mesa3d/docs/egl.html

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<h1>Mesa EGL</h1>

<p>The current version of EGL in Mesa implements EGL 1.4.  More information
about EGL can be found at
<a href="http://www.khronos.org/egl/" target="_parent">
http://www.khronos.org/egl/</a>.</p>

<p>The Mesa's implementation of EGL uses a driver architecture.  The main
library (<code>libEGL</code>) is window system neutral.  It provides the EGL
API entry points and helper functions for use by the drivers.  Drivers are
dynamically loaded by the main library and most of the EGL API calls are
directly dispatched to the drivers.</p>

<p>The driver in use decides the window system to support.  For drivers that
support hardware rendering, there are usually multiple drivers supporting the
same window system.  Each one of of them supports a certain range of graphics
cards.</p>

<h2>Build EGL</h2>

<ol>
<li>
<p>Run <code>configure</code> with the desired state trackers and enable
the Gallium driver for your hardware.  For example</p>

<pre>
  $ /configure --enable-gles-overlay --with-state-trackers=egl,vega --enable-gallium-intel
</pre>

<p>The main library and OpenGL is enabled by default.  The first option enables
<a href="opengles.html">OpenGL ES 1.x and 2.x</a>.  The <code>egl</code> state
tracker is needed by a number of EGL drivers.  EGL drivers will be covered
later.  The <a href="openvg.html">vega state tracker</a> provides OpenVG
1.x.</p>
</li>

<li>Build and install Mesa as usual.</li>
</ol>

<p>In the given example, it will build and install <code>libEGL</code>,
<code>libGL</code>, <code>libGLESv1_CM</code>, <code>libGLESv2</code>,
<code>libOpenVG</code>, and one or more EGL drivers.</p>

<h3>Configure Options</h3>

<p>There are several options that control the build of EGL at configuration
time</p>

<ul>
<li><code>--enable-egl</code>

<p>By default, EGL is enabled.  When disabled, the main library and the drivers
will not be built.</p>

</li>

<li><code>--with-egl-driver-dir</code>

<p>The directory EGL drivers should be installed to.  If not specified, EGL
drivers will be installed to <code>${libdir}/egl</code>.</p>

</li>

<li><code>--with-egl-platforms</code>

<p>List the platforms (window systems) to support.  Its argument is a comma
seprated string such as <code>--with-egl-platforms=x11,drm</code>.  It decides
the platforms a driver may support.  The first listed platform is also used by
the main library to decide the native platform: the platform the EGL native
types such as <code>EGLNativeDisplayType</code> or
<code>EGLNativeWindowType</code> defined for.</p>

<p>The available platforms are <code>x11</code>, <code>drm</code>,
<code>fbdev</code>, and <code>gdi</code>.  The <code>gdi</code> platform can
only be built with SCons.</p>

</li>

<li><code>--with-state-trackers</code>

<p>The argument is a comma separated string.  It is usually used to specify the
rendering APIs, such as OpenVG, to build.  But it is also used to specify
<code>egl</code> state tracker that <code>egl_gallium</code> depends on.</p>

</li>

<li><code>--enable-gles-overlay</code>

<p>OpenGL and OpenGL ES are not controlled by
<code>--with-state-trackers</code>.  OpenGL is always built.  To build OpenGL
ES, this option must be explicitly given.</p>

</li>

<li><code>--enable-gles1</code> and <code>--enable-gles2</code>

<p>Unlike <code>--enable-gles-overlay</code>, which builds one library for each
rendering API, these options enable OpenGL ES support in OpenGL.  The result is
one big library that supports multiple APIs.</p>

</li>

</ul>

<h2>Use EGL</h2>

<h3>Demos</h3>

<p>There are demos for the client APIs supported by EGL.  They can be found in
mesa/demos repository.</p>

<h3>Environment Variables</h3>

<p>There are several environment variables that control the behavior of EGL at
runtime</p>

<ul>
<li><code>EGL_DRIVERS_PATH</code>

<p>By default, the main library will look for drivers in the directory where
the drivers are installed to.  This variable specifies a list of
colon-separated directories where the main library will look for drivers, in
addition to the default directory.  This variable is ignored for setuid/setgid
binaries.</p>

</li>

<li><code>EGL_DRIVER</code>

<p>This variable specifies a full path to an EGL driver and it forces the
specified EGL driver to be loaded.  It comes in handy when one wants to test a
specific driver.  This variable is ignored for setuid/setgid binaries.</p>

<p><code>egl_gallium</code> dynamically loads hardware drivers and client API
modules found in <code>EGL_DRIVERS_PATH</code>.  Thus, specifying this variable
alone is not sufficient for <code>egl_gallium</code> for uninstalled build.</p>

</li>

<li><code>EGL_PLATFORM</code>

<p>This variable specifies the native platform.  The valid values are the same
as those for <code>--with-egl-platforms</code>.  When the variable is not set,
the main library uses the first platform listed in
<code>--with-egl-platforms</code> as the native platform</p>

</li>

<li><code>EGL_LOG_LEVEL</code>

<p>This changes the log level of the main library and the drivers.  The valid
values are: <code>debug</code>, <code>info</code>, <code>warning</code>, and
<code>fatal</code>.</p>

</li>

<li><code>EGL_SOFTWARE</code>

<p>For drivers that support both hardware and software rendering, setting this
variable to true forces the use of software rendering.</p>

</li>
</ul>

<h2>EGL Drivers</h2>

<ul>
<li><code>egl_gallium</code>

<p>This driver is based on Gallium3D.  It supports all rendering APIs and
hardwares supported by Gallium3D.  It is the only driver that supports OpenVG.
The supported platforms are X11, KMS, FBDEV, and GDI.</p>

</li>

<li><code>egl_glx</code>

<p>This driver provides a wrapper to GLX.  It uses exclusively GLX to implement
the EGL API.  It supports both direct and indirect rendering when the GLX does.
It is accelerated when the GLX is.  As such, it cannot provide functions that
is not available in GLX or GLX extensions.</p>
</li>

<li><code>egl_dri2</code>

<p>This driver supports the X Window System as its window system.  It functions
as a DRI2 driver loader.  Unlike <code>egl_glx</code>, it has no dependency on
<code>libGL</code>.  It talks to the X server directly using DRI2 protocol.</p>

</li>
<li><code>egl_dri</code>

<p>This driver lacks maintenance and does <em>not</em> build.  It is similiar
to <code>egl_dri2</code> in that it functions as a DRI(1) driver loader.  But
unlike <code>egl_dri2</code>, it supports Linux framebuffer devices as its
window system and supports EGL_MESA_screen_surface extension.  As DRI1 drivers
are phasing out, it might eventually be replaced by <code>egl_dri2</code>.</p>

</li>
</ul>

<h2>Developers</h2>

<p>The sources of the main library and the classic drivers can be found at
<code>src/egl/</code>.  The sources of the <code>egl</code> state tracker can
be found at <code>src/gallium/state_trackers/egl/</code>.</p>

<p>The suggested way to learn to write a EGL driver is to see how other drivers
are written.  <code>egl_glx</code> should be a good reference.  It works in any
environment that has GLX support, and it is simpler than most drivers.</p>

<h3>Lifetime of Display Resources</h3>

<p>Contexts and surfaces are examples of display resources.  They might live
longer than the display that creates them.</p>

<p>In EGL, when a display is terminated through <code>eglTerminate</code>, all
display resources should be destroyed.  Similarly, when a thread is released
throught <code>eglReleaseThread</code>, all current display resources should be
released.  Another way to destory or release resources is through functions
such as <code>eglDestroySurface</code> or <code>eglMakeCurrent</code>.</p>

<p>When a resource that is current to some thread is destroyed, the resource
should not be destroyed immediately.  EGL requires the resource to live until
it is no longer current.  A driver usually calls
<code>eglIs&lt;Resource&gt;Bound</code> to check if a resource is bound
(current) to any thread in the destroy callbacks.  If it is still bound, the
resource is not destroyed.</p>

<p>The main library will mark destroyed current resources as unlinked.  In a
driver's <code>MakeCurrent</code> callback,
<code>eglIs&lt;Resource&gt;Linked</code> can then be called to check if a newly
released resource is linked to a display.  If it is not, the last reference to
the resource is removed and the driver should destroy the resource.  But it
should be careful here because <code>MakeCurrent</code> might be called with an
uninitialized display.</p>

<p>This is the only mechanism provided by the main library to help manage the
resources.  The drivers are responsible to the correct behavior as defined by
EGL.</p>

<h3><code>EGL_RENDER_BUFFER</code></h3>

<p>In EGL, the color buffer a context should try to render to is decided by the
binding surface.  It should try to render to the front buffer if the binding
surface has <code>EGL_RENDER_BUFFER</code> set to
<code>EGL_SINGLE_BUFFER</code>;  If the same context is later bound to a
surface with <code>EGL_RENDER_BUFFER</code> set to
<code>EGL_BACK_BUFFER</code>, the context should try to render to the back
buffer.  However, the context is allowed to make the final decision as to which
color buffer it wants to or is able to render to.</p>

<p>For pbuffer surfaces, the render buffer is always
<code>EGL_BACK_BUFFER</code>.  And for pixmap surfaces, the render buffer is
always <code>EGL_SINGLE_BUFFER</code>.  Unlike window surfaces, EGL spec
requires their <code>EGL_RENDER_BUFFER</code> values to be honored.  As a
result, a driver should never set <code>EGL_PIXMAP_BIT</code> or
<code>EGL_PBUFFER_BIT</code> bits of a config if the contexts created with the
config won't be able to honor the <code>EGL_RENDER_BUFFER</code> of pixmap or
pbuffer surfaces.</p>

<p>It should also be noted that pixmap and pbuffer surfaces are assumed to be
single-buffered, in that <code>eglSwapBuffers</code> has no effect on them.  It
is desirable that a driver allocates a private color buffer for each pbuffer
surface created.  If the window system the driver supports has native pbuffers,
or if the native pixmaps have more than one color buffers, the driver should
carefully attach the native color buffers to the EGL surfaces, re-route them if
required.</p>

<p>There is no defined behavior as to, for example, how
<code>glDrawBuffer</code> interacts with <code>EGL_RENDER_BUFFER</code>.  Right
now, it is desired that the draw buffer in a client API be fixed for pixmap and
pbuffer surfaces.  Therefore, the driver is responsible to guarantee that the
client API renders to the specified render buffer for pixmap and pbuffer
surfaces.</p>

<h3><code>EGLDisplay</code> Mutex</h3>

The <code>EGLDisplay</code> will be locked before calling any of the dispatch
functions (well, except for GetProcAddress which does not take an
<code>EGLDisplay</code>).  This guarantees that the same dispatch function will
not be called with the sample display at the same time.  If a driver has access
to an <code>EGLDisplay</code> without going through the EGL APIs, the driver
should as well lock the display before using it.

<h3>TODOs</h3>

<ul>
<li>Pass the conformance tests</li>
<li>Reference counting in main library?</li>
<li>Mixed use of OpenGL, OpenGL ES 1.1, and OpenGL ES 2.0 is supported.  But
which one of <code>libGL.so</code>, <code>libGLESv1_CM.so</code>, and
<code>libGLESv2.so</code> should an application link to?  Bad things may happen
when, say, an application is linked to <code>libGLESv2.so</code> and
<code>libcairo</code>, which is linked to <code>libGL.so</code> instead.</li>

</ul>

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