[Nouveau] [Bug 66642] New: [nva5] Wrong provoking vertex is selected for flat attribute interpolation (clipping?)

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=66642

          Priority: medium
            Bug ID: 66642
          Assignee: nouveau at lists.freedesktop.org
           Summary: [nva5] Wrong provoking vertex is selected for flat
                    attribute interpolation (clipping?)
          Severity: normal
    Classification: Unclassified
                OS: Linux (All)
          Reporter: alzeih at gmail.com
          Hardware: x86-64 (AMD64)
            Status: NEW
           Version: git
         Component: Drivers/DRI/nouveau
           Product: Mesa

The following piglit test:

  interpolation-flat-gl_BackColor-smooth-none.shader_test

appears to select the wrong provoking vertex depending on the exact value of
v[0].y.

For v[0].y > -1.0, the triangle ends up red.
For v[0].y = -1.0, the triangle ends up green.
For v[0].y < -1.0, the triangle ends up (correctly) blue.

Hardware: nva5 (GT330M) in MacBookPro6,2
Mesa version: 1415a1884c
Kernel: 3.9.8-1-ARCH

Suspect the clipper doesn't respect the index of the provoking vertex in the
triangle?

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