docs/egl: add some more documentation
Inspired by `src/egl/main/README.txt`, which was severely outdated, but still contained valid information. Signed-off-by: Eric Engestrom <eric@engestrom.ch> Reviewed-by: Frank Binns <frank.binns@imgtec.com> Reviewed-by: Emil Velikov <emil.velikov@collabora.com> Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6130>
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docs/egl.rst
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docs/egl.rst
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@ -112,6 +112,46 @@ Developers
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The sources of the main library and drivers can be found at
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``src/egl/``.
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The code basically consists of two things:
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1. An EGL API dispatcher. This directly routes all the ``eglFooBar()``
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API calls into driver-specific functions.
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2. Two EGL drivers (``dri2`` and ``haiku``), implementing the API
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functions handling the platforms' specifics.
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Two of API functions are optional (``eglQuerySurface()`` and
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``eglSwapInterval()``); the former provides fallback for all the
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platform-agnostic attributes (ie. everything except ``EGL_WIDTH``
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& ``EGL_HEIGHT``), and the latter just silently pretends the API call
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succeeded (as per EGL spec).
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A driver _could_ implement all the other EGL API functions, but several of
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them are only needed for extensions, like ``eglSwapBuffersWithDamageEXT()``.
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Bootstrapping
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~~~~~~~~~~~~~
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When the apps calls ``eglInitialize()``, the driver's ``Initialize()``
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function is called. If the first driver initialisation attempt fails,
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a second one is tried using only software components (this can be forced
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using the ``LIBGL_ALWAYS_SOFTWARE`` environment variable). Typically,
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this function takes care of setting up visual configs, creating EGL
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devices, etc.
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Teardown
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~~~~~~~~
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When ``eglTerminate()`` is called, the ``driver->Terminate()`` function
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is called. The driver should clean up after itself.
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Subclassing
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~~~~~~~~~~~
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The internal libEGL data structures such as ``_EGLDisplay``,
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``_EGLContext``, ``_EGLSurface``, etc. should be considered base classes
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from which drivers will derive subclasses.
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EGL Drivers
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-----------
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@ -1,69 +0,0 @@
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Notes about the EGL library:
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The EGL code here basically consists of two things:
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1. An EGL API dispatcher. This directly routes all the eglFooBar() API
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calls into driver-specific functions.
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2. Fallbacks for EGL API functions. A driver _could_ implement all the
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EGL API calls from scratch. But in many cases, the fallbacks provided
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in libEGL (such as eglChooseConfig()) will do the job.
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Bootstrapping:
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When the apps calls eglInitialize() a device driver is selected and loaded
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(look for _eglAddDrivers() and _eglLoadModule() in egldriver.c).
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The built-in driver's entry point function is then called and given
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a freshly allocated and initialised _EGLDriver, with default fallback
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entrypoints set.
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As part of initialization, the dispatch table in _EGLDriver->API must be
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populated with all the EGL entrypoints. Some functions like
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driver->Initialize and driver->Terminate _must_ be implemented
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with driver-specific code (no default/fallback function is possible).
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Shortly after, the driver->Initialize() function is executed. Any additional
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driver initialization that wasn't done in the driver entry point should be
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done at this point. Typically, this will involve setting up visual configs, etc.
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Special Functions:
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Certain EGL functions _must_ be implemented by the driver. This includes:
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eglCreateContext
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eglCreateWindowSurface
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eglCreatePixmapSurface
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eglCreatePBufferSurface
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eglMakeCurrent
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eglSwapBuffers
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Most of the EGLConfig-related functions can be implemented with the
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defaults/fallbacks. Same thing for the eglGet/Query functions.
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Teardown:
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When eglTerminate() is called, the driver->Terminate() function is
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called. The driver should clean up after itself. eglTerminate() will
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then close/unload the driver (shared library).
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Subclassing:
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The internal libEGL data structures such as _EGLDisplay, _EGLContext,
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_EGLSurface, etc should be considered base classes from which drivers
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will derive subclasses.
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