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.</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 or the name of an EGL driver.  It
 152 forces the specified EGL driver to be loaded.  It comes in handy when one wants
 153 to test a specific driver.  This variable is ignored for setuid/setgid
 154 binaries.</p>
 155
 156 </li>
 157
 158 <li><code>EGL_PLATFORM</code>
 159
 160 <p>This variable specifies the native platform.  The valid values are the same
 161 as those for <code>--with-egl-platforms</code>.  When the variable is not set,
 162 the main library uses the first platform listed in
 163 <code>--with-egl-platforms</code> as the native platform.</p>
 164
 165 <p>Extensions like <code>EGL_MESA_drm_display</code> define new functions to
 166 create displays for non-native platforms.  These extensions are usually used by
 167 applications that support non-native platforms.  Setting this variable is
 168 probably required only for some of the demos found in mesa/demo repository.</p>
 169
 170 </li>
 171
 172 <li><code>EGL_LOG_LEVEL</code>
 173
 174 <p>This changes the log level of the main library and the drivers.  The valid
 175 values are: <code>debug</code>, <code>info</code>, <code>warning</code>, and
 176 <code>fatal</code>.</p>
 177
 178 </li>
 179
 180 <li><code>EGL_SOFTWARE</code>
 181
 182 <p>For drivers that support both hardware and software rendering, setting this
 183 variable to true forces the use of software rendering.</p>
 184
 185 </li>
 186 </ul>
 187
 188 <h2>EGL Drivers</h2>
 189
 190 <ul>
 191 <li><code>egl_dri2</code>
 192
 193 <p>This driver supports both <code>x11</code> and <code>drm</code> platforms.
 194 It functions as a DRI driver loader.  For <code>x11</code> support, it talks to
 195 the X server directly using (XCB-)DRI2 protocol.</p>
 196
 197 <p>This driver can share DRI drivers with <code>libGL</code>.</p>
 198
 199 </li>
 200
 201 <li><code>egl_gallium</code>
 202
 203 <p>This driver is based on Gallium3D.  It supports all rendering APIs and
 204 hardwares supported by Gallium3D.  It is the only driver that supports OpenVG.
 205 The supported platforms are X11, DRM, FBDEV, and GDI.</p>
 206
 207 <p>This driver comes with its own hardware drivers
 208 (<code>pipe_&lt;hw&gt;</code>) and client API modules
 209 (<code>st_&lt;api&gt;</code>).</p>
 210
 211 </li>
 212
 213 <li><code>egl_glx</code>
 214
 215 <p>This driver provides a wrapper to GLX.  It uses exclusively GLX to implement
 216 the EGL API.  It supports both direct and indirect rendering when the GLX does.
 217 It is accelerated when the GLX is.  As such, it cannot provide functions that
 218 is not available in GLX or GLX extensions.</p>
 219 </li>
 220 </ul>
 221
 222 <h2>Packaging</h2>
 223
 224 <p>The ABI between the main library and its drivers are not stable.  Nor is
 225 there a plan to stabilize it at the moment.  Of the EGL drivers,
 226 <code>egl_gallium</code> has its own hardware drivers and client API modules.
 227 They are considered internal to <code>egl_gallium</code> and there is also no
 228 stable ABI between them.  These should be kept in mind when packaging for
 229 distribution.</p>
 230
 231 <p>Generally, <code>egl_dri2</code> is preferred over <code>egl_gallium</code>
 232 when the system already has DRI drivers.  As <code>egl_gallium</code> is loaded
 233 before <code>egl_dri2</code> when both are available, <code>egl_gallium</code>
 234 may either be disabled with <code>--disable-gallium-egl</code> or packaged
 235 separately.</p>
 236
 237 <h2>Developers</h2>
 238
 239 <p>The sources of the main library and the classic drivers can be found at
 240 <code>src/egl/</code>.  The sources of the <code>egl</code> state tracker can
 241 be found at <code>src/gallium/state_trackers/egl/</code>.</p>
 242
 243 <p>The suggested way to learn to write a EGL driver is to see how other drivers
 244 are written.  <code>egl_glx</code> should be a good reference.  It works in any
 245 environment that has GLX support, and it is simpler than most drivers.</p>
 246
 247 <h3>Lifetime of Display Resources</h3>
 248
 249 <p>Contexts and surfaces are examples of display resources.  They might live
 250 longer than the display that creates them.</p>
 251
 252 <p>In EGL, when a display is terminated through <code>eglTerminate</code>, all
 253 display resources should be destroyed.  Similarly, when a thread is released
 254 throught <code>eglReleaseThread</code>, all current display resources should be
 255 released.  Another way to destory or release resources is through functions
 256 such as <code>eglDestroySurface</code> or <code>eglMakeCurrent</code>.</p>
 257
 258 <p>When a resource that is current to some thread is destroyed, the resource
 259 should not be destroyed immediately.  EGL requires the resource to live until
 260 it is no longer current.  A driver usually calls
 261 <code>eglIs&lt;Resource&gt;Bound</code> to check if a resource is bound
 262 (current) to any thread in the destroy callbacks.  If it is still bound, the
 263 resource is not destroyed.</p>
 264
 265 <p>The main library will mark destroyed current resources as unlinked.  In a
 266 driver's <code>MakeCurrent</code> callback,
 267 <code>eglIs&lt;Resource&gt;Linked</code> can then be called to check if a newly
 268 released resource is linked to a display.  If it is not, the last reference to
 269 the resource is removed and the driver should destroy the resource.  But it
 270 should be careful here because <code>MakeCurrent</code> might be called with an
 271 uninitialized display.</p>
 272
 273 <p>This is the only mechanism provided by the main library to help manage the
 274 resources.  The drivers are responsible to the correct behavior as defined by
 275 EGL.</p>
 276
 277 <h3><code>EGL_RENDER_BUFFER</code></h3>
 278
 279 <p>In EGL, the color buffer a context should try to render to is decided by the
 280 binding surface.  It should try to render to the front buffer if the binding
 281 surface has <code>EGL_RENDER_BUFFER</code> set to
 282 <code>EGL_SINGLE_BUFFER</code>;  If the same context is later bound to a
 283 surface with <code>EGL_RENDER_BUFFER</code> set to
 284 <code>EGL_BACK_BUFFER</code>, the context should try to render to the back
 285 buffer.  However, the context is allowed to make the final decision as to which
 286 color buffer it wants to or is able to render to.</p>
 287
 288 <p>For pbuffer surfaces, the render buffer is always
 289 <code>EGL_BACK_BUFFER</code>.  And for pixmap surfaces, the render buffer is
 290 always <code>EGL_SINGLE_BUFFER</code>.  Unlike window surfaces, EGL spec
 291 requires their <code>EGL_RENDER_BUFFER</code> values to be honored.  As a
 292 result, a driver should never set <code>EGL_PIXMAP_BIT</code> or
 293 <code>EGL_PBUFFER_BIT</code> bits of a config if the contexts created with the
 294 config won't be able to honor the <code>EGL_RENDER_BUFFER</code> of pixmap or
 295 pbuffer surfaces.</p>
 296
 297 <p>It should also be noted that pixmap and pbuffer surfaces are assumed to be
 298 single-buffered, in that <code>eglSwapBuffers</code> has no effect on them.  It
 299 is desirable that a driver allocates a private color buffer for each pbuffer
 300 surface created.  If the window system the driver supports has native pbuffers,
 301 or if the native pixmaps have more than one color buffers, the driver should
 302 carefully attach the native color buffers to the EGL surfaces, re-route them if
 303 required.</p>
 304
 305 <p>There is no defined behavior as to, for example, how
 306 <code>glDrawBuffer</code> interacts with <code>EGL_RENDER_BUFFER</code>.  Right
 307 now, it is desired that the draw buffer in a client API be fixed for pixmap and
 308 pbuffer surfaces.  Therefore, the driver is responsible to guarantee that the
 309 client API renders to the specified render buffer for pixmap and pbuffer
 310 surfaces.</p>
 311
 312 <h3><code>EGLDisplay</code> Mutex</h3>
 313
 314 The <code>EGLDisplay</code> will be locked before calling any of the dispatch
 315 functions (well, except for GetProcAddress which does not take an
 316 <code>EGLDisplay</code>).  This guarantees that the same dispatch function will
 317 not be called with the sample display at the same time.  If a driver has access
 318 to an <code>EGLDisplay</code> without going through the EGL APIs, the driver
 319 should as well lock the display before using it.
 320
 321 <h3>TODOs</h3>
 322
 323 <ul>
 324 <li>Pass the conformance tests</li>
 325 <li>Mixed use of OpenGL, OpenGL ES 1.1, and OpenGL ES 2.0 is supported.  But
 326 which one of <code>libGL.so</code>, <code>libGLESv1_CM.so</code>, and
 327 <code>libGLESv2.so</code> should an application link to?  Bad things may happen
 328 when, say, an application is linked to <code>libGLESv2.so</code> and
 329 <code>libcairo</code>, which is linked to <code>libGL.so</code> instead.</li>
 330
 331 </ul>
 332
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