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 state trackers and enable
  32 the Gallium driver for your hardware.  For example</p>
  33
  34 <pre>
  35   $ ./configure --enable-gles-overlay --with-state-trackers=egl,vega --enable-gallium-{swrast,intel}
  36 </pre>
  37
  38 <p>The main library and OpenGL is enabled by default.  The first option enables
  39 <a href="opengles.html">OpenGL ES 1.x and 2.x</a>.  The <code>egl</code> state
  40 tracker is needed by a number of EGL drivers.  EGL drivers will be covered
  41 later.  The <a href="openvg.html">vega state tracker</a> provides OpenVG
  42 1.x.</p>
  43 </li>
  44
  45 <li>Build and install Mesa as usual.</li>
  46 </ol>
  47
  48 <p>In the given example, it will build and install <code>libEGL</code>,
  49 <code>libGL</code>, <code>libGLESv1_CM</code>, <code>libGLESv2</code>,
  50 <code>libOpenVG</code>, and one or more EGL drivers.</p>
  51
  52 <h3>Configure Options</h3>
  53
  54 <p>There are several options that control the build of EGL at configuration
  55 time</p>
  56
  57 <ul>
  58 <li><code>--enable-egl</code>
  59
  60 <p>By default, EGL is enabled.  When disabled, the main library and the drivers
  61 will not be built.</p>
  62
  63 </li>
  64
  65 <li><code>--with-egl-driver-dir</code>
  66
  67 <p>The directory EGL drivers should be installed to.  If not specified, EGL
  68 drivers will be installed to <code>${libdir}/egl</code>.</p>
  69
  70 </li>
  71
  72 <li><code>--with-egl-displays</code>
  73
  74 <p>List the window system(s) to support.  It is by default <code>x11</code>,
  75 which supports the X Window System.  Its argument is a comma separated string
  76 like, for example, <code>--with-egl-displays=x11,kms</code>.  Because an EGL
  77 driver decides which window system to support, this example will enable two
  78 (sets of) EGL drivers.  One supports the X window system and the other supports
  79 bare KMS (kernel modesetting).</p>
  80
  81 </li>
  82
  83 <li><code>--with-state-trackers</code>
  84
  85 <p>The argument is a comma separated string.  It is usually used to specify the
  86 rendering APIs, such as OpenVG, to build.  But it should be noted that a number
  87 of EGL drivers depend on the <code>egl</code> state tracker.  They will
  88 <em>not</em> be built without the <code>egl</code> state tracker.</p>
  89
  90 </li>
  91
  92 <li><code>--enable-gles-overlay</code>
  93
  94 <p>OpenGL and OpenGL ES are not controlled by
  95 <code>--with-state-trackers</code>.  OpenGL is always built.  To build OpenGL
  96 ES, this option must be explicitly given.</p>
  97
  98 </li>
  99
 100 <li><code>--enable-gles1</code> and <code>--enable-gles2</code>
 101
 102 <p>Unlike <code>--enable-gles-overlay</code>, which builds one library for each
 103 rendering API, these options enable OpenGL ES support in OpenGL.  The result is
 104 one big library that supports multiple APIs.  This is used by DRI drivers and
 105 <code>egl_dri2</code> EGL driver.
 106
 107 </li>
 108
 109 <li><code>--enable-gallium-swrast</code>
 110
 111 <p>This option is not specific to EGL.  But if there is no driver for your
 112 hardware, or you are experiencing problems with the hardware driver, you can
 113 enable the swrast DRM driver.  It is a dummy driver and EGL will fallback to
 114 software rendering automatically.</p>
 115
 116 </li>
 117 </ul>
 118
 119 <h2>Use EGL</h2>
 120
 121 <p>There are demos for the client APIs supported by EGL.  They can be found in
 122 <code>progs/egl/</code>, You can use them to test your build.  For example,</p>
 123
 124 <pre>
 125   $ cd progs/egl/eglut
 126   $ make
 127   $ cd ../opengles1
 128   $ make
 129   $ ./torus_x11
 130 </pre>
 131
 132 <h3>Environment Variables</h3>
 133
 134 <p>There are several environment variables that control the behavior of EGL at
 135 runtime</p>
 136
 137 <ul>
 138 <li><code>EGL_DRIVERS_PATH</code>
 139
 140 <p>By default, the main library will look for drivers in the directory where
 141 the drivers are installed to.  This variable specifies a list of
 142 colon-separated directories where the main library will look for drivers, in
 143 addition to the default directory.  This variable is ignored for setuid/setgid
 144 binaries.</p>
 145
 146 </li>
 147
 148 <li><code>EGL_DRIVER</code>
 149
 150 <p>This variable specifies a full path to an EGL driver and it forces the
 151 specified EGL driver to be loaded.  It comes in handy when one wants to test a
 152 specific driver.  This variable is ignored for setuid/setgid binaries.</p>
 153
 154 </li>
 155
 156 <li><code>EGL_DISPLAY</code>
 157
 158 <p>When <code>EGL_DRIVER</code> is not set, the main library loads <em>all</em>
 159 EGL drivers that support a certain window system.  <code>EGL_DISPLAY</code> can
 160 be used to specify the window system and the valid values are, for example,
 161 <code>x11</code> or <code>kms</code>.  When the variable is not set, the main
 162 library defaults the value to the first window system listed in
 163 <code>--with-egl-displays</code> at configuration time.
 164
 165 </li>
 166
 167 <li><code>EGL_LOG_LEVEL</code>
 168
 169 <p>This changes the log level of the main library and the drivers.  The valid
 170 values are: <code>debug</code>, <code>info</code>, <code>warning</code>, and
 171 <code>fatal</code>.</p>
 172
 173 </li>
 174
 175 <li><code>EGL_SOFTWARE</code>
 176
 177 <p>For drivers that support both hardware and software rendering, setting this
 178 variable to true forces the use of software rendering.</p>
 179
 180 </li>
 181 </ul>
 182
 183 <h2>EGL Drivers</h2>
 184
 185 <p>There are two categories of EGL drivers: Gallium and classic.</p>
 186
 187 <p>Gallium EGL drivers supports all rendering APIs specified in EGL 1.4.  The
 188 support for optional EGL functions and EGL extensions is usually more complete
 189 than the classic ones.  These drivers depend on the <code>egl</code> state
 190 tracker to build.  The available drivers are</p>
 191
 192 <ul>
 193 <li><code>egl_&lt;dpy&gt;_i915</code></li>
 194 <li><code>egl_&lt;dpy&gt;_i965</code></li>
 195 <li><code>egl_&lt;dpy&gt;_radeon</code></li>
 196 <li><code>egl_&lt;dpy&gt;_nouveau</code></li>
 197 <li><code>egl_&lt;dpy&gt;_swrast</code></li>
 198 <li><code>egl_&lt;dpy&gt;_vmwgfx</code></li>
 199 </ul>
 200
 201 <p><code>&lt;dpy&gt;</code> is given by <code>--with-egl-displays</code> at
 202 configuration time.  There will be one EGL driver for each combination of the
 203 displays listed and the hardware drivers enabled.</p>
 204
 205 <p>Classic EGL drivers, on the other hand, supports only OpenGL as its
 206 rendering API.  They can be found under <code>src/egl/drivers/</code>.  There
 207 are 3 of them</p>
 208
 209 <ul>
 210 <li><code>egl_glx</code>
 211
 212 <p>This driver provides a wrapper to GLX.  It uses exclusively GLX to implement
 213 the EGL API.  It supports both direct and indirect rendering when the GLX does.
 214 It is accelerated when the GLX is.  As such, it cannot provide functions that
 215 is not available in GLX or GLX extensions.</p>
 216 </li>
 217
 218 <li><code>egl_dri2</code>
 219
 220 <p>This driver supports the X Window System as its window system.  It functions
 221 as a DRI2 driver loader.  Unlike <code>egl_glx</code>, it has no dependency on
 222 <code>libGL</code>.  It talks to the X server directly using DRI2 protocol.</p>
 223
 224 </li>
 225 <li><code>egl_dri</code>
 226
 227 <p>This driver lacks maintenance and does <em>not</em> build.  It is similiar
 228 to <code>egl_dri2</code> in that it functions as a DRI(1) driver loader.  But
 229 unlike <code>egl_dri2</code>, it supports Linux framebuffer devices as its
 230 window system and supports EGL_MESA_screen_surface extension.  As DRI1 drivers
 231 are phasing out, it might eventually be replaced by <code>egl_dri2</code>.</p>
 232
 233 </li>
 234 </ul>
 235
 236 <p>To use the classic drivers, one must manually set <code>EGL_DRIVER</code> at
 237 runtime.</p>
 238
 239 <h2>Developers</h2>
 240
 241 <p>The sources of the main library and the classic drivers can be found at
 242 <code>src/egl/</code>.  The sources of the <code>egl</code> state tracker can
 243 be found at <code>src/gallium/state_trackers/egl/</code>.</p>
 244
 245 <p>The suggested way to learn to write a EGL driver is to see how other drivers
 246 are written.  <code>egl_glx</code> should be a good reference.  It works in any
 247 environment that has GLX support, and it is simpler than most drivers.</p>
 248
 249 <h3>Lifetime of Display Resources</h3>
 250
 251 <p>Contexts and surfaces are examples of display resources.  They might live
 252 longer than the display that creates them.</p>
 253
 254 <p>In EGL, when a display is terminated through <code>eglTerminate</code>, all
 255 display resources should be destroyed.  Similarly, when a thread is released
 256 throught <code>eglReleaseThread</code>, all current display resources should be
 257 released.  Another way to destory or release resources is through functions
 258 such as <code>eglDestroySurface</code> or <code>eglMakeCurrent</code>.</p>
 259
 260 <p>When a resource that is current to some thread is destroyed, the resource
 261 should not be destroyed immediately.  EGL requires the resource to live until
 262 it is no longer current.  A driver usually calls
 263 <code>eglIs&lt;Resource&gt;Bound</code> to check if a resource is bound
 264 (current) to any thread in the destroy callbacks.  If it is still bound, the
 265 resource is not destroyed.</p>
 266
 267 <p>The main library will mark destroyed current resources as unlinked.  In a
 268 driver's <code>MakeCurrent</code> callback,
 269 <code>eglIs&lt;Resource&gt;Linked</code> can then be called to check if a newly
 270 released resource is linked to a display.  If it is not, the last reference to
 271 the resource is removed and the driver should destroy the resource.  But it
 272 should be careful here because <code>MakeCurrent</code> might be called with an
 273 uninitialized display.</p>
 274
 275 <p>This is the only mechanism provided by the main library to help manage the
 276 resources.  The drivers are responsible to the correct behavior as defined by
 277 EGL.</p>
 278
 279 <h3><code>EGL_RENDER_BUFFER</code></h3>
 280
 281 <p>In EGL, the color buffer a context should try to render to is decided by the
 282 binding surface.  It should try to render to the front buffer if the binding
 283 surface has <code>EGL_RENDER_BUFFER</code> set to
 284 <code>EGL_SINGLE_BUFFER</code>;  If the same context is later bound to a
 285 surface with <code>EGL_RENDER_BUFFER</code> set to
 286 <code>EGL_BACK_BUFFER</code>, the context should try to render to the back
 287 buffer.  However, the context is allowed to make the final decision as to which
 288 color buffer it wants to or is able to render to.</p>
 289
 290 <p>For pbuffer surfaces, the render buffer is always
 291 <code>EGL_BACK_BUFFER</code>.  And for pixmap surfaces, the render buffer is
 292 always <code>EGL_SINGLE_BUFFER</code>.  Unlike window surfaces, EGL spec
 293 requires their <code>EGL_RENDER_BUFFER</code> values to be honored.  As a
 294 result, a driver should never set <code>EGL_PIXMAP_BIT</code> or
 295 <code>EGL_PBUFFER_BIT</code> bits of a config if the contexts created with the
 296 config won't be able to honor the <code>EGL_RENDER_BUFFER</code> of pixmap or
 297 pbuffer surfaces.</p>
 298
 299 <p>It should also be noted that pixmap and pbuffer surfaces are assumed to be
 300 single-buffered, in that <code>eglSwapBuffers</code> has no effect on them.  It
 301 is desirable that a driver allocates a private color buffer for each pbuffer
 302 surface created.  If the window system the driver supports has native pbuffers,
 303 or if the native pixmaps have more than one color buffers, the driver should
 304 carefully attach the native color buffers to the EGL surfaces, re-route them if
 305 required.</p>
 306
 307 <p>There is no defined behavior as to, for example, how
 308 <code>glDrawBuffer</code> interacts with <code>EGL_RENDER_BUFFER</code>.  Right
 309 now, it is desired that the draw buffer in a client API be fixed for pixmap and
 310 pbuffer surfaces.  Therefore, the driver is responsible to guarantee that the
 311 client API renders to the specified render buffer for pixmap and pbuffer
 312 surfaces.</p>
 313
 314 <h3><code>EGLDisplay</code> Mutex</h3>
 315
 316 The <code>EGLDisplay</code> will be locked before calling any of the dispatch
 317 functions (well, except for GetProcAddress which does not take an
 318 <code>EGLDisplay</code>).  This guarantees that the same dispatch function will
 319 not be called with the sample display at the same time.  If a driver has access
 320 to an <code>EGLDisplay</code> without going through the EGL APIs, the driver
 321 should as well lock the display before using it.
 322
 323 <h3>TODOs</h3>
 324
 325 <ul>
 326 <li>Pass the conformance tests</li>
 327 <li>Better automatic driver selection: <code>EGL_DISPLAY</code> loads all
 328 drivers and might eat too much memory.</li>
 329
 330 </ul>
 331
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