT][GDG] Failed assertion: gtt[1024*i] == ~i in igt at prime_vgem@basic-gtt

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=102577

--- Comment #1 from Chris Wilson <chris at chris-wilson.co.uk> ---
This is just telling us that the GTT/WC on i915g isn't fully coherent, that a
write into the GTT isn't immediately visible by a read in WC. Very similar to
gem_mmap_gtt/coherency (which also fails) but different timing due to using WC
rather than WB.

Not sure what we can do to prevent userspace falling into this trap. Ban PRIME
on i915g? That seems overkill -- the same coherency issue is there on
byt/bsw/bxt/glk, we just haven't hit it on this path.

Move the fine grained coherency check (i.e. concurrent access to vgem and i915
mmaps) to another test and accept the failure.

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