Bug 97477

Summary: i915g: gl_FragCoord is always (0.0, max_y)
Product: Mesa Reporter: Nicholas Bishop <nicholasbishop>
Component: Drivers/Gallium/i915gAssignee: Default DRI bug account <dri-devel>
Status: NEW --- QA Contact: Default DRI bug account <dri-devel>
Severity: normal    
Priority: medium CC: jefferym
Version: git   
Hardware: Other   
OS: All   
i915 platform: i915 features:
Attachments: Patch: Handle I915_SEMANTIC_POS when setting texCoords for gl_FragCoord

Description Nicholas Bishop 2016-08-25 05:16:17 UTC
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 */
      debug_printf("Punting DDX/DDX\n");
Comment 1 Nicholas Bishop 2016-08-25 23:34:31 UTC
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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