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 --enable-openvg --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 second option enables
  40 <a href="openvg.html">OpenVG</a>.
  41 </p>
  42
  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>--enable-gles-overlay</code>
  88
  89 <p>OpenGL is built by default.  To build OpenGL ES, this option must be
  90 explicitly given.</p>
  91
  92 </li>
  93
  94 <li><code>--enable-gles1</code> and <code>--enable-gles2</code>
  95
  96 <p>Unlike <code>--enable-gles-overlay</code>, which builds one library for each
  97 rendering API, these options enable OpenGL ES support in OpenGL.  The result is
  98 one big library that supports multiple APIs.</p>
  99
 100 </li>
 101
 102 <li><code>--enable-openvg</code>
 103
 104 <p>OpenVG must be explicitly enabled by this option.</p>
 105
 106 </li>
 107
 108 </ul>
 109
 110 <h2>Use EGL</h2>
 111
 112 <h3>Demos</h3>
 113
 114 <p>There are demos for the client APIs supported by EGL.  They can be found in
 115 mesa/demos repository.</p>
 116
 117 <h3>Environment Variables</h3>
 118
 119 <p>There are several environment variables that control the behavior of EGL at
 120 runtime</p>
 121
 122 <ul>
 123 <li><code>EGL_DRIVERS_PATH</code>
 124
 125 <p>By default, the main library will look for drivers in the directory where
 126 the drivers are installed to.  This variable specifies a list of
 127 colon-separated directories where the main library will look for drivers, in
 128 addition to the default directory.  This variable is ignored for setuid/setgid
 129 binaries.</p>
 130
 131 </li>
 132
 133 <li><code>EGL_DRIVER</code>
 134
 135 <p>This variable specifies a full path to an EGL driver and it forces the
 136 specified EGL driver to be loaded.  It comes in handy when one wants to test a
 137 specific driver.  This variable is ignored for setuid/setgid binaries.</p>
 138
 139 </li>
 140
 141 <li><code>EGL_PLATFORM</code>
 142
 143 <p>This variable specifies the native platform.  The valid values are the same
 144 as those for <code>--with-egl-platforms</code>.  When the variable is not set,
 145 the main library uses the first platform listed in
 146 <code>--with-egl-platforms</code> as the native platform</p>
 147
 148 </li>
 149
 150 <li><code>EGL_LOG_LEVEL</code>
 151
 152 <p>This changes the log level of the main library and the drivers.  The valid
 153 values are: <code>debug</code>, <code>info</code>, <code>warning</code>, and
 154 <code>fatal</code>.</p>
 155
 156 </li>
 157
 158 <li><code>EGL_SOFTWARE</code>
 159
 160 <p>For drivers that support both hardware and software rendering, setting this
 161 variable to true forces the use of software rendering.</p>
 162
 163 </li>
 164 </ul>
 165
 166 <h2>EGL Drivers</h2>
 167
 168 <ul>
 169 <li><code>egl_gallium</code>
 170
 171 <p>This driver is based on Gallium3D.  It supports all rendering APIs and
 172 hardwares supported by Gallium3D.  It is the only driver that supports OpenVG.
 173 The supported platforms are X11, DRM, FBDEV, and GDI.</p>
 174
 175 </li>
 176
 177 <li><code>egl_glx</code>
 178
 179 <p>This driver provides a wrapper to GLX.  It uses exclusively GLX to implement
 180 the EGL API.  It supports both direct and indirect rendering when the GLX does.
 181 It is accelerated when the GLX is.  As such, it cannot provide functions that
 182 is not available in GLX or GLX extensions.</p>
 183 </li>
 184
 185 <li><code>egl_dri2</code>
 186
 187 <p>This driver supports the X Window System as its window system.  It functions
 188 as a DRI2 driver loader.  Unlike <code>egl_glx</code>, it has no dependency on
 189 <code>libGL</code>.  It talks to the X server directly using (XCB-)DRI2
 190 protocol.</p>
 191
 192 </li>
 193 </ul>
 194
 195 <h2>Developers</h2>
 196
 197 <p>The sources of the main library and the classic drivers can be found at
 198 <code>src/egl/</code>.  The sources of the <code>egl</code> state tracker can
 199 be found at <code>src/gallium/state_trackers/egl/</code>.</p>
 200
 201 <p>The suggested way to learn to write a EGL driver is to see how other drivers
 202 are written.  <code>egl_glx</code> should be a good reference.  It works in any
 203 environment that has GLX support, and it is simpler than most drivers.</p>
 204
 205 <h3>Lifetime of Display Resources</h3>
 206
 207 <p>Contexts and surfaces are examples of display resources.  They might live
 208 longer than the display that creates them.</p>
 209
 210 <p>In EGL, when a display is terminated through <code>eglTerminate</code>, all
 211 display resources should be destroyed.  Similarly, when a thread is released
 212 throught <code>eglReleaseThread</code>, all current display resources should be
 213 released.  Another way to destory or release resources is through functions
 214 such as <code>eglDestroySurface</code> or <code>eglMakeCurrent</code>.</p>
 215
 216 <p>When a resource that is current to some thread is destroyed, the resource
 217 should not be destroyed immediately.  EGL requires the resource to live until
 218 it is no longer current.  A driver usually calls
 219 <code>eglIs&lt;Resource&gt;Bound</code> to check if a resource is bound
 220 (current) to any thread in the destroy callbacks.  If it is still bound, the
 221 resource is not destroyed.</p>
 222
 223 <p>The main library will mark destroyed current resources as unlinked.  In a
 224 driver's <code>MakeCurrent</code> callback,
 225 <code>eglIs&lt;Resource&gt;Linked</code> can then be called to check if a newly
 226 released resource is linked to a display.  If it is not, the last reference to
 227 the resource is removed and the driver should destroy the resource.  But it
 228 should be careful here because <code>MakeCurrent</code> might be called with an
 229 uninitialized display.</p>
 230
 231 <p>This is the only mechanism provided by the main library to help manage the
 232 resources.  The drivers are responsible to the correct behavior as defined by
 233 EGL.</p>
 234
 235 <h3><code>EGL_RENDER_BUFFER</code></h3>
 236
 237 <p>In EGL, the color buffer a context should try to render to is decided by the
 238 binding surface.  It should try to render to the front buffer if the binding
 239 surface has <code>EGL_RENDER_BUFFER</code> set to
 240 <code>EGL_SINGLE_BUFFER</code>;  If the same context is later bound to a
 241 surface with <code>EGL_RENDER_BUFFER</code> set to
 242 <code>EGL_BACK_BUFFER</code>, the context should try to render to the back
 243 buffer.  However, the context is allowed to make the final decision as to which
 244 color buffer it wants to or is able to render to.</p>
 245
 246 <p>For pbuffer surfaces, the render buffer is always
 247 <code>EGL_BACK_BUFFER</code>.  And for pixmap surfaces, the render buffer is
 248 always <code>EGL_SINGLE_BUFFER</code>.  Unlike window surfaces, EGL spec
 249 requires their <code>EGL_RENDER_BUFFER</code> values to be honored.  As a
 250 result, a driver should never set <code>EGL_PIXMAP_BIT</code> or
 251 <code>EGL_PBUFFER_BIT</code> bits of a config if the contexts created with the
 252 config won't be able to honor the <code>EGL_RENDER_BUFFER</code> of pixmap or
 253 pbuffer surfaces.</p>
 254
 255 <p>It should also be noted that pixmap and pbuffer surfaces are assumed to be
 256 single-buffered, in that <code>eglSwapBuffers</code> has no effect on them.  It
 257 is desirable that a driver allocates a private color buffer for each pbuffer
 258 surface created.  If the window system the driver supports has native pbuffers,
 259 or if the native pixmaps have more than one color buffers, the driver should
 260 carefully attach the native color buffers to the EGL surfaces, re-route them if
 261 required.</p>
 262
 263 <p>There is no defined behavior as to, for example, how
 264 <code>glDrawBuffer</code> interacts with <code>EGL_RENDER_BUFFER</code>.  Right
 265 now, it is desired that the draw buffer in a client API be fixed for pixmap and
 266 pbuffer surfaces.  Therefore, the driver is responsible to guarantee that the
 267 client API renders to the specified render buffer for pixmap and pbuffer
 268 surfaces.</p>
 269
 270 <h3><code>EGLDisplay</code> Mutex</h3>
 271
 272 The <code>EGLDisplay</code> will be locked before calling any of the dispatch
 273 functions (well, except for GetProcAddress which does not take an
 274 <code>EGLDisplay</code>).  This guarantees that the same dispatch function will
 275 not be called with the sample display at the same time.  If a driver has access
 276 to an <code>EGLDisplay</code> without going through the EGL APIs, the driver
 277 should as well lock the display before using it.
 278
 279 <h3>TODOs</h3>
 280
 281 <ul>
 282 <li>Pass the conformance tests</li>
 283 <li>Mixed use of OpenGL, OpenGL ES 1.1, and OpenGL ES 2.0 is supported.  But
 284 which one of <code>libGL.so</code>, <code>libGLESv1_CM.so</code>, and
 285 <code>libGLESv2.so</code> should an application link to?  Bad things may happen
 286 when, say, an application is linked to <code>libGLESv2.so</code> and
 287 <code>libcairo</code>, which is linked to <code>libGL.so</code> instead.</li>
 288
 289 </ul>
 290
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