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-platforms</code>
  73
  74 <p>List the native platform window system(s) to support.  It is by default
  75 <code>x11</code>, which supports the X Window System.  Its argument is a comma
  76 separated string like, for example, <code>--with-egl-platforms=x11,kms</code>.
  77 Because an EGL driver decides which window system to support, this example will
  78 enable two (sets of) EGL drivers.  One supports the X window system and the
  79 other supports bare KMS (kernel modesetting).</p>
  80
  81 <p>The available platforms are <code>x11</code>, <code>kms</code>,
  82 <code>fbdev</code>, and <code>gdi</code>.  The <code>gdi</code> platform can
  83 only be built with SCons.</p>
  84
  85 </li>
  86
  87 <li><code>--with-state-trackers</code>
  88
  89 <p>The argument is a comma separated string.  It is usually used to specify the
  90 rendering APIs, such as OpenVG, to build.  But it should be noted that a number
  91 of EGL drivers depend on the <code>egl</code> state tracker.  They will
  92 <em>not</em> be built without the <code>egl</code> state tracker.</p>
  93
  94 </li>
  95
  96 <li><code>--enable-gles-overlay</code>
  97
  98 <p>OpenGL and OpenGL ES are not controlled by
  99 <code>--with-state-trackers</code>.  OpenGL is always built.  To build OpenGL
 100 ES, this option must be explicitly given.</p>
 101
 102 </li>
 103
 104 <li><code>--enable-gles1</code> and <code>--enable-gles2</code>
 105
 106 <p>Unlike <code>--enable-gles-overlay</code>, which builds one library for each
 107 rendering API, these options enable OpenGL ES support in OpenGL.  The result is
 108 one big library that supports multiple APIs.  This is used by DRI drivers and
 109 <code>egl_dri2</code> EGL driver.
 110
 111 </li>
 112
 113 <li><code>--enable-gallium-swrast</code>
 114
 115 <p>This option is not specific to EGL.  But if there is no driver for your
 116 hardware, or you are experiencing problems with the hardware driver, you can
 117 enable the swrast DRM driver.  It is a dummy driver and EGL will fallback to
 118 software rendering automatically.</p>
 119
 120 </li>
 121 </ul>
 122
 123 <h2>Use EGL</h2>
 124
 125 <h3>Demos</h3>
 126
 127 <p>There are demos for the client APIs supported by EGL.  They can be found in
 128 mesa/demos repository.</p>
 129
 130 <h3>Environment Variables</h3>
 131
 132 <p>There are several environment variables that control the behavior of EGL at
 133 runtime</p>
 134
 135 <ul>
 136 <li><code>EGL_DRIVERS_PATH</code>
 137
 138 <p>By default, the main library will look for drivers in the directory where
 139 the drivers are installed to.  This variable specifies a list of
 140 colon-separated directories where the main library will look for drivers, in
 141 addition to the default directory.  This variable is ignored for setuid/setgid
 142 binaries.</p>
 143
 144 </li>
 145
 146 <li><code>EGL_DRIVER</code>
 147
 148 <p>This variable specifies a full path to an EGL driver and it forces the
 149 specified EGL driver to be loaded.  It comes in handy when one wants to test a
 150 specific driver.  This variable is ignored for setuid/setgid binaries.</p>
 151
 152 </li>
 153
 154 <li><code>EGL_PLATFORM</code>
 155
 156 <p>When <code>EGL_DRIVER</code> is not set, the main library loads <em>all</em>
 157 EGL drivers that support a certain window system.  <code>EGL_PLATFORM</code>
 158 can be used to specify the window system and the valid values are, for example,
 159 <code>x11</code> or <code>kms</code>.  When the variable is not set, the main
 160 library defaults the value to the first window system listed in
 161 <code>--with-egl-platforms</code> at configuration time.
 162
 163 </li>
 164
 165 <li><code>EGL_LOG_LEVEL</code>
 166
 167 <p>This changes the log level of the main library and the drivers.  The valid
 168 values are: <code>debug</code>, <code>info</code>, <code>warning</code>, and
 169 <code>fatal</code>.</p>
 170
 171 </li>
 172
 173 <li><code>EGL_SOFTWARE</code>
 174
 175 <p>For drivers that support both hardware and software rendering, setting this
 176 variable to true forces the use of software rendering.</p>
 177
 178 </li>
 179 </ul>
 180
 181 <h2>EGL Drivers</h2>
 182
 183 <p>There are two categories of EGL drivers: Gallium and classic.</p>
 184
 185 <p>Gallium EGL drivers supports all rendering APIs specified in EGL 1.4.  These
 186 drivers depend on the <code>egl</code> state tracker to build.  The available
 187 drivers are</p>
 188
 189 <ul>
 190 <li><code>egl_&lt;dpy&gt;_i915</code></li>
 191 <li><code>egl_&lt;dpy&gt;_i965</code></li>
 192 <li><code>egl_&lt;dpy&gt;_nouveau</code></li>
 193 <li><code>egl_&lt;dpy&gt;_radeon</code></li>
 194 <li><code>egl_&lt;dpy&gt;_swrast</code></li>
 195 <li><code>egl_&lt;dpy&gt;_vmwgfx</code></li>
 196 </ul>
 197
 198 <p><code>&lt;dpy&gt;</code> is given by <code>--with-egl-platforms</code> at
 199 configuration time.  There is usually one EGL driver for each combination of
 200 the platforms listed and the pipe drivers enabled.  When the platform is pure
 201 software or pure hardware, non-working combinations will not be built.</p>
 202
 203 <p>Classic EGL drivers, on the other hand, support only a subset of the
 204 available rendering APIs.  They can be found under
 205 <code>src/egl/drivers/</code>.  There are 3 of them</p>
 206
 207 <ul>
 208 <li><code>egl_glx</code>
 209
 210 <p>This driver provides a wrapper to GLX.  It uses exclusively GLX to implement
 211 the EGL API.  It supports both direct and indirect rendering when the GLX does.
 212 It is accelerated when the GLX is.  As such, it cannot provide functions that
 213 is not available in GLX or GLX extensions.</p>
 214 </li>
 215
 216 <li><code>egl_dri2</code>
 217
 218 <p>This driver supports the X Window System as its window system.  It functions
 219 as a DRI2 driver loader.  Unlike <code>egl_glx</code>, it has no dependency on
 220 <code>libGL</code>.  It talks to the X server directly using DRI2 protocol.</p>
 221
 222 </li>
 223 <li><code>egl_dri</code>
 224
 225 <p>This driver lacks maintenance and does <em>not</em> build.  It is similiar
 226 to <code>egl_dri2</code> in that it functions as a DRI(1) driver loader.  But
 227 unlike <code>egl_dri2</code>, it supports Linux framebuffer devices as its
 228 window system and supports EGL_MESA_screen_surface extension.  As DRI1 drivers
 229 are phasing out, it might eventually be replaced by <code>egl_dri2</code>.</p>
 230
 231 </li>
 232 </ul>
 233
 234 <p>To use the classic drivers, one must manually set <code>EGL_DRIVER</code> at
 235 runtime.</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>Better automatic driver selection: <code>EGL_PLATFORM</code> loads all
 326 drivers and might eat too much memory.</li>
 327
 328 </ul>
 329
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