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 The <code>haiku</code> platform can only be built with SCons or Meson.
  90 Unless for special needs, the build system should
  91 select the right platforms automatically.</p>
  92
  93 </dd>
  94
  95 <dt><code>-D gles1=true</code> and <code>-D gles2=true</code></dt>
  96 <dd>
  97
  98 <p>These options enable OpenGL ES support in OpenGL.  The result is one big
  99 internal library that supports multiple APIs.</p>
 100
 101 </dd>
 102
 103 <dt><code>-D shared-glapi=true</code></dt>
 104 <dd>
 105
 106 <p>By default, <code>libGL</code> has its own copy of <code>libglapi</code>.
 107 This options makes <code>libGL</code> use the shared <code>libglapi</code>.  This
 108 is required if applications mix OpenGL and OpenGL ES.</p>
 109
 110 </dd>
 111
 112 </dl>
 113
 114 <h2>Use EGL</h2>
 115
 116 <h3>Demos</h3>
 117
 118 <p>There are demos for the client APIs supported by EGL.  They can be found in
 119 mesa/demos repository.</p>
 120
 121 <h3>Environment Variables</h3>
 122
 123 <p>There are several environment variables that control the behavior of EGL at
 124 runtime</p>
 125
 126 <dl>
 127 <dt><code>EGL_PLATFORM</code></dt>
 128 <dd>
 129
 130 <p>This variable specifies the native platform.  The valid values are the same
 131 as those for <code>-D platforms=...</code>.  When the variable is not set,
 132 the main library uses the first platform listed in
 133 <code>-D platforms=...</code> as the native platform.</p>
 134
 135 <p>Extensions like <code>EGL_MESA_drm_display</code> define new functions to
 136 create displays for non-native platforms.  These extensions are usually used by
 137 applications that support non-native platforms.  Setting this variable is
 138 probably required only for some of the demos found in mesa/demo repository.</p>
 139
 140 </dd>
 141
 142 <dt><code>EGL_LOG_LEVEL</code></dt>
 143 <dd>
 144
 145 <p>This changes the log level of the main library and the drivers.  The valid
 146 values are: <code>debug</code>, <code>info</code>, <code>warning</code>, and
 147 <code>fatal</code>.</p>
 148
 149 </dd>
 150 </dl>
 151
 152 <h2>EGL Drivers</h2>
 153
 154 <dl>
 155 <dt><code>egl_dri2</code></dt>
 156 <dd>
 157
 158 <p>This driver supports both <code>x11</code> and <code>drm</code> platforms.
 159 It functions as a DRI driver loader.  For <code>x11</code> support, it talks to
 160 the X server directly using (XCB-)DRI2 protocol.</p>
 161
 162 <p>This driver can share DRI drivers with <code>libGL</code>.</p>
 163
 164 </dd>
 165 </dl>
 166
 167 <h2>Packaging</h2>
 168
 169 <p>The ABI between the main library and its drivers are not stable.  Nor is
 170 there a plan to stabilize it at the moment.</p>
 171
 172 <h2>Developers</h2>
 173
 174 <p>The sources of the main library and drivers can be found at
 175 <code>src/egl/</code>.</p>
 176
 177 <h3>Lifetime of Display Resources</h3>
 178
 179 <p>Contexts and surfaces are examples of display resources.  They might live
 180 longer than the display that creates them.</p>
 181
 182 <p>In EGL, when a display is terminated through <code>eglTerminate</code>, all
 183 display resources should be destroyed.  Similarly, when a thread is released
 184 through <code>eglReleaseThread</code>, all current display resources should be
 185 released.  Another way to destroy or release resources is through functions
 186 such as <code>eglDestroySurface</code> or <code>eglMakeCurrent</code>.</p>
 187
 188 <p>When a resource that is current to some thread is destroyed, the resource
 189 should not be destroyed immediately.  EGL requires the resource to live until
 190 it is no longer current.  A driver usually calls
 191 <code>eglIs&lt;Resource&gt;Bound</code> to check if a resource is bound
 192 (current) to any thread in the destroy callbacks.  If it is still bound, the
 193 resource is not destroyed.</p>
 194
 195 <p>The main library will mark destroyed current resources as unlinked.  In a
 196 driver's <code>MakeCurrent</code> callback,
 197 <code>eglIs&lt;Resource&gt;Linked</code> can then be called to check if a newly
 198 released resource is linked to a display.  If it is not, the last reference to
 199 the resource is removed and the driver should destroy the resource.  But it
 200 should be careful here because <code>MakeCurrent</code> might be called with an
 201 uninitialized display.</p>
 202
 203 <p>This is the only mechanism provided by the main library to help manage the
 204 resources.  The drivers are responsible to the correct behavior as defined by
 205 EGL.</p>
 206
 207 <h3><code>EGL_RENDER_BUFFER</code></h3>
 208
 209 <p>In EGL, the color buffer a context should try to render to is decided by the
 210 binding surface.  It should try to render to the front buffer if the binding
 211 surface has <code>EGL_RENDER_BUFFER</code> set to
 212 <code>EGL_SINGLE_BUFFER</code>;  If the same context is later bound to a
 213 surface with <code>EGL_RENDER_BUFFER</code> set to
 214 <code>EGL_BACK_BUFFER</code>, the context should try to render to the back
 215 buffer.  However, the context is allowed to make the final decision as to which
 216 color buffer it wants to or is able to render to.</p>
 217
 218 <p>For pbuffer surfaces, the render buffer is always
 219 <code>EGL_BACK_BUFFER</code>.  And for pixmap surfaces, the render buffer is
 220 always <code>EGL_SINGLE_BUFFER</code>.  Unlike window surfaces, EGL spec
 221 requires their <code>EGL_RENDER_BUFFER</code> values to be honored.  As a
 222 result, a driver should never set <code>EGL_PIXMAP_BIT</code> or
 223 <code>EGL_PBUFFER_BIT</code> bits of a config if the contexts created with the
 224 config won't be able to honor the <code>EGL_RENDER_BUFFER</code> of pixmap or
 225 pbuffer surfaces.</p>
 226
 227 <p>It should also be noted that pixmap and pbuffer surfaces are assumed to be
 228 single-buffered, in that <code>eglSwapBuffers</code> has no effect on them.  It
 229 is desirable that a driver allocates a private color buffer for each pbuffer
 230 surface created.  If the window system the driver supports has native pbuffers,
 231 or if the native pixmaps have more than one color buffers, the driver should
 232 carefully attach the native color buffers to the EGL surfaces, re-route them if
 233 required.</p>
 234
 235 <p>There is no defined behavior as to, for example, how
 236 <code>glDrawBuffer</code> interacts with <code>EGL_RENDER_BUFFER</code>.  Right
 237 now, it is desired that the draw buffer in a client API be fixed for pixmap and
 238 pbuffer surfaces.  Therefore, the driver is responsible to guarantee that the
 239 client API renders to the specified render buffer for pixmap and pbuffer
 240 surfaces.</p>
 241
 242 <h3><code>EGLDisplay</code> Mutex</h3>
 243
 244 The <code>EGLDisplay</code> will be locked before calling any of the dispatch
 245 functions (well, except for GetProcAddress which does not take an
 246 <code>EGLDisplay</code>).  This guarantees that the same dispatch function will
 247 not be called with the sample display at the same time.  If a driver has access
 248 to an <code>EGLDisplay</code> without going through the EGL APIs, the driver
 249 should as well lock the display before using it.
 250
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