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 --with-state-trackers=egl,es,vega --enable-gallium-{swrast,intel}
  36 </pre>
  37
  38 <p>The main library will be enabled by default.  The <code>egl</code> state
  39 tracker is needed by a number of EGL drivers.  EGL drivers will be covered
  40 later.  The <a href="opengles.html">es state tracker</a> provides OpenGL ES 1.x
  41 and 2.x and 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>libGLESv1_CM</code>, <code>libGLESv2</code>, <code>libOpenVG</code>, and
  50 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, like OpenGL ES or OpenVG, to build.  But it should be noted
  87 that a number of EGL drivers depend on the <code>egl</code> state tracker.
  88 They will <em>not</em> be built without the <code>egl</code> state tracker.</p>
  89
  90 </li>
  91
  92 <li><code>--enable-gallium-swrast</code>
  93
  94 <p>This option is not specific to EGL.  But if there is no driver for your
  95 hardware, or you are experiencing problems with the hardware driver, you can
  96 enable the swrast DRM driver.  It is a dummy driver and EGL will fallback to
  97 software rendering automatically.</p>
  98
  99 </li>
 100 </ul>
 101
 102 <h3>OpenGL</h3>
 103
 104 <p>The OpenGL state tracker is not built in the above example.  It should be
 105 noted that the classic <code>libGL</code> is not a state tracker and cannot be
 106 used with EGL (unless the EGL driver in use is <code>egl_glx</code>).  To build
 107 the OpenGL state tracker, one may append <code>glx</code> to
 108 <code>--with-state-trackers</code> and manually build
 109 <code>src/gallium/winsys/xlib/</code>.</p>
 110
 111 <h2>Use EGL</h2>
 112
 113 <p> The demos for OpenGL ES and OpenVG can be found in <code>progs/es1/</code>,
 114 <code>progs/es2/</code> and <code>progs/openvg/</code>.  You can use them to
 115 test your build.  For example,</p>
 116
 117 <pre>
 118   $ cd progs/es1/xegl
 119   $ make
 120   $ ./torus
 121 </pre>
 122
 123 <h3>Environment Variables</h3>
 124
 125 <p>There are several environment variables that control the behavior of EGL at
 126 runtime</p>
 127
 128 <ul>
 129 <li><code>EGL_DRIVERS_PATH</code>
 130
 131 <p>By default, the main library will look for drivers in the directory where
 132 the drivers are installed to.  This variable specifies a list of
 133 colon-separated directories where the main library will look for drivers, in
 134 addition to the default directory.  This variable is ignored for setuid/setgid
 135 binaries.</p>
 136
 137 </li>
 138
 139 <li><code>EGL_DRIVER</code>
 140
 141 <p>This variable specifies a full path to an EGL driver and it forces the
 142 specified EGL driver to be loaded.  It comes in handy when one wants to test a
 143 specific driver.  This variable is ignored for setuid/setgid binaries.</p>
 144
 145 </li>
 146
 147 <li><code>EGL_DISPLAY</code>
 148
 149 <p>When <code>EGL_DRIVER</code> is not set, the main library loads <em>all</em>
 150 EGL drivers that support a certain window system.  <code>EGL_DISPLAY</code> can
 151 be used to specify the window system and the valid values are, for example,
 152 <code>x11</code> or <code>kms</code>.  When the variable is not set, the main
 153 library defaults the value to the first window system listed in
 154 <code>--with-egl-displays</code> at configuration time.
 155
 156 </li>
 157
 158 <li><code>EGL_LOG_LEVEL</code>
 159
 160 <p>This changes the log level of the main library and the drivers.  The valid
 161 values are: <code>debug</code>, <code>info</code>, <code>warning</code>, and
 162 <code>fatal</code>.</p>
 163
 164 </li>
 165
 166 <li><code>EGL_SOFTWARE</code>
 167
 168 <p>For drivers that support both hardware and software rendering, setting this
 169 variable to true forces the use of software rendering.</p>
 170
 171 </li>
 172 </ul>
 173
 174 <h2>EGL Drivers</h2>
 175
 176 <p>There are two categories of EGL drivers: Gallium and classic.</p>
 177
 178 <p>Gallium EGL drivers supports all rendering APIs specified in EGL 1.4.  The
 179 support for optional EGL functions and EGL extensions is usually more complete
 180 than the classic ones.  These drivers depend on the <code>egl</code> state
 181 tracker to build.  The available drivers are</p>
 182
 183 <ul>
 184 <li><code>egl_&lt;dpy&gt;_i915</code></li>
 185 <li><code>egl_&lt;dpy&gt;_i965</code></li>
 186 <li><code>egl_&lt;dpy&gt;_radeon</code></li>
 187 <li><code>egl_&lt;dpy&gt;_nouveau</code></li>
 188 <li><code>egl_&lt;dpy&gt;_swrast</code></li>
 189 <li><code>egl_&lt;dpy&gt;_vmwgfx</code></li>
 190 </ul>
 191
 192 <p><code>&lt;dpy&gt;</code> is given by <code>--with-egl-displays</code> at
 193 configuration time.  There will be one EGL driver for each combination of the
 194 displays listed and the hardware drivers enabled.</p>
 195
 196 <p>Classic EGL drivers, on the other hand, supports only OpenGL as its
 197 rendering API.  They can be found under <code>src/egl/drivers/</code>.  There
 198 are 3 of them</p>
 199
 200 <ul>
 201 <li><code>egl_glx</code>
 202
 203 <p>This driver provides a wrapper to GLX.  It uses exclusively GLX to implement
 204 the EGL API.  It supports both direct and indirect rendering when the GLX does.
 205 It is accelerated when the GLX is.  As such, it cannot provide functions that
 206 is not available in GLX or GLX extensions.</p>
 207 </li>
 208
 209 <li><code>egl_dri2</code>
 210
 211 <p>This driver supports the X Window System as its window system.  It functions
 212 as a DRI2 driver loader.  Unlike <code>egl_glx</code>, it has no dependency on
 213 <code>libGL</code>.  It talks to the X server directly using DRI2 protocol.</p>
 214
 215 </li>
 216 <li><code>egl_dri</code>
 217
 218 <p>This driver lacks maintenance and does <em>not</em> build.  It is similiar
 219 to <code>egl_dri2</code> in that it functions as a DRI(1) driver loader.  But
 220 unlike <code>egl_dri2</code>, it supports Linux framebuffer devices as its
 221 window system and supports EGL_MESA_screen_surface extension.  As DRI1 drivers
 222 are phasing out, it might eventually be replaced by <code>egl_dri2</code>.</p>
 223
 224 </li>
 225 </ul>
 226
 227 <p>To use the classic drivers, one must manually set <code>EGL_DRIVER</code> at
 228 runtime.</p>
 229
 230 <h2>Developers</h2>
 231
 232 <p>The sources of the main library and the classic drivers can be found at
 233 <code>src/egl/</code>.  The sources of the <code>egl</code> state tracker can
 234 be found at <code>src/gallium/state_trackers/egl/</code>.</p>
 235
 236 <p>The suggested way to learn to write a EGL driver is to see how other drivers
 237 are written.  <code>egl_glx</code> should be a good reference.  It works in any
 238 environment that has GLX support, and it is simpler than most drivers.</p>
 239
 240 <h3>Lifetime of Display Resources</h3>
 241
 242 <p>Contexts and surfaces are examples of display resources.  They might live
 243 longer than the display that creates them.</p>
 244
 245 <p>In EGL, when a display is terminated through <code>eglTerminate</code>, all
 246 display resources should be destroyed.  Similarly, when a thread is released
 247 throught <code>eglReleaseThread</code>, all current display resources should be
 248 released.  Another way to destory or release resources is through functions
 249 such as <code>eglDestroySurface</code> or <code>eglMakeCurrent</code>.</p>
 250
 251 <p>When a resource that is current to some thread is destroyed, the resource
 252 should not be destroyed immediately.  EGL requires the resource to live until
 253 it is no longer current.  A driver usually calls
 254 <code>eglIs&lt;Resource&gt;Bound</code> to check if a resource is bound
 255 (current) to any thread in the destroy callbacks.  If it is still bound, the
 256 resource is not destroyed.</p>
 257
 258 <p>The main library will mark destroyed current resources as unlinked.  In a
 259 driver's <code>MakeCurrent</code> callback,
 260 <code>eglIs&lt;Resource&gt;Linked</code> can then be called to check if a newly
 261 released resource is linked to a display.  If it is not, the last reference to
 262 the resource is removed and the driver should destroy the resource.  But it
 263 should be careful here because <code>MakeCurrent</code> might be called with an
 264 uninitialized display.</p>
 265
 266 <p>This is the only mechanism provided by the main library to help manage the
 267 resources.  The drivers are responsible to the correct behavior as defined by
 268 EGL.</p>
 269
 270 <h3><code>EGL_RENDER_BUFFER</code></h3>
 271
 272 <p>In EGL, the color buffer a context should try to render to is decided by the
 273 binding surface.  It should try to render to the front buffer if the binding
 274 surface has <code>EGL_RENDER_BUFFER</code> set to
 275 <code>EGL_SINGLE_BUFFER</code>;  If the same context is later bound to a
 276 surface with <code>EGL_RENDER_BUFFER</code> set to
 277 <code>EGL_BACK_BUFFER</code>, the context should try to render to the back
 278 buffer.  However, the context is allowed to make the final decision as to which
 279 color buffer it wants to or is able to render to.</p>
 280
 281 <p>For pbuffer surfaces, the render buffer is always
 282 <code>EGL_BACK_BUFFER</code>.  And for pixmap surfaces, the render buffer is
 283 always <code>EGL_SINGLE_BUFFER</code>.  Unlike window surfaces, EGL spec
 284 requires their <code>EGL_RENDER_BUFFER</code> values to be honored.  As a
 285 result, a driver should never set <code>EGL_PIXMAP_BIT</code> or
 286 <code>EGL_PBUFFER_BIT</code> bits of a config if the contexts created with the
 287 config won't be able to honor the <code>EGL_RENDER_BUFFER</code> of pixmap or
 288 pbuffer surfaces.</p>
 289
 290 <p>It should also be noted that pixmap and pbuffer surfaces are assumed to be
 291 single-buffered, in that <code>eglSwapBuffers</code> has no effect on them.  It
 292 is desirable that a driver allocates a private color buffer for each pbuffer
 293 surface created.  If the window system the driver supports has native pbuffers,
 294 or if the native pixmaps have more than one color buffers, the driver should
 295 carefully attach the native color buffers to the EGL surfaces, re-route them if
 296 required.</p>
 297
 298 <p>There is no defined behavior as to, for example, how
 299 <code>glDrawBuffer</code> interacts with <code>EGL_RENDER_BUFFER</code>.  Right
 300 now, it is desired that the draw buffer in a client API be fixed for pixmap and
 301 pbuffer surfaces.  Therefore, the driver is responsible to guarantee that the
 302 client API renders to the specified render buffer for pixmap and pbuffer
 303 surfaces.</p>
 304
 305 <h3><code>EGLDisplay</code> Mutex</h3>
 306
 307 The <code>EGLDisplay</code> will be locked before calling any of the dispatch
 308 functions (well, except for GetProcAddress which does not take an
 309 <code>EGLDisplay</code>).  This guarantees that the same dispatch function will
 310 not be called with the sample display at the same time.  If a driver has access
 311 to an <code>EGLDisplay</code> without going through the EGL APIs, the driver
 312 should as well lock the display before using it.
 313
 314 <h3>TODOs</h3>
 315
 316 <ul>
 317 <li>Pass the conformance tests</li>
 318 <li>Better automatic driver selection: <code>EGL_DISPLAY</code> loads all
 319 drivers and might eat too much memory.</li>
 320
 321 </ul>
 322
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