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

[Iago Toral Quiroga]

On mar, 2014-07-29 at 10:16 -0700, Kenneth Graunke wrote:
> On Tuesday, July 29, 2014 10:12:23 AM Iago Toral Quiroga wrote:
> > Hi,
> > 
> > running the piglit tests on my implementation of geometry shaders for
> > Sandy Bridge produces a GPU hang for the following test:
> > 
> > ./glsl-1.50-geometry-primitive-id-restart GL_TRIANGLE_STRIP_ADJACENCY
> > ffs
> > 
> > That test checks primitive restarts but the hang seems to be unrelated
> > to that, since it happens also when primitive restart is not enabled.
> > The problem, which only affects GL_TRIANGLE_STRIP_ADJACENCY and no other
> > primitive type -with our without adjacency-, is in this loop that the
> > test uses to setup the indices for the vertices:
> > 
> > elements = glMapBuffer(GL_ELEMENT_ARRAY_BUFFER, GL_READ_WRITE);
> > num_elements = 0;
> > for (i = 1; i <= LONGEST_INPUT_SEQUENCE; i++) {
> >    for (j = 0; j < i; j++) {
> >       /* Every element that isn't the primitive
> >        * restart index can just be element 0, since
> >        * we don't care about the actual vertex data.
> >        */
> >       elements[num_elements++] = 0;
> >    }
> >    elements[num_elements++] = prim_restart_index;
> > }
> > glUnmapBuffer(GL_ELEMENT_ARRAY_BUFFER);
> > 
> > Setting all elements to the same index (0 in this case) is the one thing
> > that causes the hang for GL_TRIANGLE_STRIP_ADJACENCY. A simple change
> > like this removes the hang:
> > -      elements[num_elements++] = 0;
> > +      elements[num_elements++] = j != prim_restart_index ? j : j + 1;
> > 
> > Skimming through the docs I have not seen any references to this being a
> > known problem. In fact, I don't see any references to
> > GL_TRIANGLE_STRIP_ADJACENCY being special in any way and it seems that
> > this is not a problem in IvyBridge, since the test runs correctly there.
> > 
> > Does this sound like a hardware bug specific to SandyBridge's handling
> > of GL_TRIANGLE_STRIP_ADJACENCY or is there something else I should check
> > before arriving to that conclusion?
> 
> Odd.  It could very well be, but I don't see anything in the documentation, either.  Could you post a branch with your preliminary code?  I'd be happy to look into the hang and see if I come up with any more useful information...

Samuel uploaded a branch to github here:
https://github.com/samuelig/mesa/tree/gs-and-tf-support-snb

Thanks for looking into this!

> > If it is a hardware bug I guess we want a workaround for it , at least
> > to prevent the hang or something but I am not sure what would be the
> > best option here, I think the only option for the driver would be to
> > explore the list of indices provided when this primitive type is used
> > and when we hit this scenario (I'd have to test how many repeating
> > indices we need for it to hang), error out and do not execute the
> > drawing command or something... any other suggestions? 
> > 
> > Iago