Bug 91461

Summary:	gl_TessLevel* writes have no effect for all but the last TCS invocation
Product:	Mesa	Reporter:	Daniel Scharrer <daniel>
Component:	Drivers/Gallium/radeonsi	Assignee:	Default DRI bug account <dri-devel>
Status:	RESOLVED FIXED	QA Contact:	Default DRI bug account <dri-devel>
Severity:	normal
Priority:	medium
Version:	git
Hardware:	Other
OS:	All
Whiteboard:
i915 platform:		i915 features:
Attachments:	Tessellation test case

Description Daniel Scharrer 2015-07-25 14:33:27 UTC

The tessellator ignores all writes to gl_TessLevelInner and gl_TessLevelOuter by the tessellation control shader except except those done by the last TCS invocation (gl_InvocationID == patch output vertices - 1).

This results in no primitives being generated unless all entries of gl_TessLevelOuter are written in that invocation (the 'initial' values seem to be 0 for gl_TessLevelOuter and 1 for gl_TessLevelInner).

Additionally, barrier() does not synchronize changes to the gl_TessLevel* between TCS invocations.

Neither of these problems can be reproduced with user-defined 'patch out' variables - writes by any TCS invocation are visible in the evaluation shader and barrier() makes them visible to the other TCS invocations.

GPU: Radeon HD 7950 (TAHITI)
Mesa: 10.7.0-devel (git-9deb614)
LLVM: r243203

Comment 1 Daniel Scharrer 2015-07-25 14:34:24 UTC

Created attachment 117373 [details]
Tessellation test case

I have attached a test case for this. It can be compiled and run using:
 $ sh tessellation.cpp

Use the arrow keys to change which TCS invocation writes to gl_TessLevel* (or use space/enter to toggle between the first and the last).

It should display a yellow grid no matter what TCS invocation is selected but instead displays a green grid for invocation 3 and nothing for all other invocations.

Comment 2 Daniel Scharrer 2015-08-03 14:11:36 UTC

I can confirm that the patch "radeonsi: before storing tess levels, load them from LDS instead of temporary" on the mailing list fixes the issue.

Comment 3 Marek Olšák 2015-08-06 18:51:28 UTC

Fixed by 60159bcfc66a067b50da06f5cabfa20d72e89. Closing.

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