docs/egl.html

   1 <html>
   2
   3 <title>Mesa EGL</title>
   4
   5 <head><link rel="stylesheet" type="text/css" href="mesa.css"></head>
   6
   7 <body>
   8
   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,kms</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>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 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 </li>
 143
 144 <li><code>EGL_PLATFORM</code>
 145
 146 <p>This variable specifies the native platform.  The valid values are the same
 147 as those for <code>--with-egl-platforms</code>.  When the variable is not set,
 148 the main library uses the first platform listed in
 149 <code>--with-egl-platforms</code> as the native platform</p>
 150
 151 </li>
 152
 153 <li><code>EGL_LOG_LEVEL</code>
 154
 155 <p>This changes the log level of the main library and the drivers.  The valid
 156 values are: <code>debug</code>, <code>info</code>, <code>warning</code>, and
 157 <code>fatal</code>.</p>
 158
 159 </li>
 160
 161 <li><code>EGL_SOFTWARE</code>
 162
 163 <p>For drivers that support both hardware and software rendering, setting this
 164 variable to true forces the use of software rendering.</p>
 165
 166 </li>
 167 </ul>
 168
 169 <h2>EGL Drivers</h2>
 170
 171 <ul>
 172 <li><code>egl_gallium</code>
 173
 174 <p>This driver is based on Gallium3D.  It supports all rendering APIs and
 175 hardwares supported by Gallium3D.  It is the only driver that supports OpenVG.
 176 The supported platforms are X11, KMS, FBDEV, and GDI.</p>
 177
 178 </li>
 179
 180 <li><code>egl_glx</code>
 181
 182 <p>This driver provides a wrapper to GLX.  It uses exclusively GLX to implement
 183 the EGL API.  It supports both direct and indirect rendering when the GLX does.
 184 It is accelerated when the GLX is.  As such, it cannot provide functions that
 185 is not available in GLX or GLX extensions.</p>
 186 </li>
 187
 188 <li><code>egl_dri2</code>
 189
 190 <p>This driver supports the X Window System as its window system.  It functions
 191 as a DRI2 driver loader.  Unlike <code>egl_glx</code>, it has no dependency on
 192 <code>libGL</code>.  It talks to the X server directly using DRI2 protocol.</p>
 193
 194 </li>
 195 <li><code>egl_dri</code>
 196
 197 <p>This driver lacks maintenance and does <em>not</em> build.  It is similiar
 198 to <code>egl_dri2</code> in that it functions as a DRI(1) driver loader.  But
 199 unlike <code>egl_dri2</code>, it supports Linux framebuffer devices as its
 200 window system and supports EGL_MESA_screen_surface extension.  As DRI1 drivers
 201 are phasing out, it might eventually be replaced by <code>egl_dri2</code>.</p>
 202
 203 </li>
 204 </ul>
 205
 206 <h2>Developers</h2>
 207
 208 <p>The sources of the main library and the classic drivers can be found at
 209 <code>src/egl/</code>.  The sources of the <code>egl</code> state tracker can
 210 be found at <code>src/gallium/state_trackers/egl/</code>.</p>
 211
 212 <p>The suggested way to learn to write a EGL driver is to see how other drivers
 213 are written.  <code>egl_glx</code> should be a good reference.  It works in any
 214 environment that has GLX support, and it is simpler than most drivers.</p>
 215
 216 <h3>Lifetime of Display Resources</h3>
 217
 218 <p>Contexts and surfaces are examples of display resources.  They might live
 219 longer than the display that creates them.</p>
 220
 221 <p>In EGL, when a display is terminated through <code>eglTerminate</code>, all
 222 display resources should be destroyed.  Similarly, when a thread is released
 223 throught <code>eglReleaseThread</code>, all current display resources should be
 224 released.  Another way to destory or release resources is through functions
 225 such as <code>eglDestroySurface</code> or <code>eglMakeCurrent</code>.</p>
 226
 227 <p>When a resource that is current to some thread is destroyed, the resource
 228 should not be destroyed immediately.  EGL requires the resource to live until
 229 it is no longer current.  A driver usually calls
 230 <code>eglIs&lt;Resource&gt;Bound</code> to check if a resource is bound
 231 (current) to any thread in the destroy callbacks.  If it is still bound, the
 232 resource is not destroyed.</p>
 233
 234 <p>The main library will mark destroyed current resources as unlinked.  In a
 235 driver's <code>MakeCurrent</code> callback,
 236 <code>eglIs&lt;Resource&gt;Linked</code> can then be called to check if a newly
 237 released resource is linked to a display.  If it is not, the last reference to
 238 the resource is removed and the driver should destroy the resource.  But it
 239 should be careful here because <code>MakeCurrent</code> might be called with an
 240 uninitialized display.</p>
 241
 242 <p>This is the only mechanism provided by the main library to help manage the
 243 resources.  The drivers are responsible to the correct behavior as defined by
 244 EGL.</p>
 245
 246 <h3><code>EGL_RENDER_BUFFER</code></h3>
 247
 248 <p>In EGL, the color buffer a context should try to render to is decided by the
 249 binding surface.  It should try to render to the front buffer if the binding
 250 surface has <code>EGL_RENDER_BUFFER</code> set to
 251 <code>EGL_SINGLE_BUFFER</code>;  If the same context is later bound to a
 252 surface with <code>EGL_RENDER_BUFFER</code> set to
 253 <code>EGL_BACK_BUFFER</code>, the context should try to render to the back
 254 buffer.  However, the context is allowed to make the final decision as to which
 255 color buffer it wants to or is able to render to.</p>
 256
 257 <p>For pbuffer surfaces, the render buffer is always
 258 <code>EGL_BACK_BUFFER</code>.  And for pixmap surfaces, the render buffer is
 259 always <code>EGL_SINGLE_BUFFER</code>.  Unlike window surfaces, EGL spec
 260 requires their <code>EGL_RENDER_BUFFER</code> values to be honored.  As a
 261 result, a driver should never set <code>EGL_PIXMAP_BIT</code> or
 262 <code>EGL_PBUFFER_BIT</code> bits of a config if the contexts created with the
 263 config won't be able to honor the <code>EGL_RENDER_BUFFER</code> of pixmap or
 264 pbuffer surfaces.</p>
 265
 266 <p>It should also be noted that pixmap and pbuffer surfaces are assumed to be
 267 single-buffered, in that <code>eglSwapBuffers</code> has no effect on them.  It
 268 is desirable that a driver allocates a private color buffer for each pbuffer
 269 surface created.  If the window system the driver supports has native pbuffers,
 270 or if the native pixmaps have more than one color buffers, the driver should
 271 carefully attach the native color buffers to the EGL surfaces, re-route them if
 272 required.</p>
 273
 274 <p>There is no defined behavior as to, for example, how
 275 <code>glDrawBuffer</code> interacts with <code>EGL_RENDER_BUFFER</code>.  Right
 276 now, it is desired that the draw buffer in a client API be fixed for pixmap and
 277 pbuffer surfaces.  Therefore, the driver is responsible to guarantee that the
 278 client API renders to the specified render buffer for pixmap and pbuffer
 279 surfaces.</p>
 280
 281 <h3><code>EGLDisplay</code> Mutex</h3>
 282
 283 The <code>EGLDisplay</code> will be locked before calling any of the dispatch
 284 functions (well, except for GetProcAddress which does not take an
 285 <code>EGLDisplay</code>).  This guarantees that the same dispatch function will
 286 not be called with the sample display at the same time.  If a driver has access
 287 to an <code>EGLDisplay</code> without going through the EGL APIs, the driver
 288 should as well lock the display before using it.
 289
 290 <h3>TODOs</h3>
 291
 292 <ul>
 293 <li>Pass the conformance tests</li>
 294 <li>Better automatic driver selection: <code>EGL_PLATFORM</code> loads all
 295 drivers and might eat too much memory.</li>
 296
 297 </ul>
 298
 299 </body>
 300 </html>