[Mesa-dev] SandyBridge issue likely related to fast color clears using meta operations

[Kenneth Graunke]

On Tuesday, September 02, 2014 06:16:01 PM Samuel Iglesias Gonsálvez wrote:
> Hello,
> 
> Two weeks ago, Iago and myself sent a batch of patches that added
> geometry shader support for SandyBridge [0].
> 
> Recently, we rebased our patches against master [1] and found
> that some things are not working properly any more. Particularly, we
> have plenty of cases where we only get a black screen when using a
> geometry shader, and in these cases it seems as if some outputs of the
> geometry shader got "lost" by the time they become inputs in the
> fragment shader (some input varyings in the fragment shader that come
> from the outputs of the geometry seem to be zeroed). The values we
> output in the geometry shader are correct though, since they are
> correctly captured by transform feedback.
> 
> A bisect shows that the commit that introduced this problem is
> 2f28a0dc231 i965: Implement fast color clears using meta operations [2].
> 
> Reverting this commit fixes the problem for us. The revert also fixes
> piglit's bin/gl-3.2-layered-rendering-clear-color* tests in SandyBridge
> [3].
> 
> So the question is whether the commit mentioned above introduces a
> regression that should be fixed somewhere or if our code needs to be
> updated to adapt to changes introduced by that commit in some way.
> 
> Best regards,
> 
> Sam

Hi Sam,

Layered clears now use GL_AMD_vertex_shader_layered.  If you expose the extension, then gl-3.2-layered-rendering-clear-color starts passing again.  Otherwise, the vertex shader code to set gl_LayerID = gl_InstanceID is #ifdef'd out, so only layer 0 would get affected.

I'm not sure if we can expose that extension without GL 3.2, so you might just put the patch to enable it (simply editing intel_extensions.c) near the end of your series.  Perhaps Jordan will know if we can just turn it on now.

--Ken
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