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