[Mesa-dev] [RFC] Vendor-neutral dispatch library for OpenGL

[Roland Scheidegger]

FWIW from a high level point of view this makes all sense to me. It is
however definitely not my field of expertise, but whatever makes it
easier to have multiple drivers installed is imho a good thing...
Some more feedback from mesa people which are more familiar with the gl
api dispatch stuff (albeit there's probably not that many...) would
probably be nice...

Roland

Am 30.09.2015 um 22:07 schrieb Kyle Brenneman:
> I'm working on libglvnd, a vendor-neutral dispatch library for OpenGL,
> and I wanted to see if anyone had any comments or suggestions about it.
> Right now, I'm trying to get the GLX interface wrapped up, but I'd like
> to get some feedback from the Mesa community before I'd be confidant in
> calling it ready.
> 
> A bit of background: Libglvnd was originally proposed back at XDC 2013.
> It's hosted on Github here:
> https://github.com/NVIDIA/libglvnd
> 
> The goal for libglvnd is to allow multiple OpenGL implementations from
> different vendors to coexist on the same system, without interfering
> with each other or requiring any manual configuration.
> 
> With libglvnd, libGL.so is a vendor-independent dispatch library, not
> part of any driver. Each vendor provides its OpenGL implementation in a
> separate library. An application still links to libGL.so just like it
> does now, but then libGL.so will figure out which vendor library to use,
> and will dispatch any OpenGL and GLX calls to that library.
> 
> The libglvnd libraries make as few assumptions as possible about how the
> vendor libraries work, so that (hopefully) it's easy to port any
> existing OpenGL implementation to it. In some cases, a simple wrapper
> library around an existing libGL.so library would be enough.
> 
> As it's currently implemented, libglvnd selects a vendor library for
> each X screen. So, you could have two X screens that each use a
> different vendor library, and a single process could create and use
> rendering contexts on both. It doesn't handle two different vendor
> libraries for the same X screen, although the ABI is set up such that it
> would be possible to add that capability later on.
> 
> The EGL interface is still in its really early design stages. Any
> comments or requirements that I might have forgotten are more than welcome.
> 
> In addition, I've put together a proof-of-concept version of Mesa that
> can use libglvnd. It's still a work in progress, but hopefully it will
> provide a more concrete example of how libglvnd works. I've got it
> checked into Github here:
> https://github.com/kbrenneman/mesa-libglvnd
> 
> Internally, it's broken down into a few different libraries:
> libGLX handles GLX functions, and is responsible for keeping track of
> GLX contexts, selecting and loading vendor libraries, and setting up the
> dispatch table for OpenGL functions. This is the only library that a
> vendor library directly talks to.
> 
> libGLdispatch is responsible for dispatching OpenGL calls to the correct
> library based on the thread's current rendering context. It uses a
> dispatch table derived from Mesa's GLAPI layer. libGLdispatch is purely
> internal -- the vendor libraries go through libGLX to set up dispatch
> tables, and an application would call into libGL.so or libOpenGL.so to
> call any OpenGL functions.
> 
> libGL.so is a wrapper around libGLX and libGLdispatch. Conceptually, it
> just exports GL and GLX functions and forwards them to libGLX and
> libGLdispatch to deal with. The implementation is a bit more complicated
> to avoid the overhead of an extra indirect jump every time an app calls
> an OpenGL function.
> 
> In addition, there's a new library, libOpenGL.so. It's basically the
> same as libGL.so, except that it only exports the OpenGL functions, not
> GLX. It also doesn't depend on libGLX, so it could also be used with an
> EGL or GLX application. The hope is that future applications will link
> against libOpenGL.so and either libGLX.so or libEGL.so. This makes for a
> cleaner separation of OpenGL from the window system binding. But,
> libGL.so will be kept around indefinitely for backwards compatibility.
> 
> Comments, questions, and suggestions are all welcome.
> 
> Thanks,
> -Kyle Brenneman
> 
> 
> 
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