325 lines
12 KiB
HTML
325 lines
12 KiB
HTML
<html>
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<title>Mesa EGL</title>
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<head><link rel="stylesheet" type="text/css" href="mesa.css"></head>
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<body>
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<h1>Mesa EGL</h1>
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<p>The current version of EGL in Mesa implements EGL 1.4. More information
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about EGL can be found at
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<a href="http://www.khronos.org/egl/" target="_parent">
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http://www.khronos.org/egl/</a>.</p>
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<p>The Mesa's implementation of EGL uses a driver architecture. The main
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library (<code>libEGL</code>) is window system neutral. It provides the EGL
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API entry points and helper functions for use by the drivers. Drivers are
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dynamically loaded by the main library and most of the EGL API calls are
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directly dispatched to the drivers.</p>
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<p>The driver in use decides the window system to support. For drivers that
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support hardware rendering, there are usually multiple drivers supporting the
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same window system. Each one of of them supports a certain range of graphics
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cards.</p>
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<h2>Build EGL</h2>
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<ol>
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<li>
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<p>Run <code>configure</code> with the desired state trackers and enable
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the Gallium driver for your hardware. For example</p>
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<pre>
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$ ./configure --with-state-trackers=egl,es,vega --enable-gallium-{swrast,intel}
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</pre>
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<p>The main library will be enabled by default. The <code>egl</code> state
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tracker is needed by a number of EGL drivers. EGL drivers will be covered
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later. The <a href="opengles.html">es state tracker</a> provides OpenGL ES 1.x
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and 2.x and the <a href="openvg.html">vega state tracker</a> provides OpenVG
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1.x.</p>
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</li>
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<li>Build and install Mesa as usual.</li>
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</ol>
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<p>In the given example, it will build and install <code>libEGL</code>,
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<code>libGLESv1_CM</code>, <code>libGLESv2</code>, <code>libOpenVG</code>, and
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one or more EGL drivers.</p>
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<h3>Configure Options</h3>
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<p>There are several options that control the build of EGL at configuration
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time</p>
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<ul>
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<li><code>--enable-egl</code>
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<p>By default, EGL is enabled. When disabled, the main library and the drivers
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will not be built.</p>
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</li>
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<li><code>--with-egl-driver-dir</code>
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<p>The directory EGL drivers should be installed to. If not specified, EGL
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drivers will be installed to <code>${libdir}/egl</code>.</p>
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</li>
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<li><code>--with-egl-displays</code>
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<p>List the window system(s) to support. It is by default <code>x11</code>,
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which supports the X Window System. Its argument is a comma separated string
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like, for example, <code>--with-egl-displays=x11,kms</code>. Because an EGL
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driver decides which window system to support, this example will enable two
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(sets of) EGL drivers. One supports the X window system and the other supports
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bare KMS (kernel modesetting).</p>
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</li>
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<li><code>--with-state-trackers</code>
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<p>The argument is a comma separated string. It is usually used to specify the
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rendering APIs, like OpenGL ES or OpenVG, to build. But it should be noted
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that a number of EGL drivers depend on the <code>egl</code> state tracker.
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They will <em>not</em> be built without the <code>egl</code> state tracker.</p>
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</li>
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<li><code>--enable-gallium-swrast</code>
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<p>This option is not specific to EGL. But if there is no driver for your
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hardware, or you are experiencing problems with the hardware driver, you can
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enable the swrast DRM driver. It is a dummy driver and EGL will fallback to
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software rendering automatically.</p>
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</li>
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</ul>
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<h3>OpenGL</h3>
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<p>The OpenGL state tracker is not built in the above example. It should be
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noted that the classic <code>libGL</code> is not a state tracker and cannot be
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used with EGL (unless the EGL driver in use is <code>egl_glx</code>). To build
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the OpenGL state tracker, one may append <code>glx</code> to
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<code>--with-state-trackers</code> and manually build
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<code>src/gallium/winsys/xlib/</code>.</p>
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<h2>Use EGL</h2>
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<p> The demos for OpenGL ES and OpenVG can be found in <code>progs/es1/</code>,
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<code>progs/es2/</code> and <code>progs/openvg/</code>. You can use them to
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test your build. For example,</p>
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<pre>
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$ cd progs/es1/xegl
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$ make
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$ ./torus
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</pre>
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<h3>Environment Variables</h3>
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<p>There are several environment variables that control the behavior of EGL at
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runtime</p>
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<ul>
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<li><code>EGL_DRIVERS_PATH</code>
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<p>By default, the main library will look for drivers in the directory where
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the drivers are installed to. This variable specifies a list of
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colon-separated directories where the main library will look for drivers, in
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addition to the default directory. This variable is ignored for setuid/setgid
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binaries.</p>
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</li>
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<li><code>EGL_DRIVER</code>
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<p>This variable specifies a full path to an EGL driver and it forces the
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specified EGL driver to be loaded. It comes in handy when one wants to test a
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specific driver. This variable is ignored for setuid/setgid binaries.</p>
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</li>
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<li><code>EGL_DISPLAY</code>
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<p>When <code>EGL_DRIVER</code> is not set, the main library loads <em>all</em>
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EGL drivers that support a certain window system. <code>EGL_DISPLAY</code> can
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be used to specify the window system and the valid values are, for example,
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<code>x11</code> or <code>kms</code>. When the variable is not set, the main
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library defaults the value to the first window system listed in
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<code>--with-egl-displays</code> at configuration time.
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</li>
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<li><code>EGL_LOG_LEVEL</code>
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<p>This changes the log level of the main library and the drivers. The valid
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values are: <code>debug</code>, <code>info</code>, <code>warning</code>, and
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<code>fatal</code>.</p>
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</li>
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<li><code>EGL_SOFTWARE</code>
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<p>For drivers that support both hardware and software rendering, setting this
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variable to true forces the use of software rendering.</p>
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</li>
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</ul>
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<h2>EGL Drivers</h2>
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<p>There are two categories of EGL drivers: Gallium and classic.</p>
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<p>Gallium EGL drivers supports all rendering APIs specified in EGL 1.4. The
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support for optional EGL functions and EGL extensions is usually more complete
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than the classic ones. These drivers depend on the <code>egl</code> state
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tracker to build. The available drivers are</p>
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<ul>
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<li><code>egl_<dpy>_i915</code></li>
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<li><code>egl_<dpy>_i965</code></li>
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<li><code>egl_<dpy>_radeon</code></li>
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<li><code>egl_<dpy>_nouveau</code></li>
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<li><code>egl_<dpy>_swrast</code></li>
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<li><code>egl_<dpy>_vmwgfx</code></li>
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</ul>
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<p><code><dpy></code> is given by <code>--with-egl-displays</code> at
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configuration time. There will be one EGL driver for each combination of the
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displays listed and the hardware drivers enabled.</p>
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<p>Classic EGL drivers, on the other hand, supports only OpenGL as its
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rendering API. They can be found under <code>src/egl/drivers/</code>. There
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are 3 of them</p>
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<ul>
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<li><code>egl_glx</code>
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<p>This driver provides a wrapper to GLX. It uses exclusively GLX to implement
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the EGL API. It supports both direct and indirect rendering when the GLX does.
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It is accelerated when the GLX is. As such, it cannot provide functions that
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is not available in GLX or GLX extensions.</p>
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</li>
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<li><code>egl_dri2</code>
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<p>This driver supports the X Window System as its window system. It functions
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as a DRI2 driver loader. Unlike <code>egl_glx</code>, it has no dependency on
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<code>libGL</code>. It talks to the X server directly using DRI2 protocol.</p>
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</li>
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<li><code>egl_dri</code>
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<p>This driver lacks maintenance and does <em>not</em> build. It is similiar
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to <code>egl_dri2</code> in that it functions as a DRI(1) driver loader. But
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unlike <code>egl_dri2</code>, it supports Linux framebuffer devices as its
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window system and supports EGL_MESA_screen_surface extension. As DRI1 drivers
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are phasing out, it might eventually be replaced by <code>egl_dri2</code>.</p>
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</li>
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</ul>
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<p>To use the classic drivers, one must manually set <code>EGL_DRIVER</code> at
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runtime.</p>
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<h2>Developers</h2>
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<p>The sources of the main library and the classic drivers can be found at
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<code>src/egl/</code>. The sources of the <code>egl</code> state tracker can
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be found at <code>src/gallium/state_trackers/egl/</code>.</p>
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<p>The suggested way to learn to write a EGL driver is to see how other drivers
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are written. <code>egl_glx</code> should be a good reference. It works in any
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environment that has GLX support, and it is simpler than most drivers.</p>
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<h3>Lifetime of Display Resources</h3>
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<p>Contexts and surfaces are examples of display resources. They might live
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longer than the display that creates them.</p>
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<p>In EGL, when a display is terminated through <code>eglTerminate</code>, all
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display resources should be destroyed. Similarly, when a thread is released
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throught <code>eglReleaseThread</code>, all current display resources should be
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released. Another way to destory or release resources is through functions
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such as <code>eglDestroySurface</code> or <code>eglMakeCurrent</code>.</p>
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<p>When a resource that is current to some thread is destroyed, the resource
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should not be destroyed immediately. EGL requires the resource to live until
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it is no longer current. A driver usually calls
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<code>eglIs<Resource>Bound</code> to check if a resource is bound
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(current) to any thread in the destroy callbacks. If it is still bound, the
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resource is not destroyed.</p>
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<p>The main library will mark destroyed current resources as unlinked. In a
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driver's <code>MakeCurrent</code> callback,
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<code>eglIs<Resource>Linked</code> can then be called to check if a newly
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released resource is linked to a display. If it is not, the last reference to
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the resource is removed and the driver should destroy the resource. But it
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should be careful here because <code>MakeCurrent</code> might be called with an
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uninitialized display.</p>
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<p>This is the only mechanism provided by the main library to help manage the
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resources. The drivers are responsible to the correct behavior as defined by
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EGL.</p>
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<h3><code>EGL_RENDER_BUFFER</code></h3>
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<p>In EGL, the color buffer a context should try to render to is decided by the
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binding surface. It should try to render to the front buffer if the binding
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surface has <code>EGL_RENDER_BUFFER</code> set to
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<code>EGL_SINGLE_BUFFER</code>; If the same context is later bound to a
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surface with <code>EGL_RENDER_BUFFER</code> set to
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<code>EGL_BACK_BUFFER</code>, the context should try to render to the back
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buffer. However, the context is allowed to make the final decision as to which
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color buffer it wants to or is able to render to.</p>
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<p>For pbuffer surfaces, the render buffer is always
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<code>EGL_BACK_BUFFER</code>. And for pixmap surfaces, the render buffer is
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always <code>EGL_SINGLE_BUFFER</code>. Unlike window surfaces, EGL spec
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requires their <code>EGL_RENDER_BUFFER</code> values to be honored. As a
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result, a driver should never set <code>EGL_PIXMAP_BIT</code> or
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<code>EGL_PBUFFER_BIT</code> bits of a config if the contexts created with the
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config won't be able to honor the <code>EGL_RENDER_BUFFER</code> of pixmap or
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pbuffer surfaces.</p>
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<p>It should also be noted that pixmap and pbuffer surfaces are assumed to be
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single-buffered, in that <code>eglSwapBuffers</code> has no effect on them. It
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is desirable that a driver allocates a private color buffer for each pbuffer
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surface created. If the window system the driver supports has native pbuffers,
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or if the native pixmaps have more than one color buffers, the driver should
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carefully attach the native color buffers to the EGL surfaces, re-route them if
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required.</p>
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<p>There is no defined behavior as to, for example, how
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<code>glDrawBuffer</code> interacts with <code>EGL_RENDER_BUFFER</code>. Right
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now, it is desired that the draw buffer in a client API be fixed for pixmap and
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pbuffer surfaces. Therefore, the driver is responsible to guarantee that the
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client API renders to the specified render buffer for pixmap and pbuffer
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surfaces.</p>
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<h3><code>EGLDisplay</code> Mutex</h3>
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The <code>EGLDisplay</code> will be locked before calling any of the dispatch
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functions (well, except for GetProcAddress which does not take an
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<code>EGLDisplay</code>). This guarantees that the same dispatch function will
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not be called with the sample display at the same time. If a driver has access
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to an <code>EGLDisplay</code> without going through the EGL APIs, the driver
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should as well lock the display before using it.
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<h3>TODOs</h3>
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<ul>
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<li>Pass the conformance tests</li>
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<li>Better automatic driver selection: <code>EGL_DISPLAY</code> loads all
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drivers and might eat too much memory.</li>
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</ul>
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</body>
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</html>
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