MESA_multithread_makecurrent.spec revision 49d7e48b33264d94e30af6129c281b6acafa9427 
1Name
2
3    MESA_multithread_makecurrent
4
5Name Strings
6
7    GLX_MESA_multithread_makecurrent
8
9Contact
10
11    Eric Anholt (eric@anholt.net)
12
13Status
14
15    Not shipping.
16
17Version
18
19    Last Modified Date:  21 February 2011
20
21Number
22
23    TBD
24
25Dependencies
26
27    OpenGL 1.0 or later is required.
28    GLX 1.3 or later is required.
29
30Overview
31
32    The GLX context setup encourages multithreaded applications to
33    create a context per thread which each operate on their own
34    objects in parallel, and leaves synchronization for write access
35    to shared objects up to the application.
36
37    For some applications, maintaining per-thread contexts and
38    ensuring that the glFlush happens in one thread before another
39    thread starts working on that object is difficult.  For them,
40    using the same context across multiple threads and protecting its
41    usage with a mutex is both higher performance and easier to
42    implement.  This extension gives those applications that option by
43    relaxing the context binding requirements.
44
45    This new behavior matches the requirements of AGL, while providing
46    a feature not specified in WGL.
47
48IP Status
49
50    Open-source; freely implementable.
51
52Issues
53
54    None.
55
56New Procedures and Functions
57
58    None.
59
60New Tokens
61
62    None.
63
64Changes to Chapter 3 of the GLX 1.3 Specification (Functions and Errors)
65
66    Remove the following sentence from section 3.3.7 Rendering Contexts:
67	If ctx is current to some other thread, then
68	glXMakeContextCurrent will generate a BadAccess error.
69
70    Remove the following sentence from section 3.5 Rendering Contexts:
71	If ctx is current to some other thread, then
72	glXMakeCurrent will generate a BadAccess error.
73
74GLX Protocol
75
76    None.  The GLX extension is client-side.
77
78Errors
79
80    None.
81
82New State
83
84    None.
85
86Issues
87
88    (1) What happens if the app binds a context/drawable in multiple
89	threads, then binds a different context/thread in one of them?
90
91    As with binding a new context from the current thread, the old
92    context's refcount is reduced and the new context's refcount is
93    increased.
94
95    (2) What happens if the app binds a context/drawable in multiple
96	threads, then binds None/None in one of them?
97
98    The GLX context is unreferenced from that thread, and the other
99    threads retain their GLX context binding.
100
101    (3) What happens if the app binds a context/drawable in 7 threads,
102	then destroys the context in one of them?
103
104    As with GLX context destruction previously, the XID is destroyed
105    but the context remains usable by threads that have the context
106    current.
107
108    (4) What happens if the app binds a new drawable/readable with
109        glXMakeCurrent() when it is already bound to another thread?
110
111    The context becomes bound to the new drawable/readable, and
112    further rendering in either thread will use the new
113    drawable/readable.
114
115    (5) What requirements should be placed on the user managing contexts
116        from multiple threads?
117
118    The intention is to allow multithreaded access to the GL at the
119    minimal performance cost, so requiring that the GL do general
120    synchronization (beyond that already required by context sharing)
121    is not an option, and synchronizing of GL's access to the GL
122    context between multiple threads is left to the application to do
123    across GL calls.  However, it would be unfortunate for a library
124    doing multithread_makecurrent to require that other libraries
125    share in synchronization for binding of their own contexts, so the
126    refcounting of the contexts is required to be threadsafe.
127
128Revision History
129
130    20 November 2009 Eric Anholt - initial specification
131    22 November 2009 Eric Anholt - added issues from Ian Romanick.
132    3 February 2011 Eric Anholt - updated with resolution to issues 1-3
133    3 February 2011 Eric Anholt - added issue 4, 5
134    21 February 2011 Eric Anholt - Include glXMakeCurrent() sentence
135    along with glXMakeContextCurrent() for removal.
136