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lib/mesa/docs/_extra/specs/MESA_configless_context.spec

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+Name
+
+    MESA_configless_context
+
+Name Strings
+
+    EGL_MESA_configless_context
+
+Contact
+
+    Neil Roberts <neil.s.roberts@intel.com>
+
+Status
+
+    Superseded by the functionally identical EGL_KHR_no_config_context
+    extension.
+
+Version
+
+    Version 2, September 9, 2016
+
+Number
+
+    EGL Extension #not assigned
+
+Dependencies
+
+    Requires EGL 1.4 or later.  This extension is written against the
+    wording of the EGL 1.4 specification.
+
+Overview
+
+    This extension provides a means to use a single context to render to
+    multiple surfaces which have different EGLConfigs. Without this extension
+    the EGLConfig for every surface used by the context must be compatible
+    with the one used by the context. The only way to render to surfaces with
+    different formats would be to create multiple contexts but this is
+    inefficient with modern GPUs where this restriction is unnecessary.
+
+IP Status
+
+    Open-source; freely implementable.
+
+New Procedures and Functions
+
+    None.
+
+New Tokens
+
+    Accepted as <config> in eglCreateContext
+
+        EGL_NO_CONFIG_MESA                  ((EGLConfig)0)
+
+Additions to the EGL Specification section "2.2 Rendering Contexts and Drawing
+Surfaces"
+
+    Add the following to the 3rd paragraph:
+
+   "EGLContexts can also optionally be created with respect to an EGLConfig
+    depending on the parameters used at creation time. If a config is provided
+    then additional restrictions apply on what surfaces can be used with the
+    context."
+
+    Replace the last sentence of the 6th paragraph with:
+
+   "In order for a context to be compatible with a surface they both must have
+    been created with respect to the same EGLDisplay. If the context was
+    created without respect to an EGLConfig then there are no further
+    constraints. Otherwise they are only compatible if:"
+
+    Remove the last bullet point in the list of constraints.
+
+Additions to the EGL Specification section "3.7.1 Creating Rendering Contexts"
+
+    Replace the paragraph starting "If config is not a valid EGLConfig..."
+    with
+
+   "The config argument can either be a valid EGLConfig or EGL_NO_CONFIG_MESA.
+    If it is neither of these then an EGL_BAD_CONFIG error is generated. If a
+    valid config is passed then the error will also be generated if the config
+    does not support the requested client API (this includes requesting
+    creation of an OpenGL ES 1.x context when the EGL_RENDERABLE_TYPE
+    attribute of config does not contain EGL_OPENGL_ES_BIT, or creation of an
+    OpenGL ES 2.x context when the attribute does not contain
+    EGL_OPENGL_ES2_BIT).
+
+    Passing EGL_NO_CONFIG_MESA will create a configless context. When a
+    configless context is used with the OpenGL API it can be assumed that the
+    initial values of the context's state will be decided when the context is
+    first made current. In particular this means that the decision of whether
+    to use GL_BACK or GL_FRONT for the initial value of the first output in
+    glDrawBuffers will be decided based on the config of the draw surface when
+    it is first bound."
+
+Additions to the EGL Specification section "3.7.3 Binding Contexts and
+Drawables"
+
+    Replace the first bullet point with the following:
+
+   "* If draw or read are not compatible with ctx as described in section 2.2,
+      then an EGL_BAD_MATCH error is generated."
+
+    Add a second bullet point after that:
+
+   "* If draw and read are not compatible with each other as described in
+      section 2.2, then an EGL_BAD_MATCH error is generated."
+
+Issues
+
+    1.  What happens when an OpenGL context with a double-buffered surface and
+        draw buffer set to GL_BACK is made current with a single-buffered
+        surface?
+
+        NOT RESOLVED: There are a few options here.  An implementation can
+        raise an error, change the drawbuffer state to GL_FRONT or just do
+        nothing, expecting the application to set GL_FRONT drawbuffer before
+        drawing.  However, this extension deliberately does not specify any
+        required behavior in this corner case and applications should avoid
+        mixing single- and double-buffered surfaces with configless contexts.
+
+        Future extensions may specify required behavior in this case.
+
+Revision History
+
+    Version 2, September 9, 2016
+        Defer to EGL_KHR_no_config_context (Adam Jackson)
+
+    Version 1, February 28, 2014
+        Initial draft (Neil Roberts)