[Piglit] Core vs compat 3.1 contexts

[Emil Velikov]

On 17/06/14 23:50, Ian Romanick wrote:
> 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.
> 
Waffle itself does not check for GL_ARB_compat when creating a context. The
decision of what to do if the extension is available is left to the user -
which imho is the only sane solution.

>From the man page
   If the chosen API is <constant>WAFFLE_CONTEXT_OPENGL</constant>
   and version 3.1 is requested,
   then the returned context may implement any of:

    * Version 3.1, with or without the GL_ARB_compatibility extension.
    * The core profile of version 3.2 or greater.

-Emil

> 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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> 
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