Mesa EGL
The current version of EGL in Mesa implements EGL 1.4. More information about EGL can be found at http://www.khronos.org/egl/.
The Mesa's implementation of EGL uses a driver architecture. The main
library (libEGL
) 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.
The driver in use decides the window system to support.
Build EGL
-
Run
configure
with the desired client APIs and enable the driver for your hardware. For example$ ./configure --enable-gles1 --enable-gles2 \ --with-dri-drivers=... \ --with-gallium-drivers=...
The main library and OpenGL is enabled by default. The first two options above enables OpenGL ES 1.x and 2.x. The last two options enables the listed classic and and Gallium drivers respectively.
- Build and install Mesa as usual.
In the given example, it will build and install libEGL
,
libGL
, libGLESv1_CM
, libGLESv2
, and one
or more EGL drivers.
Configure Options
There are several options that control the build of EGL at configuration time
--enable-egl
-
By default, EGL is enabled. When disabled, the main library and the drivers will not be built.
--with-egl-driver-dir
-
The directory EGL drivers should be installed to. If not specified, EGL drivers will be installed to
${libdir}/egl
. --with-egl-platforms
-
List the platforms (window systems) to support. Its argument is a comma separated string such as
--with-egl-platforms=x11,drm
. 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 asEGLNativeDisplayType
orEGLNativeWindowType
defined for.The available platforms are
x11
,drm
,fbdev
, andgdi
. Thegdi
platform can only be built with SCons. Unless for special needs, the build system should select the right platforms automatically. --enable-gles1
--enable-gles2
-
These options enable OpenGL ES support in OpenGL. The result is one big internal library that supports multiple APIs.
--enable-shared-glapi
-
By default,
libGL
has its own copy oflibglapi
. This options makeslibGL
use the sharedlibglapi
. This is required if applications mix OpenGL and OpenGL ES.
Use EGL
Demos
There are demos for the client APIs supported by EGL. They can be found in mesa/demos repository.
Environment Variables
There are several environment variables that control the behavior of EGL at runtime
EGL_DRIVERS_PATH
-
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.
This variable is usually set to test an uninstalled build. For example, one may set
$ export LD_LIBRARY_PATH=$mesa/lib $ export EGL_DRIVERS_PATH=$mesa/lib/egl
to test a build without installation
EGL_DRIVER
-
This variable specifies a full path to or the name of an EGL driver. 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.
EGL_PLATFORM
-
This variable specifies the native platform. The valid values are the same as those for
--with-egl-platforms
. When the variable is not set, the main library uses the first platform listed in--with-egl-platforms
as the native platform.Extensions like
EGL_MESA_drm_display
define new functions to create displays for non-native platforms. These extensions are usually used by applications that support non-native platforms. Setting this variable is probably required only for some of the demos found in mesa/demo repository. EGL_LOG_LEVEL
-
This changes the log level of the main library and the drivers. The valid values are:
debug
,info
,warning
, andfatal
. EGL_SOFTWARE
-
For drivers that support both hardware and software rendering, setting this variable to true forces the use of software rendering.
EGL Drivers
egl_dri2
-
This driver supports both
x11
anddrm
platforms. It functions as a DRI driver loader. Forx11
support, it talks to the X server directly using (XCB-)DRI2 protocol.This driver can share DRI drivers with
libGL
.
Packaging
The ABI between the main library and its drivers are not stable. Nor is there a plan to stabilize it at the moment.
Developers
The sources of the main library and drivers can be found at
src/egl/
.
Lifetime of Display Resources
Contexts and surfaces are examples of display resources. They might live longer than the display that creates them.
In EGL, when a display is terminated through eglTerminate
, all
display resources should be destroyed. Similarly, when a thread is released
through eglReleaseThread
, all current display resources should be
released. Another way to destroy or release resources is through functions
such as eglDestroySurface
or eglMakeCurrent
.
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
eglIs<Resource>Bound
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.
The main library will mark destroyed current resources as unlinked. In a
driver's MakeCurrent
callback,
eglIs<Resource>Linked
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 MakeCurrent
might be called with an
uninitialized display.
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.
EGL_RENDER_BUFFER
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 EGL_RENDER_BUFFER
set to
EGL_SINGLE_BUFFER
; If the same context is later bound to a
surface with EGL_RENDER_BUFFER
set to
EGL_BACK_BUFFER
, 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.
For pbuffer surfaces, the render buffer is always
EGL_BACK_BUFFER
. And for pixmap surfaces, the render buffer is
always EGL_SINGLE_BUFFER
. Unlike window surfaces, EGL spec
requires their EGL_RENDER_BUFFER
values to be honored. As a
result, a driver should never set EGL_PIXMAP_BIT
or
EGL_PBUFFER_BIT
bits of a config if the contexts created with the
config won't be able to honor the EGL_RENDER_BUFFER
of pixmap or
pbuffer surfaces.
It should also be noted that pixmap and pbuffer surfaces are assumed to be
single-buffered, in that eglSwapBuffers
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.
There is no defined behavior as to, for example, how
glDrawBuffer
interacts with EGL_RENDER_BUFFER
. 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.
EGLDisplay
Mutex
The EGLDisplay
will be locked before calling any of the dispatch
functions (well, except for GetProcAddress which does not take an
EGLDisplay
). 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 EGLDisplay
without going through the EGL APIs, the driver
should as well lock the display before using it.