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

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+Name
+
+    MESA_multithread_makecurrent
+
+Name Strings
+
+    GLX_MESA_multithread_makecurrent
+
+Contact
+
+    Eric Anholt (eric@anholt.net)
+
+Status
+
+    Not shipping.
+
+Version
+
+    Last Modified Date:  21 February 2011
+
+Number
+
+    TBD
+
+Dependencies
+
+    OpenGL 1.0 or later is required.
+    GLX 1.3 or later is required.
+
+Overview
+
+    The GLX context setup encourages multithreaded applications to
+    create a context per thread which each operate on their own
+    objects in parallel, and leaves synchronization for write access
+    to shared objects up to the application.
+
+    For some applications, maintaining per-thread contexts and
+    ensuring that the glFlush happens in one thread before another
+    thread starts working on that object is difficult.  For them,
+    using the same context across multiple threads and protecting its
+    usage with a mutex is both higher performance and easier to
+    implement.  This extension gives those applications that option by
+    relaxing the context binding requirements.
+
+    This new behavior matches the requirements of AGL, while providing
+    a feature not specified in WGL.
+
+IP Status
+
+    Open-source; freely implementable.
+
+Issues
+
+    None.
+
+New Procedures and Functions
+
+    None.
+
+New Tokens
+
+    None.
+
+Changes to Chapter 2 of the GLX 1.3 Specification (Functions and Errors)
+
+    Replace the following sentence from section 2.2 Rendering Contexts:
+	In addition, a rendering context can be current for only one
+	thread at a time.
+    with:
+	In addition, an indirect rendering context can be current for
+	only one thread at a time.  A direct rendering context may be
+	current to multiple threads, with synchronization of access to
+	the context thruogh the GL managed by the application through
+	mutexes.
+
+Changes to Chapter 3 of the GLX 1.3 Specification (Functions and Errors)
+
+    Replace the following sentence from section 3.3.7 Rendering Contexts:
+	If ctx is current to some other thread, then
+	glXMakeContextCurrent will generate a BadAccess error.
+    with:
+	If ctx is an indirect context current to some other thread,
+	then glXMakeContextCurrent will generate a BadAccess error.
+
+    Replace the following sentence from section 3.5 Rendering Contexts:
+	If ctx is current to some other thread, then
+	glXMakeCurrent will generate a BadAccess error.
+    with:
+	If ctx is an indirect context current to some other thread,
+	then glXMakeCurrent will generate a BadAccess error.
+
+GLX Protocol
+
+    None.  The GLX extension only extends to direct rendering contexts.
+
+Errors
+
+    None.
+
+New State
+
+    None.
+
+Issues
+
+    (1) What happens if the app binds a context/drawable in multiple
+	threads, then binds a different context/thread in one of them?
+
+    As with binding a new context from the current thread, the old
+    context's refcount is reduced and the new context's refcount is
+    increased.
+
+    (2) What happens if the app binds a context/drawable in multiple
+	threads, then binds None/None in one of them?
+
+    The GLX context is unreferenced from that thread, and the other
+    threads retain their GLX context binding.
+
+    (3) What happens if the app binds a context/drawable in 7 threads,
+	then destroys the context in one of them?
+
+    As with GLX context destruction previously, the XID is destroyed
+    but the context remains usable by threads that have the context
+    current.
+
+    (4) What happens if the app binds a new drawable/readable with
+        glXMakeCurrent() when it is already bound to another thread?
+
+    The context becomes bound to the new drawable/readable, and
+    further rendering in either thread will use the new
+    drawable/readable.
+
+    (5) What requirements should be placed on the user managing contexts
+        from multiple threads?
+
+    The intention is to allow multithreaded access to the GL at the
+    minimal performance cost, so requiring that the GL do general
+    synchronization (beyond that already required by context sharing)
+    is not an option, and synchronizing of GL's access to the GL
+    context between multiple threads is left to the application to do
+    across GL calls.  However, it would be unfortunate for a library
+    doing multithread_makecurrent to require that other libraries
+    share in synchronization for binding of their own contexts, so the
+    refcounting of the contexts is required to be threadsafe.
+
+    (6) Does this apply to indirect contexts?
+
+    This was ignored in the initial revision of the spec.  Behavior
+    for indirect contexts is left as-is.
+
+Revision History
+
+    20 November 2009 Eric Anholt - initial specification
+    22 November 2009 Eric Anholt - added issues from Ian Romanick.
+    3 February 2011 Eric Anholt - updated with resolution to issues 1-3
+    3 February 2011 Eric Anholt - added issue 4, 5
+    21 February 2011 Eric Anholt - Include glXMakeCurrent() sentence
+    along with glXMakeContextCurrent() for removal.