docs/egl.rst

   1 EGL
   2 ===
   3
   4 The current version of EGL in Mesa implements EGL 1.4. More information
   5 about EGL can be found at https://www.khronos.org/egl/.
   6
   7 The Mesa's implementation of EGL uses a driver architecture. The main
   8 library (``libEGL``) is window system neutral. It provides the EGL API
   9 entry points and helper functions for use by the drivers. Drivers are
  10 dynamically loaded by the main library and most of the EGL API calls are
  11 directly dispatched to the drivers.
  12
  13 The driver in use decides the window system to support.
  14
  15 Build EGL
  16 ---------
  17
  18 #. Configure your build with the desired client APIs and enable the
  19    driver for your hardware. For example:
  20
  21    .. code-block:: console
  22
  23       $ meson configure \
  24               -D egl=true \
  25               -D gles1=true \
  26               -D gles2=true \
  27               -D dri-drivers=... \
  28               -D gallium-drivers=...
  29
  30    The main library and OpenGL is enabled by default. The first two
  31    options above enables :doc:`OpenGL ES 1.x and 2.x <opengles>`. The
  32    last two options enables the listed classic and Gallium drivers
  33    respectively.
  34
  35 #. Build and install Mesa as usual.
  36
  37 In the given example, it will build and install ``libEGL``, ``libGL``,
  38 ``libGLESv1_CM``, ``libGLESv2``, and one or more EGL drivers.
  39
  40 Configure Options
  41 ~~~~~~~~~~~~~~~~~
  42
  43 There are several options that control the build of EGL at configuration
  44 time
  45
  46 ``-D egl=true``
  47    By default, EGL is enabled. When disabled, the main library and the
  48    drivers will not be built.
  49
  50 ``-D platforms=...``
  51    List the platforms (window systems) to support. Its argument is a
  52    comma separated string such as ``-D platforms=x11,wayland``. It decides
  53    the platforms a driver may support. The first listed platform is also
  54    used by the main library to decide the native platform.
  55
  56    The available platforms are ``x11``, ``wayland``,
  57    ``android``, and ``haiku``. The ``android`` platform
  58    can either be built as a system component, part of AOSP, using
  59    ``Android.mk`` files, or cross-compiled using appropriate options.
  60    Unless for special needs, the build system should select the right
  61    platforms automatically.
  62
  63 ``-D gles1=true`` and ``-D gles2=true``
  64    These options enable OpenGL ES support in OpenGL. The result is one
  65    big internal library that supports multiple APIs.
  66
  67 ``-D shared-glapi=true``
  68    By default, ``libGL`` has its own copy of ``libglapi``. This options
  69    makes ``libGL`` use the shared ``libglapi``. This is required if
  70    applications mix OpenGL and OpenGL ES.
  71
  72 Use EGL
  73 -------
  74
  75 Demos
  76 ~~~~~
  77
  78 There are demos for the client APIs supported by EGL. They can be found
  79 in mesa/demos repository.
  80
  81 Environment Variables
  82 ~~~~~~~~~~~~~~~~~~~~~
  83
  84 There are several environment variables that control the behavior of EGL
  85 at runtime
  86
  87 ``EGL_PLATFORM``
  88    This variable specifies the native platform. The valid values are the
  89    same as those for ``-D platforms=...``. When the variable is not set,
  90    the main library uses the first platform listed in
  91    ``-D platforms=...`` as the native platform.
  92
  93    Extensions like ``EGL_MESA_drm_display`` define new functions to
  94    create displays for non-native platforms. These extensions are
  95    usually used by applications that support non-native platforms.
  96    Setting this variable is probably required only for some of the demos
  97    found in mesa/demo repository.
  98
  99 ``EGL_LOG_LEVEL``
 100    This changes the log level of the main library and the drivers. The
 101    valid values are: ``debug``, ``info``, ``warning``, and ``fatal``.
 102
 103 Packaging
 104 ---------
 105
 106 The ABI between the main library and its drivers are not stable. Nor is
 107 there a plan to stabilize it at the moment.
 108
 109 Developers
 110 ----------
 111
 112 The sources of the main library and drivers can be found at
 113 ``src/egl/``.
 114
 115 The code basically consists of two things:
 116
 117 1. An EGL API dispatcher. This directly routes all the ``eglFooBar()``
 118    API calls into driver-specific functions.
 119
 120 2. Two EGL drivers (``dri2`` and ``haiku``), implementing the API
 121    functions handling the platforms' specifics.
 122
 123 Two of API functions are optional (``eglQuerySurface()`` and
 124 ``eglSwapInterval()``); the former provides fallback for all the
 125 platform-agnostic attributes (ie. everything except ``EGL_WIDTH``
 126 & ``EGL_HEIGHT``), and the latter just silently pretends the API call
 127 succeeded (as per EGL spec).
 128
 129 A driver _could_ implement all the other EGL API functions, but several of
 130 them are only needed for extensions, like ``eglSwapBuffersWithDamageEXT()``.
 131 See ``src/egl/main/egldriver.h`` to see which driver hooks are only
 132 required by extensions.
 133
 134 Bootstrapping
 135 ~~~~~~~~~~~~~
 136
 137 When the apps calls ``eglInitialize()``, the driver's ``Initialize()``
 138 function is called. If the first driver initialisation attempt fails,
 139 a second one is tried using only software components (this can be forced
 140 using the ``LIBGL_ALWAYS_SOFTWARE`` environment variable). Typically,
 141 this function takes care of setting up visual configs, creating EGL
 142 devices, etc.
 143
 144 Teardown
 145 ~~~~~~~~
 146
 147 When ``eglTerminate()`` is called, the ``driver->Terminate()`` function
 148 is called. The driver should clean up after itself.
 149
 150 Subclassing
 151 ~~~~~~~~~~~
 152
 153 The internal libEGL data structures such as ``_EGLDisplay``,
 154 ``_EGLContext``, ``_EGLSurface``, etc. should be considered base classes
 155 from which drivers will derive subclasses.
 156
 157 EGL Drivers
 158 -----------
 159
 160 ``egl_dri2``
 161    This driver supports several platforms: ``android``, ``device``,
 162    ``drm, ``surfaceless``, ``wayland`` and ``x11``. It functions as
 163    a DRI driver loader. For ``x11`` support, it talks to the X server
 164    directly using (XCB-)DRI3 protocol when available, and falls back to
 165    DRI2 if necessary (can be forced with ``LIBGL_DRI3_DISABLE``).
 166
 167    This driver can share DRI drivers with ``libGL``.
 168
 169 ``haiku``
 170    This driver supports only the `Haiku <https://haiku-os.org>`__
 171    platform. It is also much less feature-complete than ``egl_dri2``,
 172    supporting only part of EGL 1.4 and none of the extensions beyond it.
 173
 174 Lifetime of Display Resources
 175 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 176
 177 Contexts and surfaces are examples of display resources. They might live
 178 longer than the display that creates them.
 179
 180 In EGL, when a display is terminated through ``eglTerminate``, all
 181 display resources should be destroyed. Similarly, when a thread is
 182 released through ``eglReleaseThread``, all current display resources
 183 should be released. Another way to destroy or release resources is
 184 through functions such as ``eglDestroySurface`` or ``eglMakeCurrent``.
 185
 186 When a resource that is current to some thread is destroyed, the
 187 resource should not be destroyed immediately. EGL requires the resource
 188 to live until it is no longer current. A driver usually calls
 189 ``eglIs<Resource>Bound`` to check if a resource is bound (current) to
 190 any thread in the destroy callbacks. If it is still bound, the resource
 191 is not destroyed.
 192
 193 The main library will mark destroyed current resources as unlinked. In a
 194 driver's ``MakeCurrent`` callback, ``eglIs<Resource>Linked`` can then be
 195 called to check if a newly released resource is linked to a display. If
 196 it is not, the last reference to the resource is removed and the driver
 197 should destroy the resource. But it should be careful here because
 198 ``MakeCurrent`` might be called with an uninitialized display.
 199
 200 This is the only mechanism provided by the main library to help manage
 201 the resources. The drivers are responsible to the correct behavior as
 202 defined by EGL.
 203
 204 ``EGL_RENDER_BUFFER``
 205 ~~~~~~~~~~~~~~~~~~~~~
 206
 207 In EGL, the color buffer a context should try to render to is decided by
 208 the binding surface. It should try to render to the front buffer if the
 209 binding surface has ``EGL_RENDER_BUFFER`` set to ``EGL_SINGLE_BUFFER``;
 210 If the same context is later bound to a surface with
 211 ``EGL_RENDER_BUFFER`` set to ``EGL_BACK_BUFFER``, the context should try
 212 to render to the back buffer. However, the context is allowed to make
 213 the final decision as to which color buffer it wants to or is able to
 214 render to.
 215
 216 For pbuffer surfaces, the render buffer is always ``EGL_BACK_BUFFER``.
 217 And for pixmap surfaces, the render buffer is always
 218 ``EGL_SINGLE_BUFFER``. Unlike window surfaces, EGL spec requires their
 219 ``EGL_RENDER_BUFFER`` values to be honored. As a result, a driver should
 220 never set ``EGL_PIXMAP_BIT`` or ``EGL_PBUFFER_BIT`` bits of a config if
 221 the contexts created with the config won't be able to honor the
 222 ``EGL_RENDER_BUFFER`` of pixmap or pbuffer surfaces.
 223
 224 It should also be noted that pixmap and pbuffer surfaces are assumed to
 225 be single-buffered, in that ``eglSwapBuffers`` has no effect on them. It
 226 is desirable that a driver allocates a private color buffer for each
 227 pbuffer surface created. If the window system the driver supports has
 228 native pbuffers, or if the native pixmaps have more than one color
 229 buffers, the driver should carefully attach the native color buffers to
 230 the EGL surfaces, re-route them if required.
 231
 232 There is no defined behavior as to, for example, how ``glDrawBuffer``
 233 interacts with ``EGL_RENDER_BUFFER``. Right now, it is desired that the
 234 draw buffer in a client API be fixed for pixmap and pbuffer surfaces.
 235 Therefore, the driver is responsible to guarantee that the client API
 236 renders to the specified render buffer for pixmap and pbuffer surfaces.
 237
 238 ``EGLDisplay`` Mutex
 239 ~~~~~~~~~~~~~~~~~~~~
 240
 241 The ``EGLDisplay`` will be locked before calling any of the dispatch
 242 functions (well, except for GetProcAddress which does not take an
 243 ``EGLDisplay``). This guarantees that the same dispatch function will
 244 not be called with the same display at the same time. If a driver has
 245 access to an ``EGLDisplay`` without going through the EGL APIs, the
 246 driver should as well lock the display before using it.