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-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 platforms (window systems) to support.  Its argument is a comma
  75 seprated string such as <code>--with-egl-platforms=x11,drm</code>.  It decides
  76 the platforms a driver may support.  The first listed platform is also used by
  77 the main library to decide the native platform: the platform the EGL native
  78 types such as <code>EGLNativeDisplayType</code> or
  79 <code>EGLNativeWindowType</code> defined for.</p>
  80
  81 <p>The available platforms are <code>x11</code>, <code>drm</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 is also used to specify
  91 <code>egl</code> state tracker that <code>egl_gallium</code> depends on.</p>
  92
  93 </li>
  94
  95 <li><code>--enable-gles-overlay</code>
  96
  97 <p>OpenGL and OpenGL ES are not controlled by
  98 <code>--with-state-trackers</code>.  OpenGL is always built.  To build OpenGL
  99 ES, this option must be explicitly given.</p>
 100
 101 </li>
 102
 103 <li><code>--enable-gles1</code> and <code>--enable-gles2</code>
 104
 105 <p>Unlike <code>--enable-gles-overlay</code>, which builds one library for each
 106 rendering API, these options enable OpenGL ES support in OpenGL.  The result is
 107 one big library that supports multiple APIs.</p>
 108
 109 </li>
 110
 111 </ul>
 112
 113 <h2>Use EGL</h2>
 114
 115 <h3>Demos</h3>
 116
 117 <p>There are demos for the client APIs supported by EGL.  They can be found in
 118 mesa/demos repository.</p>
 119
 120 <h3>Environment Variables</h3>
 121
 122 <p>There are several environment variables that control the behavior of EGL at
 123 runtime</p>
 124
 125 <ul>
 126 <li><code>EGL_DRIVERS_PATH</code>
 127
 128 <p>By default, the main library will look for drivers in the directory where
 129 the drivers are installed to.  This variable specifies a list of
 130 colon-separated directories where the main library will look for drivers, in
 131 addition to the default directory.  This variable is ignored for setuid/setgid
 132 binaries.</p>
 133
 134 </li>
 135
 136 <li><code>EGL_DRIVER</code>
 137
 138 <p>This variable specifies a full path to an EGL driver and it forces the
 139 specified EGL driver to be loaded.  It comes in handy when one wants to test a
 140 specific driver.  This variable is ignored for setuid/setgid binaries.</p>
 141
 142 <p><code>egl_gallium</code> dynamically loads hardware drivers and client API
 143 modules found in <code>EGL_DRIVERS_PATH</code>.  Thus, specifying this variable
 144 alone is not sufficient for <code>egl_gallium</code> for uninstalled build.</p>
 145
 146 </li>
 147
 148 <li><code>EGL_PLATFORM</code>
 149
 150 <p>This variable specifies the native platform.  The valid values are the same
 151 as those for <code>--with-egl-platforms</code>.  When the variable is not set,
 152 the main library uses the first platform listed in
 153 <code>--with-egl-platforms</code> as the native platform</p>
 154
 155 </li>
 156
 157 <li><code>EGL_LOG_LEVEL</code>
 158
 159 <p>This changes the log level of the main library and the drivers.  The valid
 160 values are: <code>debug</code>, <code>info</code>, <code>warning</code>, and
 161 <code>fatal</code>.</p>
 162
 163 </li>
 164
 165 <li><code>EGL_SOFTWARE</code>
 166
 167 <p>For drivers that support both hardware and software rendering, setting this
 168 variable to true forces the use of software rendering.</p>
 169
 170 </li>
 171 </ul>
 172
 173 <h2>EGL Drivers</h2>
 174
 175 <ul>
 176 <li><code>egl_gallium</code>
 177
 178 <p>This driver is based on Gallium3D.  It supports all rendering APIs and
 179 hardwares supported by Gallium3D.  It is the only driver that supports OpenVG.
 180 The supported platforms are X11, KMS, FBDEV, and GDI.</p>
 181
 182 </li>
 183
 184 <li><code>egl_glx</code>
 185
 186 <p>This driver provides a wrapper to GLX.  It uses exclusively GLX to implement
 187 the EGL API.  It supports both direct and indirect rendering when the GLX does.
 188 It is accelerated when the GLX is.  As such, it cannot provide functions that
 189 is not available in GLX or GLX extensions.</p>
 190 </li>
 191
 192 <li><code>egl_dri2</code>
 193
 194 <p>This driver supports the X Window System as its window system.  It functions
 195 as a DRI2 driver loader.  Unlike <code>egl_glx</code>, it has no dependency on
 196 <code>libGL</code>.  It talks to the X server directly using DRI2 protocol.</p>
 197
 198 </li>
 199 <li><code>egl_dri</code>
 200
 201 <p>This driver lacks maintenance and does <em>not</em> build.  It is similiar
 202 to <code>egl_dri2</code> in that it functions as a DRI(1) driver loader.  But
 203 unlike <code>egl_dri2</code>, it supports Linux framebuffer devices as its
 204 window system and supports EGL_MESA_screen_surface extension.  As DRI1 drivers
 205 are phasing out, it might eventually be replaced by <code>egl_dri2</code>.</p>
 206
 207 </li>
 208 </ul>
 209
 210 <h2>Developers</h2>
 211
 212 <p>The sources of the main library and the classic drivers can be found at
 213 <code>src/egl/</code>.  The sources of the <code>egl</code> state tracker can
 214 be found at <code>src/gallium/state_trackers/egl/</code>.</p>
 215
 216 <p>The suggested way to learn to write a EGL driver is to see how other drivers
 217 are written.  <code>egl_glx</code> should be a good reference.  It works in any
 218 environment that has GLX support, and it is simpler than most drivers.</p>
 219
 220 <h3>Lifetime of Display Resources</h3>
 221
 222 <p>Contexts and surfaces are examples of display resources.  They might live
 223 longer than the display that creates them.</p>
 224
 225 <p>In EGL, when a display is terminated through <code>eglTerminate</code>, all
 226 display resources should be destroyed.  Similarly, when a thread is released
 227 throught <code>eglReleaseThread</code>, all current display resources should be
 228 released.  Another way to destory or release resources is through functions
 229 such as <code>eglDestroySurface</code> or <code>eglMakeCurrent</code>.</p>
 230
 231 <p>When a resource that is current to some thread is destroyed, the resource
 232 should not be destroyed immediately.  EGL requires the resource to live until
 233 it is no longer current.  A driver usually calls
 234 <code>eglIs&lt;Resource&gt;Bound</code> to check if a resource is bound
 235 (current) to any thread in the destroy callbacks.  If it is still bound, the
 236 resource is not destroyed.</p>
 237
 238 <p>The main library will mark destroyed current resources as unlinked.  In a
 239 driver's <code>MakeCurrent</code> callback,
 240 <code>eglIs&lt;Resource&gt;Linked</code> can then be called to check if a newly
 241 released resource is linked to a display.  If it is not, the last reference to
 242 the resource is removed and the driver should destroy the resource.  But it
 243 should be careful here because <code>MakeCurrent</code> might be called with an
 244 uninitialized display.</p>
 245
 246 <p>This is the only mechanism provided by the main library to help manage the
 247 resources.  The drivers are responsible to the correct behavior as defined by
 248 EGL.</p>
 249
 250 <h3><code>EGL_RENDER_BUFFER</code></h3>
 251
 252 <p>In EGL, the color buffer a context should try to render to is decided by the
 253 binding surface.  It should try to render to the front buffer if the binding
 254 surface has <code>EGL_RENDER_BUFFER</code> set to
 255 <code>EGL_SINGLE_BUFFER</code>;  If the same context is later bound to a
 256 surface with <code>EGL_RENDER_BUFFER</code> set to
 257 <code>EGL_BACK_BUFFER</code>, the context should try to render to the back
 258 buffer.  However, the context is allowed to make the final decision as to which
 259 color buffer it wants to or is able to render to.</p>
 260
 261 <p>For pbuffer surfaces, the render buffer is always
 262 <code>EGL_BACK_BUFFER</code>.  And for pixmap surfaces, the render buffer is
 263 always <code>EGL_SINGLE_BUFFER</code>.  Unlike window surfaces, EGL spec
 264 requires their <code>EGL_RENDER_BUFFER</code> values to be honored.  As a
 265 result, a driver should never set <code>EGL_PIXMAP_BIT</code> or
 266 <code>EGL_PBUFFER_BIT</code> bits of a config if the contexts created with the
 267 config won't be able to honor the <code>EGL_RENDER_BUFFER</code> of pixmap or
 268 pbuffer surfaces.</p>
 269
 270 <p>It should also be noted that pixmap and pbuffer surfaces are assumed to be
 271 single-buffered, in that <code>eglSwapBuffers</code> has no effect on them.  It
 272 is desirable that a driver allocates a private color buffer for each pbuffer
 273 surface created.  If the window system the driver supports has native pbuffers,
 274 or if the native pixmaps have more than one color buffers, the driver should
 275 carefully attach the native color buffers to the EGL surfaces, re-route them if
 276 required.</p>
 277
 278 <p>There is no defined behavior as to, for example, how
 279 <code>glDrawBuffer</code> interacts with <code>EGL_RENDER_BUFFER</code>.  Right
 280 now, it is desired that the draw buffer in a client API be fixed for pixmap and
 281 pbuffer surfaces.  Therefore, the driver is responsible to guarantee that the
 282 client API renders to the specified render buffer for pixmap and pbuffer
 283 surfaces.</p>
 284
 285 <h3><code>EGLDisplay</code> Mutex</h3>
 286
 287 The <code>EGLDisplay</code> will be locked before calling any of the dispatch
 288 functions (well, except for GetProcAddress which does not take an
 289 <code>EGLDisplay</code>).  This guarantees that the same dispatch function will
 290 not be called with the sample display at the same time.  If a driver has access
 291 to an <code>EGLDisplay</code> without going through the EGL APIs, the driver
 292 should as well lock the display before using it.
 293
 294 <h3>TODOs</h3>
 295
 296 <ul>
 297 <li>Pass the conformance tests</li>
 298 <li>Reference counting in main library?</li>
 299 <li>Mixed use of OpenGL, OpenGL ES 1.1, and OpenGL ES 2.0 is supported.  But
 300 which one of <code>libGL.so</code>, <code>libGLESv1_CM.so</code>, and
 301 <code>libGLESv2.so</code> should an application link to?  Bad things may happen
 302 when, say, an application is linked to <code>libGLESv2.so</code> and
 303 <code>libcairo</code>, which is linked to <code>libGL.so</code> instead.</li>
 304
 305 </ul>
 306
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