docs/egl.html

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  11   The Mesa 3D Graphics Library
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  16
  17 <h1>EGL</h1>
  18
  19 <p>The current version of EGL in Mesa implements EGL 1.4.  More information
  20 about EGL can be found at
  21 <a href="https://www.khronos.org/egl/">
  22 https://www.khronos.org/egl/</a>.</p>
  23
  24 <p>The Mesa's implementation of EGL uses a driver architecture.  The main
  25 library (<code>libEGL</code>) is window system neutral.  It provides the EGL
  26 API entry points and helper functions for use by the drivers.  Drivers are
  27 dynamically loaded by the main library and most of the EGL API calls are
  28 directly dispatched to the drivers.</p>
  29
  30 <p>The driver in use decides the window system to support.</p>
  31
  32 <h2>Build EGL</h2>
  33
  34 <ol>
  35 <li>
  36 <p>Configure your build with the desired client APIs and enable
  37 the driver for your hardware.  For example:</p>
  38
  39 <pre>
  40 $ meson configure \
  41         -D egl=true \
  42         -D gles1=true \
  43         -D gles2=true \
  44         -D dri-drivers=... \
  45         -D gallium-drivers=...
  46 </pre>
  47
  48 <p>The main library and OpenGL is enabled by default.  The first two options
  49 above enables <a href="opengles.html">OpenGL ES 1.x and 2.x</a>.  The last two
  50 options enables the listed classic and Gallium drivers respectively.</p>
  51
  52 </li>
  53
  54 <li>Build and install Mesa as usual.</li>
  55 </ol>
  56
  57 <p>In the given example, it will build and install <code>libEGL</code>,
  58 <code>libGL</code>, <code>libGLESv1_CM</code>, <code>libGLESv2</code>, and one
  59 or more EGL drivers.</p>
  60
  61 <h3>Configure Options</h3>
  62
  63 <p>There are several options that control the build of EGL at configuration
  64 time</p>
  65
  66 <dl>
  67 <dt><code>-D egl=true</code></dt>
  68 <dd>
  69
  70 <p>By default, EGL is enabled.  When disabled, the main library and the drivers
  71 will not be built.</p>
  72
  73 </dd>
  74
  75 <dt><code>-D platforms=...</code></dt>
  76 <dd>
  77
  78 <p>List the platforms (window systems) to support.  Its argument is a comma
  79 separated string such as <code>-D platforms=x11,drm</code>.  It decides
  80 the platforms a driver may support.  The first listed platform is also used by
  81 the main library to decide the native platform.</p>
  82
  83 <p>The available platforms are <code>x11</code>, <code>drm</code>,
  84 <code>wayland</code>, <code>surfaceless</code>, <code>android</code>,
  85 and <code>haiku</code>.
  86 The <code>android</code> platform can either be built as a system
  87 component, part of AOSP, using <code>Android.mk</code> files, or
  88 cross-compiled using appropriate options.
  89 Unless for special needs, the build system should
  90 select the right platforms automatically.</p>
  91
  92 </dd>
  93
  94 <dt><code>-D gles1=true</code> and <code>-D gles2=true</code></dt>
  95 <dd>
  96
  97 <p>These options enable OpenGL ES support in OpenGL.  The result is one big
  98 internal library that supports multiple APIs.</p>
  99
 100 </dd>
 101
 102 <dt><code>-D shared-glapi=true</code></dt>
 103 <dd>
 104
 105 <p>By default, <code>libGL</code> has its own copy of <code>libglapi</code>.
 106 This options makes <code>libGL</code> use the shared <code>libglapi</code>.  This
 107 is required if applications mix OpenGL and OpenGL ES.</p>
 108
 109 </dd>
 110
 111 </dl>
 112
 113 <h2>Use EGL</h2>
 114
 115 <h3>Demos</h3>
 116
 117 <p>There are demos for the client APIs supported by EGL.  They can be found in
 118 mesa/demos repository.</p>
 119
 120 <h3>Environment Variables</h3>
 121
 122 <p>There are several environment variables that control the behavior of EGL at
 123 runtime</p>
 124
 125 <dl>
 126 <dt><code>EGL_PLATFORM</code></dt>
 127 <dd>
 128
 129 <p>This variable specifies the native platform.  The valid values are the same
 130 as those for <code>-D platforms=...</code>.  When the variable is not set,
 131 the main library uses the first platform listed in
 132 <code>-D platforms=...</code> as the native platform.</p>
 133
 134 <p>Extensions like <code>EGL_MESA_drm_display</code> define new functions to
 135 create displays for non-native platforms.  These extensions are usually used by
 136 applications that support non-native platforms.  Setting this variable is
 137 probably required only for some of the demos found in mesa/demo repository.</p>
 138
 139 </dd>
 140
 141 <dt><code>EGL_LOG_LEVEL</code></dt>
 142 <dd>
 143
 144 <p>This changes the log level of the main library and the drivers.  The valid
 145 values are: <code>debug</code>, <code>info</code>, <code>warning</code>, and
 146 <code>fatal</code>.</p>
 147
 148 </dd>
 149 </dl>
 150
 151 <h2>EGL Drivers</h2>
 152
 153 <dl>
 154 <dt><code>egl_dri2</code></dt>
 155 <dd>
 156
 157 <p>This driver supports both <code>x11</code> and <code>drm</code> platforms.
 158 It functions as a DRI driver loader.  For <code>x11</code> support, it talks to
 159 the X server directly using (XCB-)DRI2 protocol.</p>
 160
 161 <p>This driver can share DRI drivers with <code>libGL</code>.</p>
 162
 163 </dd>
 164 </dl>
 165
 166 <h2>Packaging</h2>
 167
 168 <p>The ABI between the main library and its drivers are not stable.  Nor is
 169 there a plan to stabilize it at the moment.</p>
 170
 171 <h2>Developers</h2>
 172
 173 <p>The sources of the main library and drivers can be found at
 174 <code>src/egl/</code>.</p>
 175
 176 <h3>Lifetime of Display Resources</h3>
 177
 178 <p>Contexts and surfaces are examples of display resources.  They might live
 179 longer than the display that creates them.</p>
 180
 181 <p>In EGL, when a display is terminated through <code>eglTerminate</code>, all
 182 display resources should be destroyed.  Similarly, when a thread is released
 183 through <code>eglReleaseThread</code>, all current display resources should be
 184 released.  Another way to destroy or release resources is through functions
 185 such as <code>eglDestroySurface</code> or <code>eglMakeCurrent</code>.</p>
 186
 187 <p>When a resource that is current to some thread is destroyed, the resource
 188 should not be destroyed immediately.  EGL requires the resource to live until
 189 it is no longer current.  A driver usually calls
 190 <code>eglIs&lt;Resource&gt;Bound</code> to check if a resource is bound
 191 (current) to any thread in the destroy callbacks.  If it is still bound, the
 192 resource is not destroyed.</p>
 193
 194 <p>The main library will mark destroyed current resources as unlinked.  In a
 195 driver's <code>MakeCurrent</code> callback,
 196 <code>eglIs&lt;Resource&gt;Linked</code> can then be called to check if a newly
 197 released resource is linked to a display.  If it is not, the last reference to
 198 the resource is removed and the driver should destroy the resource.  But it
 199 should be careful here because <code>MakeCurrent</code> might be called with an
 200 uninitialized display.</p>
 201
 202 <p>This is the only mechanism provided by the main library to help manage the
 203 resources.  The drivers are responsible to the correct behavior as defined by
 204 EGL.</p>
 205
 206 <h3><code>EGL_RENDER_BUFFER</code></h3>
 207
 208 <p>In EGL, the color buffer a context should try to render to is decided by the
 209 binding surface.  It should try to render to the front buffer if the binding
 210 surface has <code>EGL_RENDER_BUFFER</code> set to
 211 <code>EGL_SINGLE_BUFFER</code>;  If the same context is later bound to a
 212 surface with <code>EGL_RENDER_BUFFER</code> set to
 213 <code>EGL_BACK_BUFFER</code>, the context should try to render to the back
 214 buffer.  However, the context is allowed to make the final decision as to which
 215 color buffer it wants to or is able to render to.</p>
 216
 217 <p>For pbuffer surfaces, the render buffer is always
 218 <code>EGL_BACK_BUFFER</code>.  And for pixmap surfaces, the render buffer is
 219 always <code>EGL_SINGLE_BUFFER</code>.  Unlike window surfaces, EGL spec
 220 requires their <code>EGL_RENDER_BUFFER</code> values to be honored.  As a
 221 result, a driver should never set <code>EGL_PIXMAP_BIT</code> or
 222 <code>EGL_PBUFFER_BIT</code> bits of a config if the contexts created with the
 223 config won't be able to honor the <code>EGL_RENDER_BUFFER</code> of pixmap or
 224 pbuffer surfaces.</p>
 225
 226 <p>It should also be noted that pixmap and pbuffer surfaces are assumed to be
 227 single-buffered, in that <code>eglSwapBuffers</code> has no effect on them.  It
 228 is desirable that a driver allocates a private color buffer for each pbuffer
 229 surface created.  If the window system the driver supports has native pbuffers,
 230 or if the native pixmaps have more than one color buffers, the driver should
 231 carefully attach the native color buffers to the EGL surfaces, re-route them if
 232 required.</p>
 233
 234 <p>There is no defined behavior as to, for example, how
 235 <code>glDrawBuffer</code> interacts with <code>EGL_RENDER_BUFFER</code>.  Right
 236 now, it is desired that the draw buffer in a client API be fixed for pixmap and
 237 pbuffer surfaces.  Therefore, the driver is responsible to guarantee that the
 238 client API renders to the specified render buffer for pixmap and pbuffer
 239 surfaces.</p>
 240
 241 <h3><code>EGLDisplay</code> Mutex</h3>
 242
 243 The <code>EGLDisplay</code> will be locked before calling any of the dispatch
 244 functions (well, except for GetProcAddress which does not take an
 245 <code>EGLDisplay</code>).  This guarantees that the same dispatch function will
 246 not be called with the sample display at the same time.  If a driver has access
 247 to an <code>EGLDisplay</code> without going through the EGL APIs, the driver
 248 should as well lock the display before using it.
 249
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