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Bug 3415 - SDL_VIDEODRIVER=wayland makes SDL_GL_GetProcAddress return NULL on many modern opengl functions
Summary: SDL_VIDEODRIVER=wayland makes SDL_GL_GetProcAddress return NULL on many moder...
Status: RESOLVED INVALID
Alias: None
Product: SDL
Classification: Unclassified
Component: video (show other bugs)
Version: HG 2.1
Hardware: x86_64 Linux
: P2 normal
Assignee: Sam Lantinga
QA Contact: Sam Lantinga
URL:
Keywords:
Depends on:
Blocks:
 
Reported: 2016-08-26 10:08 UTC by eliashogstvedt
Modified: 2016-09-07 10:32 UTC (History)
2 users (show)

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Description eliashogstvedt 2016-08-26 10:08:21 UTC 
This only happens when using the environment variable and it does not happen when using the default video driver (xorg). For example direct state access functions like SDL_GL_GetProcAddress("glCreateVertexArrays"); do not exist.
Comment 1 eliashogstvedt 2016-08-26 10:38:29 UTC 
Having had a look at the function I saw that it uses SDL_SetError on failure SDL_GetError returns :

Failed loading _glCreateVertexArrays: /usr/lib/x86_64-linux-gnu/mesa/libGL.so.1: undefined symbol: __glCreateVertexArrays

I don't know much about how symbols are loaded in opengl or sdl but there is no such symbol in libGL.so however I did found "00000000002f3400 D epoxy_glCreateVertexArrays" in /usr/lib/x86_64-linux-gnu/libepoxy.so. Might be a clue?
Comment 2 Alex Szpakowski 2016-08-28 13:49:35 UTC 
Does your OpenGL implementation provide either OpenGL 4.5 or the GL_ARB_direct_state_access extension, when you use wayland? That function requires at least one of those at runtime.
Comment 3 eliashogstvedt 2016-08-31 11:02:14 UTC 
(In reply to Alex Szpakowski from comment #2)
> Does your OpenGL implementation provide either OpenGL 4.5 or the
> GL_ARB_direct_state_access extension, when you use wayland? That function
> requires at least one of those at runtime.

It provides 4.4 (using intel hd graphcis 5500/i915 with mesa 12.1.0-devel)

I think this is a misunderstanding on my end on how SDL_GL_GetProcAddress works. With X SDL_GL_ExtensionSupported("GL_ARB_direct_state_access") does not return true but SDL_GL_GetProcAddress returns the DSA function pointers and they work as expected. This had me confused so I had worked around it in my renderer but I wasn't sure if this was related to this issue so I didn't mention it. I also thought DSA was core from 4.2-3 and up.

With wayland (SDL_VIDEODRIVER=wayland) the DSA extension is also false but the function pointers are NULL.

So I don't know if this is a bug or expected behavior anymore. Now my issue is: How do I get access to the DSA functions under OpenGL 4.4 using wayland?
Comment 4 Alex Szpakowski 2016-08-31 20:57:43 UTC 
Mesa 12.1 isn't officially released yet. Maybe it's a driver bug?
Comment 5 Ryan C. Gordon 2016-09-07 03:07:50 UTC 
> So I don't know if this is a bug or expected behavior anymore.

Mesa on X11 will always return a non-NULL pointer, even if the extension isn't supported, because you can legally ask for a pointer before you have a context and they know what's actually available to you. These pointers are trampolines that will bounce to the real implementation once you have a current context.

https://dri.freedesktop.org/wiki/glXGetProcAddressNeverReturnsNULL/

_Always_ check the extension, not whether you get a NULL pointer!

I don't know if Wayland cleaned this mess up, but it _would_ use a different mechanism for finding GL entry points.

--ryan.
Comment 6 eliashogstvedt 2016-09-07 10:32:47 UTC 
Thanks for the feedback. 

This has been a mistake on my end where I had cached the result of what SDL_GL_ExtensionSupported returned and called the function too early making me believe direct_state_access was not available.

But for some reason now I can also use the DSA functions under wayland but I'm not sure how that started working. Maybe it's because I'm using the oibaf ppa to get the latest mesa drivers?

I also wasn't able to compile SDL with wayland whereas a week ago I was able to. I had had a look at the configure file and installed the packages it was looking for. This might have been the latest cursor addition to wayland though.