1 2 3 4 Introduction 5 ------------ 6 7 This document describes the implementation of the XFree86 4.0 libGL.so 8 library defined by the Linux/OpenGL Base specification found at 9 http://reality.sgi.com/opengl/linux/linuxbase.html. 10 11 The documentation is divided into two sections: 12 User's Guide 13 Driver Developer's Guide 14 15 Author: Brian Paul (brian (a] precisioninsight.com) 16 Date: February 2000 17 18 19 20 User's Guide 21 ------------ 22 23 Using libGL.so 24 25 The libGL.so library defines the gl- and glX-prefixed functions needed to 26 run OpenGL programs. OpenGL client applications should link with the 27 -lGL option to use it. 28 29 libGL.so serves two primary functions: GLX protocol generation for indirect 30 rendering and loading/management of hardware drivers for direct rendering. 31 32 When libGL.so initializes itself it uses the DRI to determine the 33 appropriate hardware driver for each screen on the local X display. 34 The hardware drivers are expected to be in the /usr/X11R6/lib/modules/dri/ 35 directory. Drivers are named with the convention <name>_dri.so where 36 <name> is a driver such as "tdfx", "i810", "gamma", etc. 37 38 The LIBGL_DRIVERS_DIR environment variable may be used to specify a 39 different DRI modules directory, overriding /usr/X11R6/lib/modules/dri/. 40 This environment variable is ignored in setuid programs for security 41 reasons. 42 43 When libGL.so is unable to locate appropriate hardware drivers it will 44 fall back to using indirect GLX rendering. 45 46 To aid in solving problems, libGL.so will print diagnostic messages to 47 stderr if the LIBGL_DEBUG environment variable is defined. 48 49 libGL.so is thread safe. The overhead of thread safety for common, 50 single-thread clients is negligible. However, the overhead of thread 51 safety for multi-threaded clients is significant. Each GL API call 52 requires two calls to pthread_get_specific() which can noticably 53 impact performance. Warning: libGL.so is thread safe but individual 54 DRI drivers may not be. Please consult the documentation for a driver 55 to learn if it is thread safe. 56 57 58 59 Indirect Rendering 60 61 You can force indirect rendering mode by setting the LIBGL_ALWAYS_INDIRECT 62 environment variable. Hardware acceleration will not be used. 63 64 65 66 libGL.so Extensibility 67 68 libGL.so is designed to be extended without upgrading. That is, 69 drivers may install new OpenGL extension functions into libGL.so 70 without requiring libGL.so to be replaced. Clients of libGL.so should 71 use the glXGetProcAddressEXT() function to obtain the address of 72 functions by name. For more details of GLX_ARB_get_proc_address see 73 http://oss.sgi.com/projects/ogl-sample/registry/ARB/get_proc_address.spec 74 75 libGL.so is also designed with flexibility such that it may be used 76 with many generations of hardware drivers to come. 77 78 79 80 81 Driver Developer's Guide 82 ------------------------ 83 84 This section describes the requirements to make an XFree86 4.0 85 libGL.so-compatible hardware driver. It is not intended for end 86 users of libGL.so. 87 88 89 XFree86 source files 90 91 libGL.so is built inside XFree86 with sources found in xc/lib/GL/. 92 Specifically, libGL.so is built from: 93 94 xc/lib/GL/glx/*.c 95 xc/lib/dri/XF86dri.c 96 xc/lib/dri/dri_glx.c 97 xc/lib/GL/mesa/src/glapi.c 98 xc/lib/GL/mesa/src/glapitemp.h 99 xc/lib/GL/mesa/src/glapitable.h 100 xc/lib/GL/mesa/src/glapioffsets.h 101 xc/lib/GL/mesa/src/glapinoop.c 102 xc/lib/GL/mesa/src/glheader.h 103 xc/lib/GL/mesa/src/glthread.c 104 xc/lib/GL/mesa/src/glthread.h 105 xc/lib/GL/mesa/src/X86/glapi_x86.S 106 xc/lib/GL/mesa/src/X86/assyntax.h 107 108 Understand that the mesa/src/gl*.[ch] files are not tied to Mesa. They 109 have no dependencies on the rest of Mesa and are designed to be reusable 110 in a number of projects. 111 112 The glapi_x86.X and assyntax.h files implement x86-optimized dispatch 113 of GL functions. They are not required; C-based dispatch can be used 114 instead, with a slight performance penalty. 115 116 117 118 Driver loading and binding 119 120 When libGL.so initializes itself (via the __glXInitialize function) a 121 call is made to driCreateDisplay(). This function uses DRI facilities 122 to determine the driver file appropriate for each screen on the local 123 display. Each screen's driver is then opened with dlopen() and asked 124 for its __driCreateScreen() function. The pointers to the __driCreateScreen() 125 functions are kept in an array, indexed by screen number, in the 126 __DRIdisplayRec struct. 127 128 When a driver's __driCreateScreen() function is called, it must initialize 129 a __DRIscreenRec struct. This struct acts as the root of a tree of 130 function pointers which are called to create and destroy contexts and 131 drawables and perform all the operations needed by the GLX interface. 132 See the xc/lib/GL/glx/glxclient.h file for details. 133 134 135 136 Dynamic Extension Function Registration 137 138 In order to provide forward compatibility with future drivers, libGL.so 139 allows drivers to register new OpenGL extension functions which weren't 140 known when libGL.so was built. 141 142 The register_extensions() function in xc/lib/GL/dri/dri_glx.c is called 143 as soon as libGL.so is loaded. This is done with gcc's constructor 144 attribute. This mechanism will likely have to be changed for other compilers. 145 146 register_extensions() loops over all local displays and screens, determines 147 the DRI driver for each, and calls the driver's __driRegisterExtensions() 148 function, if present. 149 150 The __driRegisterExtensions() function can add new entrypoints to libGL 151 by calling: 152 153 GLboolean _glapi_add_entrypoint(const char *funcName, GLuint offset) 154 155 The parameters are the name of the function (such as "glFoobarEXT") and the 156 offset of the dispatch slot in the API dispatch table. The return value 157 indicates success (GL_TRUE) or failure (GL_FALSE). 158 159 _glapi_add_entrypoint() will synthesize entrypoint code in assembly 160 language. Assembly languages is required since parameter passing 161 can't be handled correctly using a C-based solution. 162 163 The address of the new entrypoint is obtained by calling the 164 glXGetProcAddressARB() function. 165 166 The dispatch offset number MUST be a number allocated by SGI in the same 167 manner in which new GL_* constants are allocated. Using an arbitrary 168 offset number will result in many problems. 169 170 171 172 Dispatch Management 173 174 When a GL context is made current, the driver must install its dispatch 175 table as the current dispatch table. This is done by calling 176 177 void _glapi_set_dispatch(struct _glapi_table *dispatch); 178 179 This will install the named dispatch table for the calling thread. 180 The current dispatch table for a thread can be obtained by calling 181 182 struct _glapi_table *_glapi_get_dispatch(void); 183 184 For higher performance in the common single-thread case, the global 185 variable _glapi_Dispatch will point to the current dispatch table. 186 This variable will be NULL when in multi-thread mode. 187 188 189 190 Context Management 191 192 libGL.so uses the XFree86 xthreads package to manage a thread-specific 193 current context pointer. See __glXGet/SetCurrentContext() in glext.c 194 195 Drivers may use the _glapi_set/get_context() functions to maintain 196 a private thread-specific context pointer. 197 198
