[igt-dev] [PATCH] selftests/i915_gem_gtt: Create igt_ggtt_scratch subtest

[Chris Wilson]

Quoting Ewelina Musial (2018-04-11 11:20:56)
> On Wed, Apr 11, 2018 at 09:48:21AM +0100, Chris Wilson wrote:
> > Quoting Ewelina Musial (2018-04-11 09:27:12)
> > > Test have similar functionality that gem_cs_prefetch IGT test
> > > but gem_cs_prefetch is not up to date and there was an idea
> > > to move this test to kselftests so this is respond for this
> > > request.
> > 
> > gem_cs_prefetch itself does one thing: verify that we cannot cross the
> > page boundary beyond the last page of the GTT. It is about the Command
> > Streamer prefetch; there is no command streamer here.
> > 
> > gem_cs_prefetch could be simplified by EXEC_OBJECT_PINNED, I think the
> > notes got muddled up.
> > -Chris
> 
> Probably you are right. I focused on some other point of view. My test is checking
> that if we cross the boundary beyond the last page values are stored in scratch.

If we cross the boundary beyond the end of the last page of the GTT, there are no
more pages. Hitting scratch is not a sensible test; scratch is just a
figment of the imagination, the only reason it may exist in some
circumstances is to prevent page faults. And other than vtd w/a that
was a bad idea.
-Chris