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diff --git a/docs/libGL.txt b/docs/libGL.txt new file mode 100644 index 0000000000..cb98840437 --- /dev/null +++ b/docs/libGL.txt @@ -0,0 +1,197 @@ + + + +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. + |