[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 #6 from Chris Wilson <chris at chris-wilson.co.uk> ---
(In reply to Martin Peres from comment #5)
> (In reply to Chris Wilson from comment #4)
> > For a similar issue within the kernel (i.e. not this test but others):
> > commit c5ba5b24657e473b1c64b0a614b168a635a2c935
> > Author: Chris Wilson <chris at chris-wilson.co.uk>
> > Date:   Thu Sep 7 19:45:20 2017 +0100
> > 
> >     drm/i915: Apply the GTT write flush for all !llc machines
> >     
> >     We also see the delayed GTT write issue on i915g/i915gm, so let's
> >     presume that it is a universal problem for all !llc machines, and that we
> >     just haven't yet noticed on g33, gen4 and gen5 machines.
> 
> This does not seem to be sufficient...

It's for the same issue but inside the kernel (should show up in the pwrite
tests, some reloc tests). Here userspace is able to expose the coherency issue
using two mmaps that are expected to coherent, there's no kernel intervention
in the test; there's an equivalent test in gem_mmap_gtt to show how the same
issue can be detected during pure i915 mmappings.

It is theorectically possible to prevent it, we would have to trap every fault
and do exclusive domain management. We would have to intercept all contending
mmaps -- challenging.

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