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-openvg</code>
  92
  93 <p>OpenVG must be explicitly enabled by this option.</p>
  94
  95 </li>
  96
  97 <li><code>--enable-gallium-egl</code>
  98
  99 <p>Explicitly enable or disable <code>egl_gallium</code>.</p>
 100
 101 </li>
 102
 103 </ul>
 104
 105 <h2>Use EGL</h2>
 106
 107 <h3>Demos</h3>
 108
 109 <p>There are demos for the client APIs supported by EGL.  They can be found in
 110 mesa/demos repository.</p>
 111
 112 <h3>Environment Variables</h3>
 113
 114 <p>There are several environment variables that control the behavior of EGL at
 115 runtime</p>
 116
 117 <ul>
 118 <li><code>EGL_DRIVERS_PATH</code>
 119
 120 <p>By default, the main library will look for drivers in the directory where
 121 the drivers are installed to.  This variable specifies a list of
 122 colon-separated directories where the main library will look for drivers, in
 123 addition to the default directory.  This variable is ignored for setuid/setgid
 124 binaries.</p>
 125
 126 <p>This variable is usually set to test an uninstalled build.  For example, one
 127 may set</p>
 128
 129 <pre>
 130   $ export LD_LIBRARY_PATH=$mesa/lib
 131   $ export EGL_DRIVERS_PATH=$mesa/lib/egl
 132 </pre>
 133
 134 <p>to test a build without installation</p>
 135
 136 </li>
 137
 138 <li><code>EGL_DRIVER</code>
 139
 140 <p>This variable specifies a full path to or the name of an EGL driver.  It
 141 forces the specified EGL driver to be loaded.  It comes in handy when one wants
 142 to test a specific driver.  This variable is ignored for setuid/setgid
 143 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 DRI driver loader.  For <code>x11</code> support, it talks to
 184 the X server directly using (XCB-)DRI2 protocol.</p>
 185
 186 <p>This driver can share DRI drivers with <code>libGL</code>.</p>
 187
 188 </li>
 189
 190 <li><code>egl_gallium</code>
 191
 192 <p>This driver is based on Gallium3D.  It supports all rendering APIs and
 193 hardwares supported by Gallium3D.  It is the only driver that supports OpenVG.
 194 The supported platforms are X11, DRM, FBDEV, and GDI.</p>
 195
 196 <p>This driver comes with its own hardware drivers
 197 (<code>pipe_&lt;hw&gt;</code>) and client API modules
 198 (<code>st_&lt;api&gt;</code>).</p>
 199
 200 </li>
 201
 202 <li><code>egl_glx</code>
 203
 204 <p>This driver provides a wrapper to GLX.  It uses exclusively GLX to implement
 205 the EGL API.  It supports both direct and indirect rendering when the GLX does.
 206 It is accelerated when the GLX is.  As such, it cannot provide functions that
 207 is not available in GLX or GLX extensions.</p>
 208 </li>
 209 </ul>
 210
 211 <h2>Packaging</h2>
 212
 213 <p>The ABI between the main library and its drivers are not stable.  Nor is
 214 there a plan to stabilize it at the moment.  Of the EGL drivers,
 215 <code>egl_gallium</code> has its own hardware drivers and client API modules.
 216 They are considered internal to <code>egl_gallium</code> and there is also no
 217 stable ABI between them.  These should be kept in mind when packaging for
 218 distribution.</p>
 219
 220 <p>Generally, <code>egl_dri2</code> is preferred over <code>egl_gallium</code>
 221 when the system already has DRI drivers.  As <code>egl_gallium</code> is loaded
 222 before <code>egl_dri2</code> when both are available, <code>egl_gallium</code>
 223 may either be disabled with <code>--disable-gallium-egl</code> or packaged
 224 separately.</p>
 225
 226 <h2>Developers</h2>
 227
 228 <p>The sources of the main library and the classic drivers can be found at
 229 <code>src/egl/</code>.  The sources of the <code>egl</code> state tracker can
 230 be found at <code>src/gallium/state_trackers/egl/</code>.</p>
 231
 232 <p>The suggested way to learn to write a EGL driver is to see how other drivers
 233 are written.  <code>egl_glx</code> should be a good reference.  It works in any
 234 environment that has GLX support, and it is simpler than most drivers.</p>
 235
 236 <h3>Lifetime of Display Resources</h3>
 237
 238 <p>Contexts and surfaces are examples of display resources.  They might live
 239 longer than the display that creates them.</p>
 240
 241 <p>In EGL, when a display is terminated through <code>eglTerminate</code>, all
 242 display resources should be destroyed.  Similarly, when a thread is released
 243 throught <code>eglReleaseThread</code>, all current display resources should be
 244 released.  Another way to destory or release resources is through functions
 245 such as <code>eglDestroySurface</code> or <code>eglMakeCurrent</code>.</p>
 246
 247 <p>When a resource that is current to some thread is destroyed, the resource
 248 should not be destroyed immediately.  EGL requires the resource to live until
 249 it is no longer current.  A driver usually calls
 250 <code>eglIs&lt;Resource&gt;Bound</code> to check if a resource is bound
 251 (current) to any thread in the destroy callbacks.  If it is still bound, the
 252 resource is not destroyed.</p>
 253
 254 <p>The main library will mark destroyed current resources as unlinked.  In a
 255 driver's <code>MakeCurrent</code> callback,
 256 <code>eglIs&lt;Resource&gt;Linked</code> can then be called to check if a newly
 257 released resource is linked to a display.  If it is not, the last reference to
 258 the resource is removed and the driver should destroy the resource.  But it
 259 should be careful here because <code>MakeCurrent</code> might be called with an
 260 uninitialized display.</p>
 261
 262 <p>This is the only mechanism provided by the main library to help manage the
 263 resources.  The drivers are responsible to the correct behavior as defined by
 264 EGL.</p>
 265
 266 <h3><code>EGL_RENDER_BUFFER</code></h3>
 267
 268 <p>In EGL, the color buffer a context should try to render to is decided by the
 269 binding surface.  It should try to render to the front buffer if the binding
 270 surface has <code>EGL_RENDER_BUFFER</code> set to
 271 <code>EGL_SINGLE_BUFFER</code>;  If the same context is later bound to a
 272 surface with <code>EGL_RENDER_BUFFER</code> set to
 273 <code>EGL_BACK_BUFFER</code>, the context should try to render to the back
 274 buffer.  However, the context is allowed to make the final decision as to which
 275 color buffer it wants to or is able to render to.</p>
 276
 277 <p>For pbuffer surfaces, the render buffer is always
 278 <code>EGL_BACK_BUFFER</code>.  And for pixmap surfaces, the render buffer is
 279 always <code>EGL_SINGLE_BUFFER</code>.  Unlike window surfaces, EGL spec
 280 requires their <code>EGL_RENDER_BUFFER</code> values to be honored.  As a
 281 result, a driver should never set <code>EGL_PIXMAP_BIT</code> or
 282 <code>EGL_PBUFFER_BIT</code> bits of a config if the contexts created with the
 283 config won't be able to honor the <code>EGL_RENDER_BUFFER</code> of pixmap or
 284 pbuffer surfaces.</p>
 285
 286 <p>It should also be noted that pixmap and pbuffer surfaces are assumed to be
 287 single-buffered, in that <code>eglSwapBuffers</code> has no effect on them.  It
 288 is desirable that a driver allocates a private color buffer for each pbuffer
 289 surface created.  If the window system the driver supports has native pbuffers,
 290 or if the native pixmaps have more than one color buffers, the driver should
 291 carefully attach the native color buffers to the EGL surfaces, re-route them if
 292 required.</p>
 293
 294 <p>There is no defined behavior as to, for example, how
 295 <code>glDrawBuffer</code> interacts with <code>EGL_RENDER_BUFFER</code>.  Right
 296 now, it is desired that the draw buffer in a client API be fixed for pixmap and
 297 pbuffer surfaces.  Therefore, the driver is responsible to guarantee that the
 298 client API renders to the specified render buffer for pixmap and pbuffer
 299 surfaces.</p>
 300
 301 <h3><code>EGLDisplay</code> Mutex</h3>
 302
 303 The <code>EGLDisplay</code> will be locked before calling any of the dispatch
 304 functions (well, except for GetProcAddress which does not take an
 305 <code>EGLDisplay</code>).  This guarantees that the same dispatch function will
 306 not be called with the sample display at the same time.  If a driver has access
 307 to an <code>EGLDisplay</code> without going through the EGL APIs, the driver
 308 should as well lock the display before using it.
 309
 310 <h3>TODOs</h3>
 311
 312 <ul>
 313 <li>Pass the conformance tests</li>
 314 <li>Mixed use of OpenGL, OpenGL ES 1.1, and OpenGL ES 2.0 is supported.  But
 315 which one of <code>libGL.so</code>, <code>libGLESv1_CM.so</code>, and
 316 <code>libGLESv2.so</code> should an application link to?  Bad things may happen
 317 when, say, an application is linked to <code>libGLESv2.so</code> and
 318 <code>libcairo</code>, which is linked to <code>libGL.so</code> instead.</li>
 319
 320 </ul>
 321
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