Mesa EGL

# The Mesa 3D Graphics Library

# Mesa 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. 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 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

`--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-platforms`

  List the platforms (window systems) to support. Its argument is a comma separated string such as `--with-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 available platforms are `x11`, `drm`, `wayland`, `surfaceless`, `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 `configure` options. The `haiku` 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 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_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-platforms`. When the variable is not set, the main library uses the first platform listed in `--with-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`.

## EGL Drivers

`egl_dri2`

  This driver supports both `x11` and `drm` platforms. It functions as a DRI driver loader. For `x11` 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.