EGL

The current version of EGL in Mesa implements EGL 1.4. More information about EGL can be found at https://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

1. Configure your build with the desired client APIs and enable the driver for your hardware. For example:

   $ meson configure \
           -D egl=enabled \
           -D gles1=enabled \
           -D gles2=enabled \
           -D dri-drivers=... \
           -D 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 Gallium drivers respectively.

2. 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

-D egl=enabled

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

-D platforms=...

List the platforms (window systems) to support. Its argument is a comma separated string such as -D platforms=x11,wayland. 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 available platforms are x11, wayland, android, and haiku. The android platform can either be built as a system component, part of AOSP, using Android.mk files, or cross-compiled using appropriate options. Unless for special needs, the build system should select the right platforms automatically.

-D gles1=enabled and -D gles2=enabled

These options enable OpenGL ES support in OpenGL. The result is one big internal library that supports multiple APIs.

-D shared-glapi=enabled

By default, libGL has its own copy of libglapi. This options makes libGL use the shared libglapi. 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_PLATFORM

This variable specifies the native platform. The valid values are the same as those for -D platforms=.... When the variable is not set, the main library uses the first platform listed in -D 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, and fatal.

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/.

The code basically consists of two things:

1. An EGL API dispatcher. This directly routes all the eglFooBar() API calls into driver-specific functions.

2. Two EGL drivers (dri2 and haiku), implementing the API functions handling the platforms’ specifics.

Two of API functions are optional (eglQuerySurface() and eglSwapInterval()); the former provides fallback for all the platform-agnostic attributes (i.e. everything except EGL_WIDTH & EGL_HEIGHT), and the latter just silently pretends the API call succeeded (as per EGL spec).

A driver _could_ implement all the other EGL API functions, but several of them are only needed for extensions, like eglSwapBuffersWithDamageEXT(). See src/egl/main/egldriver.h to see which driver hooks are only required by extensions.

Bootstrapping

When the apps calls eglInitialize(), the driver’s Initialize() function is called. If the first driver initialization attempt fails, a second one is tried using only software components (this can be forced using the LIBGL_ALWAYS_SOFTWARE environment variable). Typically, this function takes care of setting up visual configs, creating EGL devices, etc.

Teardown

When eglTerminate() is called, the driver->Terminate() function is called. The driver should clean up after itself.

Subclassing

The internal libEGL data structures such as _EGLDisplay, _EGLContext, _EGLSurface, etc. should be considered base classes from which drivers will derive subclasses.

EGL Drivers

egl_dri2

This driver supports several platforms: android, device, drm, surfaceless, wayland and x11. It functions as a DRI driver loader. For x11 support, it talks to the X server directly using (XCB-)DRI3 protocol when available, and falls back to DRI2 if necessary (can be forced with LIBGL_DRI3_DISABLE).

This driver can share DRI drivers with libGL.

haiku

This driver supports only the Haiku platform. It is also much less feature-complete than egl_dri2, supporting only part of EGL 1.4 and none of the extensions beyond it.

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 same 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.