docs/egl.html

   1 <html>
   2
   3 <title>Mesa EGL</title>
   4
   5 <head><link rel="stylesheet" type="text/css" href="mesa.css"></head>
   6
   7 <body>
   8
   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.</p>
  23
  24 <h2>Build EGL</h2>
  25
  26 <ol>
  27 <li>
  28 <p>Run <code>configure</code> with the desired client APIs and enable
  29 the driver for your hardware.  For example</p>
  30
  31 <pre>
  32   $ ./configure --enable-gles2 --enable-openvg --enable-gallium-nouveau
  33 </pre>
  34
  35 <p>The main library and OpenGL is enabled by default.  The first option above
  36 enables <a href="opengles.html">OpenGL ES 2.x</a>.  The second option enables
  37 <a href="openvg.html">OpenVG</a>.</p>
  38
  39 </li>
  40
  41 <li>Build and install Mesa as usual.</li>
  42 </ol>
  43
  44 <p>In the given example, it will build and install <code>libEGL</code>,
  45 <code>libGL</code>, <code>libGLESv1_CM</code>, <code>libGLESv2</code>,
  46 <code>libOpenVG</code>, and one or more EGL drivers.</p>
  47
  48 <h3>Configure Options</h3>
  49
  50 <p>There are several options that control the build of EGL at configuration
  51 time</p>
  52
  53 <ul>
  54 <li><code>--enable-egl</code>
  55
  56 <p>By default, EGL is enabled.  When disabled, the main library and the drivers
  57 will not be built.</p>
  58
  59 </li>
  60
  61 <li><code>--with-egl-driver-dir</code>
  62
  63 <p>The directory EGL drivers should be installed to.  If not specified, EGL
  64 drivers will be installed to <code>${libdir}/egl</code>.</p>
  65
  66 </li>
  67
  68 <li><code>--with-egl-platforms</code>
  69
  70 <p>List the platforms (window systems) to support.  Its argument is a comma
  71 seprated string such as <code>--with-egl-platforms=x11,drm</code>.  It decides
  72 the platforms a driver may support.  The first listed platform is also used by
  73 the main library to decide the native platform: the platform the EGL native
  74 types such as <code>EGLNativeDisplayType</code> or
  75 <code>EGLNativeWindowType</code> defined for.</p>
  76
  77 <p>The available platforms are <code>x11</code>, <code>drm</code>,
  78 <code>fbdev</code>, and <code>gdi</code>.  The <code>gdi</code> platform can
  79 only be built with SCons.  Unless for special needs, the build system should
  80 select the right platforms automatically.</p>
  81
  82 </li>
  83
  84 <li><code>--enable-gles1</code> and <code>--enable-gles2</code>
  85
  86 <p>These options enable OpenGL ES support in OpenGL.  The result is one big
  87 internal library that supports multiple APIs.</p>
  88
  89 </li>
  90
  91 <li><code>--enable-gles-overlay</code>
  92
  93 <p>This option enables OpenGL ES as separate internal libraries.  This is an
  94 alternative approach to enable OpenGL ES.</p>
  95
  96 <p>This is only supported by <code>egl_gallium</code>.  For systems using DRI
  97 drivers, <code>--enable-gles1</code> and <code>--enable-gles2</code> are
  98 suggested instead as all drivers will benefit.</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 <p>This variable is usually set to test an uninstalled build.  For example, one
 138 may set</p>
 139
 140 <pre>
 141   $ export LD_LIBRARY_PATH=$mesa/lib
 142   $ export EGL_DRIVERS_PATH=$mesa/lib/egl
 143 </pre>
 144
 145 <p>to test a build without installation</p>
 146
 147 </li>
 148
 149 <li><code>EGL_DRIVER</code>
 150
 151 <p>This variable specifies a full path to an EGL driver and it forces the
 152 specified EGL driver to be loaded.  It comes in handy when one wants to test a
 153 specific driver.  This variable is ignored for setuid/setgid binaries.</p>
 154
 155 </li>
 156
 157 <li><code>EGL_PLATFORM</code>
 158
 159 <p>This variable specifies the native platform.  The valid values are the same
 160 as those for <code>--with-egl-platforms</code>.  When the variable is not set,
 161 the main library uses the first platform listed in
 162 <code>--with-egl-platforms</code> as the native platform.</p>
 163
 164 <p>Extensions like <code>EGL_MESA_drm_display</code> define new functions to
 165 create displays for non-native platforms.  These extensions are usually used by
 166 applications that support non-native platforms.  Setting this variable is
 167 probably required only for some of the demos found in mesa/demo repository.</p>
 168
 169 </li>
 170
 171 <li><code>EGL_LOG_LEVEL</code>
 172
 173 <p>This changes the log level of the main library and the drivers.  The valid
 174 values are: <code>debug</code>, <code>info</code>, <code>warning</code>, and
 175 <code>fatal</code>.</p>
 176
 177 </li>
 178
 179 <li><code>EGL_SOFTWARE</code>
 180
 181 <p>For drivers that support both hardware and software rendering, setting this
 182 variable to true forces the use of software rendering.</p>
 183
 184 </li>
 185 </ul>
 186
 187 <h2>EGL Drivers</h2>
 188
 189 <ul>
 190 <li><code>egl_dri2</code>
 191
 192 <p>This driver supports both <code>x11</code> and <code>drm</code> platforms.
 193 It functions as a DRI driver loader.  For <code>x11</code> support, it talks to
 194 the X server directly using (XCB-)DRI2 protocol.</p>
 195
 196 <p>This driver can share DRI drivers with <code>libGL</code>.</p>
 197
 198 </li>
 199
 200 <li><code>egl_gallium</code>
 201
 202 <p>This driver is based on Gallium3D.  It supports all rendering APIs and
 203 hardwares supported by Gallium3D.  It is the only driver that supports OpenVG.
 204 The supported platforms are X11, DRM, FBDEV, and GDI.</p>
 205
 206 <p>This driver comes with its own hardware drivers
 207 (<code>pipe_&lt;hw&gt;</code>) and client API modules
 208 (<code>st_&lt;api&gt;</code>).</p>
 209
 210 </li>
 211
 212 <li><code>egl_glx</code>
 213
 214 <p>This driver provides a wrapper to GLX.  It uses exclusively GLX to implement
 215 the EGL API.  It supports both direct and indirect rendering when the GLX does.
 216 It is accelerated when the GLX is.  As such, it cannot provide functions that
 217 is not available in GLX or GLX extensions.</p>
 218 </li>
 219 </ul>
 220
 221 <h2>Packaging</h2>
 222
 223 <p>The ABI between the main library and its drivers are not stable.  Nor is
 224 there a plan to stabilize it at the moment.  Of the EGL drivers,
 225 <code>egl_gallium</code> has its own hardware drivers and client API modules.
 226 They are considered internal to <code>egl_gallium</code> and there is also no
 227 stable ABI between them.  These should be kept in mind when packaging for
 228 distribution.</p>
 229
 230 <p>Generally, <code>egl_dri2</code> is preferred over <code>egl_gallium</code>
 231 when the system already has DRI drivers.  As <code>egl_gallium</code> is loaded
 232 before <code>egl_dri2</code> when both are available, <code>egl_gallium</code>
 233 may either be disabled with <code>--disable-gallium-egl</code> or packaged
 234 separately.</p>
 235
 236 <h2>Developers</h2>
 237
 238 <p>The sources of the main library and the classic drivers can be found at
 239 <code>src/egl/</code>.  The sources of the <code>egl</code> state tracker can
 240 be found at <code>src/gallium/state_trackers/egl/</code>.</p>
 241
 242 <p>The suggested way to learn to write a EGL driver is to see how other drivers
 243 are written.  <code>egl_glx</code> should be a good reference.  It works in any
 244 environment that has GLX support, and it is simpler than most drivers.</p>
 245
 246 <h3>Lifetime of Display Resources</h3>
 247
 248 <p>Contexts and surfaces are examples of display resources.  They might live
 249 longer than the display that creates them.</p>
 250
 251 <p>In EGL, when a display is terminated through <code>eglTerminate</code>, all
 252 display resources should be destroyed.  Similarly, when a thread is released
 253 throught <code>eglReleaseThread</code>, all current display resources should be
 254 released.  Another way to destory or release resources is through functions
 255 such as <code>eglDestroySurface</code> or <code>eglMakeCurrent</code>.</p>
 256
 257 <p>When a resource that is current to some thread is destroyed, the resource
 258 should not be destroyed immediately.  EGL requires the resource to live until
 259 it is no longer current.  A driver usually calls
 260 <code>eglIs&lt;Resource&gt;Bound</code> to check if a resource is bound
 261 (current) to any thread in the destroy callbacks.  If it is still bound, the
 262 resource is not destroyed.</p>
 263
 264 <p>The main library will mark destroyed current resources as unlinked.  In a
 265 driver's <code>MakeCurrent</code> callback,
 266 <code>eglIs&lt;Resource&gt;Linked</code> can then be called to check if a newly
 267 released resource is linked to a display.  If it is not, the last reference to
 268 the resource is removed and the driver should destroy the resource.  But it
 269 should be careful here because <code>MakeCurrent</code> might be called with an
 270 uninitialized display.</p>
 271
 272 <p>This is the only mechanism provided by the main library to help manage the
 273 resources.  The drivers are responsible to the correct behavior as defined by
 274 EGL.</p>
 275
 276 <h3><code>EGL_RENDER_BUFFER</code></h3>
 277
 278 <p>In EGL, the color buffer a context should try to render to is decided by the
 279 binding surface.  It should try to render to the front buffer if the binding
 280 surface has <code>EGL_RENDER_BUFFER</code> set to
 281 <code>EGL_SINGLE_BUFFER</code>;  If the same context is later bound to a
 282 surface with <code>EGL_RENDER_BUFFER</code> set to
 283 <code>EGL_BACK_BUFFER</code>, the context should try to render to the back
 284 buffer.  However, the context is allowed to make the final decision as to which
 285 color buffer it wants to or is able to render to.</p>
 286
 287 <p>For pbuffer surfaces, the render buffer is always
 288 <code>EGL_BACK_BUFFER</code>.  And for pixmap surfaces, the render buffer is
 289 always <code>EGL_SINGLE_BUFFER</code>.  Unlike window surfaces, EGL spec
 290 requires their <code>EGL_RENDER_BUFFER</code> values to be honored.  As a
 291 result, a driver should never set <code>EGL_PIXMAP_BIT</code> or
 292 <code>EGL_PBUFFER_BIT</code> bits of a config if the contexts created with the
 293 config won't be able to honor the <code>EGL_RENDER_BUFFER</code> of pixmap or
 294 pbuffer surfaces.</p>
 295
 296 <p>It should also be noted that pixmap and pbuffer surfaces are assumed to be
 297 single-buffered, in that <code>eglSwapBuffers</code> has no effect on them.  It
 298 is desirable that a driver allocates a private color buffer for each pbuffer
 299 surface created.  If the window system the driver supports has native pbuffers,
 300 or if the native pixmaps have more than one color buffers, the driver should
 301 carefully attach the native color buffers to the EGL surfaces, re-route them if
 302 required.</p>
 303
 304 <p>There is no defined behavior as to, for example, how
 305 <code>glDrawBuffer</code> interacts with <code>EGL_RENDER_BUFFER</code>.  Right
 306 now, it is desired that the draw buffer in a client API be fixed for pixmap and
 307 pbuffer surfaces.  Therefore, the driver is responsible to guarantee that the
 308 client API renders to the specified render buffer for pixmap and pbuffer
 309 surfaces.</p>
 310
 311 <h3><code>EGLDisplay</code> Mutex</h3>
 312
 313 The <code>EGLDisplay</code> will be locked before calling any of the dispatch
 314 functions (well, except for GetProcAddress which does not take an
 315 <code>EGLDisplay</code>).  This guarantees that the same dispatch function will
 316 not be called with the sample display at the same time.  If a driver has access
 317 to an <code>EGLDisplay</code> without going through the EGL APIs, the driver
 318 should as well lock the display before using it.
 319
 320 <h3>TODOs</h3>
 321
 322 <ul>
 323 <li>Pass the conformance tests</li>
 324 <li>Mixed use of OpenGL, OpenGL ES 1.1, and OpenGL ES 2.0 is supported.  But
 325 which one of <code>libGL.so</code>, <code>libGLESv1_CM.so</code>, and
 326 <code>libGLESv2.so</code> should an application link to?  Bad things may happen
 327 when, say, an application is linked to <code>libGLESv2.so</code> and
 328 <code>libcairo</code>, which is linked to <code>libGL.so</code> instead.</li>
 329
 330 </ul>
 331
 332 </body>
 333 </html>