Created attachment 126029 [details] [review]
Patch: Handle I915_SEMANTIC_POS when setting texCoords for gl_FragCoord
On Intel Pineview M hardware, the i915 gallium driver doesn't output the correct gl_FragCoord. It seems to always have an X coord of 0.0 and a Y coord of the window's height in pixels, e.g. 600.0f or such.
I believe this is a regression caused in part by commit afa035031ff9e0c07a2297d864e46c76f7bfff58 (https://lists.freedesktop.org/archives/mesa-commit/2016-January/061181.html)
The old behavior used output zero, while the new behavior uses actual zeroes. In the case of gl_FragCoord output zero happened to be the correct one.
I've attached a patch that seems to fix the issue for me. I ran the piglit "quick" tests and observed 17 new tests passing, and one regression. I suspect the regression is unimportant since it involves derivatives that aren't implemented for this driver anyway:
"fs-dfdy-accuracy" test failed ... in i915_fpc_translate.c:
/* XXX We just output 0 here */
Posted the patch to the mailing list with a somewhat clearer commit message: https://lists.freedesktop.org/archives/mesa-dev/2016-August/127065.html
FWIW, the process of submitting a patch via the mailing list is a really big barrier to entry for someone new to the project.
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