[Piglit] Core vs compat 3.1 contexts

[Ian Romanick]

On 06/16/2014 11:29 PM, Ilia Mirkin wrote:
> All the arb_shader_atomic_counters tests start with
> 
>         config.supports_gl_core_version = 31;
> 
> However when I try to run them on the NVIDIA proprietary driver, I get:
> 
> piglit: info: Requested a OpenGL 3.1 Context, but the actual context
> is a 3.1 context that exposes the GL_ARB_compatibility extension
> piglit: info: Failed to create any GL context
> PIGLIT: {'result': 'skip' }

Some back story:  OpenGL 3.0 marked a bunch of stuff deprecated.  All of
the deprecated functionality was scheduled to be removed in the next GL
version.  OpenGL 3.1 removed all of that functionalty, but... one vendor
was unwilling to let go of the past.  As a compromise, the
GL_ARB_compatibility extension was created.

If you create an OpenGL 3.1 context you may or may not get a context
with GL_ARB_compatibility.  There is no way to request one way or the
other at context creation... you just get what you get.

OpenGL 3.2 introduced profiles.  With profiles an application can ask
for a context with or without the compatibility functionality.

The only implementation of OpenGL 3.1 that does not provide
GL_ARB_compatibility is Mesa.  Apple supports either 2.1 or 3.2 core
profile.  Quite a few drivers in Mesa support only OpenGL 3.1.  Quite a
few of those are gaining 3.2 support, but some, like the Sandybridge
driver, will probably never get 3.2.

This poses a problem for piglit.  We want to make tests for things that
don't have GL_ARB_compatibility, but we can't actually ask for that.
When a test says

    config.supports_gl_core_version = 31;

Waffle uses glXCreateContextAttribsARB to create 3.1 context.  It then
checks to see whether or not that context has GL_ARB_compatibility.  If
it does, then context creation fails.  Waffle will then try to create
some other kind of context if you set another supports_gl_* value.

I think this is a bug in Waffle (or maybe piglit's usage of Waffle).  If
a 3.1 context with GL_ARB_compatibility was created, Waffle should try
again by asking for a 3.2 core profile.

> This seems to come from piglit_wfl_framework.c:special_case_gl_31.
> 
> Unfortunately I know ~nothing about the subtleties of all this, but
> what's the way forward? Is the driver at fault? Piglit? FTR, changing
> the supports_gl_core_version to 32 fixes it.
> 
>   -ilia
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