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