workbench/libs/mesa/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-gles1 --enable-gles2 \
  33                 --with-dri-drivers=... \
  34                 --with-gallium-drivers=...
  35 </pre>
  36
  37 <p>The main library and OpenGL is enabled by default.  The first two options
  38 above enables <a href="opengles.html">OpenGL ES 1.x and 2.x</a>.  The last two
  39 options enables the listed classic and and Gallium drivers respectively.</p>
  40
  41 </li>
  42
  43 <li>Build and install Mesa as usual.</li>
  44 </ol>
  45
  46 <p>In the given example, it will build and install <code>libEGL</code>,
  47 <code>libGL</code>, <code>libGLESv1_CM</code>, <code>libGLESv2</code>, and one
  48 or more EGL drivers.</p>
  49
  50 <h3>Configure Options</h3>
  51
  52 <p>There are several options that control the build of EGL at configuration
  53 time</p>
  54
  55 <ul>
  56 <li><code>--enable-egl</code>
  57
  58 <p>By default, EGL is enabled.  When disabled, the main library and the drivers
  59 will not be built.</p>
  60
  61 </li>
  62
  63 <li><code>--with-egl-driver-dir</code>
  64
  65 <p>The directory EGL drivers should be installed to.  If not specified, EGL
  66 drivers will be installed to <code>${libdir}/egl</code>.</p>
  67
  68 </li>
  69
  70 <li><code>--enable-gallium-egl</code>
  71
  72 <p>Enable the optional <code>egl_gallium</code> driver.</p>
  73
  74 </li>
  75
  76 <li><code>--with-egl-platforms</code>
  77
  78 <p>List the platforms (window systems) to support.  Its argument is a comma
  79 seprated string such as <code>--with-egl-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: the platform the EGL native
  82 types such as <code>EGLNativeDisplayType</code> or
  83 <code>EGLNativeWindowType</code> defined for.</p>
  84
  85 <p>The available platforms are <code>x11</code>, <code>drm</code>,
  86 <code>fbdev</code>, and <code>gdi</code>.  The <code>gdi</code> platform can
  87 only be built with SCons.  Unless for special needs, the build system should
  88 select the right platforms automatically.</p>
  89
  90 </li>
  91
  92 <li><code>--enable-gles1</code> and <code>--enable-gles2</code>
  93
  94 <p>These options enable OpenGL ES support in OpenGL.  The result is one big
  95 internal library that supports multiple APIs.</p>
  96
  97 </li>
  98
  99 <li><code>--enable-shared-glapi</code>
 100
 101 <p>By default, <code>libGL</code> has its own copy of <code>libglapi</code>.
 102 This options makes <code>libGL</code> use the shared <code>libglapi</code>.  This
 103 is required if applications mix OpenGL and OpenGL ES.</p>
 104
 105 </li>
 106
 107 <li><code>--enable-openvg</code>
 108
 109 <p>OpenVG must be explicitly enabled by this option.</p>
 110
 111 </li>
 112
 113 </ul>
 114
 115 <h2>Use EGL</h2>
 116
 117 <h3>Demos</h3>
 118
 119 <p>There are demos for the client APIs supported by EGL.  They can be found in
 120 mesa/demos repository.</p>
 121
 122 <h3>Environment Variables</h3>
 123
 124 <p>There are several environment variables that control the behavior of EGL at
 125 runtime</p>
 126
 127 <ul>
 128 <li><code>EGL_DRIVERS_PATH</code>
 129
 130 <p>By default, the main library will look for drivers in the directory where
 131 the drivers are installed to.  This variable specifies a list of
 132 colon-separated directories where the main library will look for drivers, in
 133 addition to the default directory.  This variable is ignored for setuid/setgid
 134 binaries.</p>
 135
 136 <p>This variable is usually set to test an uninstalled build.  For example, one
 137 may set</p>
 138
 139 <pre>
 140   $ export LD_LIBRARY_PATH=$mesa/lib
 141   $ export EGL_DRIVERS_PATH=$mesa/lib/egl
 142 </pre>
 143
 144 <p>to test a build without installation</p>
 145
 146 </li>
 147
 148 <li><code>EGL_DRIVER</code>
 149
 150 <p>This variable specifies a full path to or the name of an EGL driver.  It
 151 forces the specified EGL driver to be loaded.  It comes in handy when one wants
 152 to test a specific driver.  This variable is ignored for setuid/setgid
 153 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 is disabled by default.</p>
 234
 235 <h2>Developers</h2>
 236
 237 <p>The sources of the main library and the classic drivers can be found at
 238 <code>src/egl/</code>.  The sources of the <code>egl</code> state tracker can
 239 be found at <code>src/gallium/state_trackers/egl/</code>.</p>
 240
 241 <p>The suggested way to learn to write a EGL driver is to see how other drivers
 242 are written.  <code>egl_glx</code> should be a good reference.  It works in any
 243 environment that has GLX support, and it is simpler than most drivers.</p>
 244
 245 <h3>Lifetime of Display Resources</h3>
 246
 247 <p>Contexts and surfaces are examples of display resources.  They might live
 248 longer than the display that creates them.</p>
 249
 250 <p>In EGL, when a display is terminated through <code>eglTerminate</code>, all
 251 display resources should be destroyed.  Similarly, when a thread is released
 252 throught <code>eglReleaseThread</code>, all current display resources should be
 253 released.  Another way to destory or release resources is through functions
 254 such as <code>eglDestroySurface</code> or <code>eglMakeCurrent</code>.</p>
 255
 256 <p>When a resource that is current to some thread is destroyed, the resource
 257 should not be destroyed immediately.  EGL requires the resource to live until
 258 it is no longer current.  A driver usually calls
 259 <code>eglIs&lt;Resource&gt;Bound</code> to check if a resource is bound
 260 (current) to any thread in the destroy callbacks.  If it is still bound, the
 261 resource is not destroyed.</p>
 262
 263 <p>The main library will mark destroyed current resources as unlinked.  In a
 264 driver's <code>MakeCurrent</code> callback,
 265 <code>eglIs&lt;Resource&gt;Linked</code> can then be called to check if a newly
 266 released resource is linked to a display.  If it is not, the last reference to
 267 the resource is removed and the driver should destroy the resource.  But it
 268 should be careful here because <code>MakeCurrent</code> might be called with an
 269 uninitialized display.</p>
 270
 271 <p>This is the only mechanism provided by the main library to help manage the
 272 resources.  The drivers are responsible to the correct behavior as defined by
 273 EGL.</p>
 274
 275 <h3><code>EGL_RENDER_BUFFER</code></h3>
 276
 277 <p>In EGL, the color buffer a context should try to render to is decided by the
 278 binding surface.  It should try to render to the front buffer if the binding
 279 surface has <code>EGL_RENDER_BUFFER</code> set to
 280 <code>EGL_SINGLE_BUFFER</code>;  If the same context is later bound to a
 281 surface with <code>EGL_RENDER_BUFFER</code> set to
 282 <code>EGL_BACK_BUFFER</code>, the context should try to render to the back
 283 buffer.  However, the context is allowed to make the final decision as to which
 284 color buffer it wants to or is able to render to.</p>
 285
 286 <p>For pbuffer surfaces, the render buffer is always
 287 <code>EGL_BACK_BUFFER</code>.  And for pixmap surfaces, the render buffer is
 288 always <code>EGL_SINGLE_BUFFER</code>.  Unlike window surfaces, EGL spec
 289 requires their <code>EGL_RENDER_BUFFER</code> values to be honored.  As a
 290 result, a driver should never set <code>EGL_PIXMAP_BIT</code> or
 291 <code>EGL_PBUFFER_BIT</code> bits of a config if the contexts created with the
 292 config won't be able to honor the <code>EGL_RENDER_BUFFER</code> of pixmap or
 293 pbuffer surfaces.</p>
 294
 295 <p>It should also be noted that pixmap and pbuffer surfaces are assumed to be
 296 single-buffered, in that <code>eglSwapBuffers</code> has no effect on them.  It
 297 is desirable that a driver allocates a private color buffer for each pbuffer
 298 surface created.  If the window system the driver supports has native pbuffers,
 299 or if the native pixmaps have more than one color buffers, the driver should
 300 carefully attach the native color buffers to the EGL surfaces, re-route them if
 301 required.</p>
 302
 303 <p>There is no defined behavior as to, for example, how
 304 <code>glDrawBuffer</code> interacts with <code>EGL_RENDER_BUFFER</code>.  Right
 305 now, it is desired that the draw buffer in a client API be fixed for pixmap and
 306 pbuffer surfaces.  Therefore, the driver is responsible to guarantee that the
 307 client API renders to the specified render buffer for pixmap and pbuffer
 308 surfaces.</p>
 309
 310 <h3><code>EGLDisplay</code> Mutex</h3>
 311
 312 The <code>EGLDisplay</code> will be locked before calling any of the dispatch
 313 functions (well, except for GetProcAddress which does not take an
 314 <code>EGLDisplay</code>).  This guarantees that the same dispatch function will
 315 not be called with the sample display at the same time.  If a driver has access
 316 to an <code>EGLDisplay</code> without going through the EGL APIs, the driver
 317 should as well lock the display before using it.
 318
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