docs/egl.html

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   9 <h1>Mesa EGL</h1>
  10
  11 <p>The current version of EGL in Mesa implements EGL 1.4.  More information
  12 about EGL can be found at
  13 <a href="http://www.khronos.org/egl/" target="_parent">
  14 http://www.khronos.org/egl/</a>.</p>
  15
  16 <p>The Mesa's implementation of EGL uses a driver architecture.  The main
  17 library (<code>libEGL</code>) is window system neutral.  It provides the EGL
  18 API entry points and helper functions for use by the drivers.  Drivers are
  19 dynamically loaded by the main library and most of the EGL API calls are
  20 directly dispatched to the drivers.</p>
  21
  22 <p>The driver in use decides the window system to support.  For drivers that
  23 support hardware rendering, there are usually multiple drivers supporting the
  24 same window system.  Each one of of them supports a certain range of graphics
  25 cards.</p>
  26
  27 <h2>Build EGL</h2>
  28
  29 <ol>
  30 <li>
  31 <p>Run <code>configure</code> with the desired client APIs and enable
  32 the driver for your hardware.  For example</p>
  33
  34 <pre>
  35   $ ./configure --enable-gles2 --enable-openvg --enable-gallium-nouveau
  36 </pre>
  37
  38 <p>The main library and OpenGL is enabled by default.  The first option above
  39 enables <a href="opengles.html">OpenGL ES 2.x</a>.  The second option enables
  40 <a href="openvg.html">OpenVG</a>.</p>
  41
  42 </li>
  43
  44 <li>Build and install Mesa as usual.</li>
  45 </ol>
  46
  47 <p>In the given example, it will build and install <code>libEGL</code>,
  48 <code>libGL</code>, <code>libGLESv1_CM</code>, <code>libGLESv2</code>,
  49 <code>libOpenVG</code>, and one or more EGL drivers.</p>
  50
  51 <h3>Configure Options</h3>
  52
  53 <p>There are several options that control the build of EGL at configuration
  54 time</p>
  55
  56 <ul>
  57 <li><code>--enable-egl</code>
  58
  59 <p>By default, EGL is enabled.  When disabled, the main library and the drivers
  60 will not be built.</p>
  61
  62 </li>
  63
  64 <li><code>--with-egl-driver-dir</code>
  65
  66 <p>The directory EGL drivers should be installed to.  If not specified, EGL
  67 drivers will be installed to <code>${libdir}/egl</code>.</p>
  68
  69 </li>
  70
  71 <li><code>--with-egl-platforms</code>
  72
  73 <p>List the platforms (window systems) to support.  Its argument is a comma
  74 seprated string such as <code>--with-egl-platforms=x11,drm</code>.  It decides
  75 the platforms a driver may support.  The first listed platform is also used by
  76 the main library to decide the native platform: the platform the EGL native
  77 types such as <code>EGLNativeDisplayType</code> or
  78 <code>EGLNativeWindowType</code> defined for.</p>
  79
  80 <p>The available platforms are <code>x11</code>, <code>drm</code>,
  81 <code>fbdev</code>, and <code>gdi</code>.  The <code>gdi</code> platform can
  82 only be built with SCons.  Unless for special needs, the build system should
  83 select the right platforms automatically.</p>
  84
  85 </li>
  86
  87 <li><code>--enable-gles1</code> and <code>--enable-gles2</code>
  88
  89 <p>These options enable OpenGL ES support in OpenGL.  The result is
  90 one big library that supports multiple APIs.</p>
  91
  92 </li>
  93
  94 <li><code>--enable-gles-overlay</code>
  95
  96 <p>This option enables OpenGL ES as separate libraries.  This is an alternative
  97 approach to enable OpenGL ES.  It is only supported by
  98 <code>egl_gallium</code>.</p>
  99
 100 </li>
 101
 102 <li><code>--enable-openvg</code>
 103
 104 <p>OpenVG must be explicitly enabled by this option.</p>
 105
 106 </li>
 107
 108 <li><code>--enable-gallium-egl</code>
 109
 110 <p>Explicitly enable or disable <code>egl_gallium</code>.</p>
 111
 112 </li>
 113
 114 </ul>
 115
 116 <h2>Use EGL</h2>
 117
 118 <h3>Demos</h3>
 119
 120 <p>There are demos for the client APIs supported by EGL.  They can be found in
 121 mesa/demos repository.</p>
 122
 123 <h3>Environment Variables</h3>
 124
 125 <p>There are several environment variables that control the behavior of EGL at
 126 runtime</p>
 127
 128 <ul>
 129 <li><code>EGL_DRIVERS_PATH</code>
 130
 131 <p>By default, the main library will look for drivers in the directory where
 132 the drivers are installed to.  This variable specifies a list of
 133 colon-separated directories where the main library will look for drivers, in
 134 addition to the default directory.  This variable is ignored for setuid/setgid
 135 binaries.</p>
 136
 137 </li>
 138
 139 <li><code>EGL_DRIVER</code>
 140
 141 <p>This variable specifies a full path to an EGL driver and it forces the
 142 specified EGL driver to be loaded.  It comes in handy when one wants to test a
 143 specific driver.  This variable is ignored for setuid/setgid binaries.</p>
 144
 145 </li>
 146
 147 <li><code>EGL_PLATFORM</code>
 148
 149 <p>This variable specifies the native platform.  The valid values are the same
 150 as those for <code>--with-egl-platforms</code>.  When the variable is not set,
 151 the main library uses the first platform listed in
 152 <code>--with-egl-platforms</code> as the native platform.</p>
 153
 154 <p>Extensions like <code>EGL_MESA_drm_display</code> define new functions to
 155 create displays for non-native platforms.  These extensions are usually used by
 156 applications that support non-native platforms.  Setting this variable is
 157 probably required only for some of the demos found in mesa/demo repository.</p>
 158
 159 </li>
 160
 161 <li><code>EGL_LOG_LEVEL</code>
 162
 163 <p>This changes the log level of the main library and the drivers.  The valid
 164 values are: <code>debug</code>, <code>info</code>, <code>warning</code>, and
 165 <code>fatal</code>.</p>
 166
 167 </li>
 168
 169 <li><code>EGL_SOFTWARE</code>
 170
 171 <p>For drivers that support both hardware and software rendering, setting this
 172 variable to true forces the use of software rendering.</p>
 173
 174 </li>
 175 </ul>
 176
 177 <h2>EGL Drivers</h2>
 178
 179 <ul>
 180 <li><code>egl_dri2</code>
 181
 182 <p>This driver supports both <code>x11</code> and <code>drm</code> platforms.
 183 It functions as a DRI2 driver loader.  For <code>x11</code> support, it talks
 184 to the X server directly using (XCB-)DRI2 protocol.</p>
 185
 186 </li>
 187
 188 <li><code>egl_gallium</code>
 189
 190 <p>This driver is based on Gallium3D.  It supports all rendering APIs and
 191 hardwares supported by Gallium3D.  It is the only driver that supports OpenVG.
 192 The supported platforms are X11, DRM, FBDEV, and GDI.</p>
 193
 194 </li>
 195
 196 <li><code>egl_glx</code>
 197
 198 <p>This driver provides a wrapper to GLX.  It uses exclusively GLX to implement
 199 the EGL API.  It supports both direct and indirect rendering when the GLX does.
 200 It is accelerated when the GLX is.  As such, it cannot provide functions that
 201 is not available in GLX or GLX extensions.</p>
 202 </li>
 203 </ul>
 204
 205 <h2>Developers</h2>
 206
 207 <p>The sources of the main library and the classic drivers can be found at
 208 <code>src/egl/</code>.  The sources of the <code>egl</code> state tracker can
 209 be found at <code>src/gallium/state_trackers/egl/</code>.</p>
 210
 211 <p>The suggested way to learn to write a EGL driver is to see how other drivers
 212 are written.  <code>egl_glx</code> should be a good reference.  It works in any
 213 environment that has GLX support, and it is simpler than most drivers.</p>
 214
 215 <h3>Lifetime of Display Resources</h3>
 216
 217 <p>Contexts and surfaces are examples of display resources.  They might live
 218 longer than the display that creates them.</p>
 219
 220 <p>In EGL, when a display is terminated through <code>eglTerminate</code>, all
 221 display resources should be destroyed.  Similarly, when a thread is released
 222 throught <code>eglReleaseThread</code>, all current display resources should be
 223 released.  Another way to destory or release resources is through functions
 224 such as <code>eglDestroySurface</code> or <code>eglMakeCurrent</code>.</p>
 225
 226 <p>When a resource that is current to some thread is destroyed, the resource
 227 should not be destroyed immediately.  EGL requires the resource to live until
 228 it is no longer current.  A driver usually calls
 229 <code>eglIs&lt;Resource&gt;Bound</code> to check if a resource is bound
 230 (current) to any thread in the destroy callbacks.  If it is still bound, the
 231 resource is not destroyed.</p>
 232
 233 <p>The main library will mark destroyed current resources as unlinked.  In a
 234 driver's <code>MakeCurrent</code> callback,
 235 <code>eglIs&lt;Resource&gt;Linked</code> can then be called to check if a newly
 236 released resource is linked to a display.  If it is not, the last reference to
 237 the resource is removed and the driver should destroy the resource.  But it
 238 should be careful here because <code>MakeCurrent</code> might be called with an
 239 uninitialized display.</p>
 240
 241 <p>This is the only mechanism provided by the main library to help manage the
 242 resources.  The drivers are responsible to the correct behavior as defined by
 243 EGL.</p>
 244
 245 <h3><code>EGL_RENDER_BUFFER</code></h3>
 246
 247 <p>In EGL, the color buffer a context should try to render to is decided by the
 248 binding surface.  It should try to render to the front buffer if the binding
 249 surface has <code>EGL_RENDER_BUFFER</code> set to
 250 <code>EGL_SINGLE_BUFFER</code>;  If the same context is later bound to a
 251 surface with <code>EGL_RENDER_BUFFER</code> set to
 252 <code>EGL_BACK_BUFFER</code>, the context should try to render to the back
 253 buffer.  However, the context is allowed to make the final decision as to which
 254 color buffer it wants to or is able to render to.</p>
 255
 256 <p>For pbuffer surfaces, the render buffer is always
 257 <code>EGL_BACK_BUFFER</code>.  And for pixmap surfaces, the render buffer is
 258 always <code>EGL_SINGLE_BUFFER</code>.  Unlike window surfaces, EGL spec
 259 requires their <code>EGL_RENDER_BUFFER</code> values to be honored.  As a
 260 result, a driver should never set <code>EGL_PIXMAP_BIT</code> or
 261 <code>EGL_PBUFFER_BIT</code> bits of a config if the contexts created with the
 262 config won't be able to honor the <code>EGL_RENDER_BUFFER</code> of pixmap or
 263 pbuffer surfaces.</p>
 264
 265 <p>It should also be noted that pixmap and pbuffer surfaces are assumed to be
 266 single-buffered, in that <code>eglSwapBuffers</code> has no effect on them.  It
 267 is desirable that a driver allocates a private color buffer for each pbuffer
 268 surface created.  If the window system the driver supports has native pbuffers,
 269 or if the native pixmaps have more than one color buffers, the driver should
 270 carefully attach the native color buffers to the EGL surfaces, re-route them if
 271 required.</p>
 272
 273 <p>There is no defined behavior as to, for example, how
 274 <code>glDrawBuffer</code> interacts with <code>EGL_RENDER_BUFFER</code>.  Right
 275 now, it is desired that the draw buffer in a client API be fixed for pixmap and
 276 pbuffer surfaces.  Therefore, the driver is responsible to guarantee that the
 277 client API renders to the specified render buffer for pixmap and pbuffer
 278 surfaces.</p>
 279
 280 <h3><code>EGLDisplay</code> Mutex</h3>
 281
 282 The <code>EGLDisplay</code> will be locked before calling any of the dispatch
 283 functions (well, except for GetProcAddress which does not take an
 284 <code>EGLDisplay</code>).  This guarantees that the same dispatch function will
 285 not be called with the sample display at the same time.  If a driver has access
 286 to an <code>EGLDisplay</code> without going through the EGL APIs, the driver
 287 should as well lock the display before using it.
 288
 289 <h3>TODOs</h3>
 290
 291 <ul>
 292 <li>Pass the conformance tests</li>
 293 <li>Mixed use of OpenGL, OpenGL ES 1.1, and OpenGL ES 2.0 is supported.  But
 294 which one of <code>libGL.so</code>, <code>libGLESv1_CM.so</code>, and
 295 <code>libGLESv2.so</code> should an application link to?  Bad things may happen
 296 when, say, an application is linked to <code>libGLESv2.so</code> and
 297 <code>libcairo</code>, which is linked to <code>libGL.so</code> instead.</li>
 298
 299 </ul>
 300
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