Re: [PATCH 2/2] drm/i915: drop the guard page at the end of the gtt

[Daniel Vetter <daniel@ffwll.ch>]

On Mon, Jan 30, 2012 at 04:55:49PM +0100, Daniel Vetter wrote:
> Chris Wilson reported that with a bunch of patches to no longer force
> batchbuffer (in most cases at least) into the mappable part of gtt
> that his Pineview died while prefetching the last page of the gtt.
> 
> Turns out that since my intel-gtt rewrite we don't actually put a
> dummy pte in that last page anymore. Which left me rather puzzled
> because Chris' error_state was the first ever since that rewrite that
> indicated a dead gpu due to prefetch.
> 
> So I've gone ahead and created the gem_cs_prefetch testcase which
> reliably manages to execute a batch on the last page. Of all the
> machines Chris and I have tried this on only his Pineview fell over,
> all the others handled the invalid pte right after the end of the
> batch correctly.
> 
> The other thing is that due to my intel-gtt we've also started to use
> the non-mappable part of the gtt. So my theory is that this guard page
> was only necessary when we didn't use and hence also didn't set up any
> dummy ptes to the scratch page in that area. In that case the cs would
> prefetch into the unmappable gtt area and fail over on the invalid
> pte.
> 
> So given that this guard page smells like it just duct-taped over an
> issue in our code I've simply removed it. And this seems to work!
> 
> Now we always run the risk that an odd machine we don't have in our
> test labs needs this, but
> - we now have an excellent testcase to diagnose any such issues
> - and the patch is easy to revert.
> Hence I prefer if we try to get rid of this.
> 
> Tested-by: Chris Wilson <chris@chris-wilson.co.uk>
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=44748
> Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

Ok, I've seriously botched things up at testing. This actually blows up my
g33 (which works fine without the patch), so we need that darn guard page.

I'll fix this mess up.
-Daniel
-- 
Daniel Vetter
Mail: daniel@ffwll.ch
Mobile: +41 (0)79 365 57 48