Re: [PATCH 1/2] drm/i915: Remove use of the autoreported ringbuffer HEAD position

[Daniel Vetter <daniel@ffwll.ch>]

On Wed, Feb 08, 2012 at 06:17:00PM +0000, Chris Wilson wrote:
> On Wed, 8 Feb 2012 19:02:47 +0100, Daniel Vetter <daniel@ffwll.ch> wrote:
> > The issue is that the first one introduces a pretty decent perf
> > regression, iirc Chris mentions something much large than 10x slowdown
> > on certain cairo traces on sna. So we can only merge the first one
> > together with the second one.
> 
> As it turns out, the cairo-traces were successfully exploiting the
> ringbuffer overflow to avoid work. The reason why it eventually broke is
> that when I experimented with using the BLT and semaphores, I introduced
> a dependency on accurate seqno processing, which was obviously broken by
> the overwriting of commands in the ringbuffer. And so is consistent with
> the hangs reported by Eugeni and Ben, but we have yet to see many more
> examples in the wild. Otoh, I don't think this explains the
> all-generation use-after-free bug.
> 
> Net result, the 0.9s firefox-paintball result was a fantasy and the perf
> gains provided by the second patch are only around 20% for the same
> trace. Ergo -next material only if it passes scrutiny.

Well, that explains a few things. In that case I'm fine with just merging
the first patch for -fixes, I think there're quite a few bugs this does
fix.
-Daniel
-- 
Daniel Vetter
Mail: daniel@ffwll.ch
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