[Bug 82885] [i965]Piglit spec_ARB_shader_texture_lod_execution_tex-miplevel-selection_*GradARB_Cube fails

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=82885

--- Comment #7 from Samuel Iglesias <siglesias at igalia.com> ---
I spent some time looking into this issue and I have some discoveries to share:

* This issue is reproducible in Ironlake with master branch
(66a2fe4cf95f6d4f5bdbfd740314113e87e5649b).

* I first though that the issue could be related something specific to cube map
textures but test "bin/tex-miplevel-selection *Lod Cube" passes. So, I compared
the INTEL_DEBUG=fs of the latter program with the reported in the bug, they are
almost the same: the difference is in the number of parameters sent to the
sampler and the uniforms received in the payload. I also read the SNB
documentation to check that INTEL_DEBUG=fs is giving the expected output for
both cases.

* Then I was suspicious of a misconfigured state message (3DSTATE_WM,
3DSTATE_CONSTANT_PS, SURFACE_STATE, sampling engine messages). After comparing
its setup in the source code with SNB documentation, I have not found any
misconfigured flag but I could be wrong as it might be something very subtle to
be seen at first glance.

* In gen6_update_renderbuffer_surface(), when the gl_target is a cube map
texture, the surface type is changed to SURFACE_2D with its depth multiply by 6
(as the cube map has six faces). I modified it but no change in the output.
Furthermore, as it is working for other cases (see test in second bullet), I
don't think this is related to this bug.

* As this tests renders into each texture by using only glClear() commands,
then I looked at the meta fast color clear path. I found that commenting out
the meta fast color clear path in brw_clear(), then some color patterns
appeared in the output... but they are still wrong (see attachments).

So, I'm running out of ideas to continue analyzing this bug. Maybe someone with
more experience in that part of the source code has some ideas that could be
worth to try or point out where I made a mistake in my analysis.

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