docs: libGL documentation, from the xorg-docs tree

This is pretty old and needs some updating but might be useful or
interesting to some people.

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+
+Introduction
+------------
+
+This document describes the implementation of the XFree86 4.0 libGL.so
+library defined by the Linux/OpenGL Base specification found at
+http://reality.sgi.com/opengl/linux/linuxbase.html.
+
+The documentation is divided into two sections:
+    User's Guide
+    Driver Developer's Guide
+
+Author:  Brian Paul     (brian@precisioninsight.com)
+Date:    February 2000
+
+
+
+User's Guide
+------------
+
+Using libGL.so
+
+The libGL.so library defines the gl- and glX-prefixed functions needed to
+run OpenGL programs.  OpenGL client applications should link with the
+-lGL option to use it.
+
+libGL.so serves two primary functions: GLX protocol generation for indirect
+rendering and loading/management of hardware drivers for direct rendering.
+
+When libGL.so initializes itself it uses the DRI to determine the
+appropriate hardware driver for each screen on the local X display.
+The hardware drivers are expected to be in the /usr/X11R6/lib/modules/dri/
+directory.  Drivers are named with the convention <name>_dri.so where
+<name> is a driver such as "tdfx", "i810", "gamma", etc.
+
+The LIBGL_DRIVERS_DIR environment variable may be used to specify a
+different DRI modules directory, overriding /usr/X11R6/lib/modules/dri/.
+This environment variable is ignored in setuid programs for security
+reasons.
+
+When libGL.so is unable to locate appropriate hardware drivers it will
+fall back to using indirect GLX rendering.
+
+To aid in solving problems, libGL.so will print diagnostic messages to
+stderr if the LIBGL_DEBUG environment variable is defined.
+
+libGL.so is thread safe.  The overhead of thread safety for common,
+single-thread clients is negligible.  However, the overhead of thread
+safety for multi-threaded clients is significant.  Each GL API call
+requires two calls to pthread_get_specific() which can noticably
+impact performance.  Warning:  libGL.so is thread safe but individual
+DRI drivers may not be.  Please consult the documentation for a driver
+to learn if it is thread safe.
+
+
+
+Indirect Rendering
+
+You can force indirect rendering mode by setting the LIBGL_ALWAYS_INDIRECT
+environment variable.  Hardware acceleration will not be used.
+
+
+
+libGL.so Extensibility
+
+libGL.so is designed to be extended without upgrading.  That is,
+drivers may install new OpenGL extension functions into libGL.so
+without requiring libGL.so to be replaced.  Clients of libGL.so should
+use the glXGetProcAddressEXT() function to obtain the address of
+functions by name.  For more details of GLX_ARB_get_proc_address see
+http://oss.sgi.com/projects/ogl-sample/registry/ARB/get_proc_address.spec
+
+libGL.so is also designed with flexibility such that it may be used
+with many generations of hardware drivers to come.
+
+
+
+
+Driver Developer's Guide
+------------------------
+
+This section describes the requirements to make an XFree86 4.0
+libGL.so-compatible hardware driver.  It is not intended for end
+users of libGL.so.
+
+
+XFree86 source files
+
+libGL.so is built inside XFree86 with sources found in xc/lib/GL/.
+Specifically, libGL.so is built from:
+
+	xc/lib/GL/glx/*.c
+	xc/lib/dri/XF86dri.c
+	xc/lib/dri/dri_glx.c
+	xc/lib/GL/mesa/src/glapi.c
+	xc/lib/GL/mesa/src/glapitemp.h
+	xc/lib/GL/mesa/src/glapitable.h
+	xc/lib/GL/mesa/src/glapioffsets.h
+	xc/lib/GL/mesa/src/glapinoop.c
+	xc/lib/GL/mesa/src/glheader.h
+	xc/lib/GL/mesa/src/glthread.c
+	xc/lib/GL/mesa/src/glthread.h
+	xc/lib/GL/mesa/src/X86/glapi_x86.S
+	xc/lib/GL/mesa/src/X86/assyntax.h
+
+Understand that the mesa/src/gl*.[ch] files are not tied to Mesa.  They
+have no dependencies on the rest of Mesa and are designed to be reusable
+in a number of projects.
+
+The glapi_x86.X and assyntax.h files implement x86-optimized dispatch
+of GL functions.  They are not required; C-based dispatch can be used
+instead, with a slight performance penalty.
+
+
+
+Driver loading and binding
+
+When libGL.so initializes itself (via the __glXInitialize function) a
+call is made to driCreateDisplay().  This function uses DRI facilities
+to determine the driver file appropriate for each screen on the local
+display.  Each screen's driver is then opened with dlopen() and asked
+for its __driCreateScreen() function.  The pointers to the __driCreateScreen()
+functions are kept in an array, indexed by screen number, in the
+__DRIdisplayRec struct.
+
+When a driver's __driCreateScreen() function is called, it must initialize
+a __DRIscreenRec struct.  This struct acts as the root of a tree of
+function pointers which are called to create and destroy contexts and
+drawables and perform all the operations needed by the GLX interface.
+See the xc/lib/GL/glx/glxclient.h file for details.
+
+
+
+Dynamic Extension Function Registration
+
+In order to provide forward compatibility with future drivers, libGL.so
+allows drivers to register new OpenGL extension functions which weren't
+known when libGL.so was built.
+
+The register_extensions() function in xc/lib/GL/dri/dri_glx.c is called
+as soon as libGL.so is loaded.  This is done with gcc's constructor
+attribute.  This mechanism will likely have to be changed for other compilers.
+
+register_extensions() loops over all local displays and screens, determines
+the DRI driver for each, and calls the driver's __driRegisterExtensions()
+function, if present.
+
+The __driRegisterExtensions() function can add new entrypoints to libGL
+by calling:
+
+    GLboolean _glapi_add_entrypoint(const char *funcName, GLuint offset)
+
+The parameters are the name of the function (such as "glFoobarEXT") and the
+offset of the dispatch slot in the API dispatch table.  The return value
+indicates success (GL_TRUE) or failure (GL_FALSE).
+
+_glapi_add_entrypoint() will synthesize entrypoint code in assembly
+language.  Assembly languages is required since parameter passing
+can't be handled correctly using a C-based solution.
+
+The address of the new entrypoint is obtained by calling the
+glXGetProcAddressARB() function.
+
+The dispatch offset number MUST be a number allocated by SGI in the same
+manner in which new GL_* constants are allocated.  Using an arbitrary
+offset number will result in many problems.
+
+
+
+Dispatch Management
+
+When a GL context is made current, the driver must install its dispatch
+table as the current dispatch table.  This is done by calling
+
+	void _glapi_set_dispatch(struct _glapi_table *dispatch);
+
+This will install the named dispatch table for the calling thread.
+The current dispatch table for a thread can be obtained by calling
+
+	struct _glapi_table *_glapi_get_dispatch(void);
+
+For higher performance in the common single-thread case, the global
+variable _glapi_Dispatch will point to the current dispatch table.
+This variable will be NULL when in multi-thread mode.
+
+
+
+Context Management
+
+libGL.so uses the XFree86 xthreads package to manage a thread-specific
+current context pointer.  See __glXGet/SetCurrentContext() in glext.c
+
+Drivers may use the _glapi_set/get_context() functions to maintain
+a private thread-specific context pointer.
+