[Mesa-dev] SandyBridge not handling GL_TRIANGLE_STRIP_ADJACENCY with repeating vertex indices correctly

[Kenneth Graunke]

On Friday, August 22, 2014 12:59:57 PM Samuel Iglesias Gonsálvez wrote:
> On Thu, 2014-08-14 at 14:28 +0200, Iago Toral Quiroga wrote:
> [...]
> > At this point I'd like to hear suggestions for things we could try next
> > to confirm whether this is a hardware problem or a driver problem, or,
> > if we agree that this is enough evidence that this must be a hardware
> > problem, how we can limit its impact, starting, probably, by rewriting
> > the piglit test so that we don't alter its purpose but avoid the hang on
> > gen6. We should also discuss if there is a way to work around this
> > problem so that at least developers running into it (as unlikely as that
> > may be) don't hang their systems.
> > 
> > I am going to be on holidays starting tomorrow and will have difficult
> > and limited Internet access for the most part, but Samuel (in the CC)
> > will be available next week to try any suggestions you may have.
> 
> As Iago said, I'm available for trying any suggestions you may have.
> 
> I'm wondering if you need more information about the issue or if I can
> run more tests in order to confirm/discard this is a HW problem and the
> next steps to follow.
> 
> Best regards,
> 
> Sam

I just came across an interesting looking errata...

From the Sandybridge PRM, Volume 2 Part 1, Section 5.5.1 "Partial Object Removal":

"An object is considered ‘partial’ if the last vertex of the primitive topology
 is encountered (i.e., PrimEnd is set) before a complete set of vertices for
 that object have been received. Given that only LINESTRIP and TRISTRIP primitive
 types are subject to CLIP unit partial object removal, the only supported cases
 of partial objects are 1-vertex LINESTRIPs and 1 or 2-vertex TRISTRIPs.

 [errata DevSNB]: Possible hang if final output from GS kernel is 2 vertex
 triangle. If it is possible for the final output from GS kernel to be a 2
 vertex triangle, then have the GS kernel always output an extra single vertex
 triangle as the final output."

Do you think this could be related?  It's a SNB specific hang closely tied to triangle strips, which sounds promising, but the symptoms don't sound quite right either...

--Ken
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