Name KHR_reusable_sync Name Strings EGL_KHR_reusable_sync Contributors Acorn Pooley Gary King Gregory Prisament Jon Leech Robert Palmer Contacts Acorn Pooley, NVIDIA Corporation (apooley 'at' nvidia.com) Gary King, NVIDIA Corporation (gking 'at' nvidia.com) Gregory Prisament, NVIDIA Corporation (gprisament 'at' nvidia.com) Jon Leech (jon 'at' alumni.caltech.edu) Robert Palmer (robert.palmer 'at' nokia.com) Notice Copyright (c) 2006-2013 The Khronos Group Inc. Copyright terms at http://www.khronos.org/registry/speccopyright.html Status Complete. Approved by the Khronos Board of Promoters on August 28, 2009. Version Version 22, January 31, 2014 Number EGL Extension #6 Dependencies Requires EGL 1.1 This extension is written against the wording of the EGL 1.2 Specification. Overview This extension introduces the concept of "sync objects" into EGL. Sync objects are a synchronization primitive, representing events whose completion can be tested or waited upon. This extension borrows from the GL_ARB_sync extension but introduces a type of sync object known as "reusable sync object" comparable to an OS semaphore. The specification is designed to allow additional types of sync objects to be easily introduced in later extensions. Reusable sync objects may be used to synchronize activity between threads or between client APIs. Synchronization is accomplished by explicitly changing the status of a reusable object using EGL API commands. New Types /* * EGLSyncKHR is an opaque handle to an EGL sync object */ typedef void* EGLSyncKHR; /* * EGLTimeKHR is a 64-bit unsigned integer representing intervals * in nanoseconds. */ #include typedef khronos_utime_nanoseconds_t EGLTimeKHR; New Procedures and Functions EGLSyncKHR eglCreateSyncKHR( EGLDisplay dpy, EGLenum type, const EGLint *attrib_list); EGLBoolean eglDestroySyncKHR( EGLDisplay dpy, EGLSyncKHR sync); EGLint eglClientWaitSyncKHR( EGLDisplay dpy, EGLSyncKHR sync, EGLint flags, EGLTimeKHR timeout); EGLBoolean eglSignalSyncKHR( EGLDisplay dpy, EGLSyncKHR sync, EGLenum mode); EGLBoolean eglGetSyncAttribKHR( EGLDisplay dpy, EGLSyncKHR sync, EGLint attribute, EGLint *value); New Tokens Accepted by the parameter of eglCreateSyncKHR, and returned in when eglGetSyncAttribKHR is called with EGL_SYNC_TYPE_KHR: EGL_SYNC_REUSABLE_KHR 0x30FA Accepted by the parameter of eglGetSyncAttribKHR: EGL_SYNC_TYPE_KHR 0x30F7 EGL_SYNC_STATUS_KHR 0x30F1 Accepted by the parameter of eglSignalSyncKHR and returned in when eglGetSyncAttribKHR is called with EGL_SYNC_STATUS_KHR: EGL_SIGNALED_KHR 0x30F2 EGL_UNSIGNALED_KHR 0x30F3 Accepted in the parameter of eglClientWaitSyncKHR: EGL_SYNC_FLUSH_COMMANDS_BIT_KHR 0x0001 Accepted in the parameter of eglClientWaitSyncKHR: EGL_FOREVER_KHR 0xFFFFFFFFFFFFFFFFull Returned by eglClientWaitSyncKHR: EGL_TIMEOUT_EXPIRED_KHR 0x30F5 EGL_CONDITION_SATISFIED_KHR 0x30F6 Returned by eglCreateSyncKHR in the event of an error: EGL_NO_SYNC_KHR ((EGLSyncKHR)0) Changes to Chapter 3 of the EGL 1.2 Specification (EGL Functions and Errors) Add a new subsection at the end of Section 3.8, page 43 (Synchronization Primitives) "3.8.1 Sync Objects In addition to the aforementioned synchronization functions, which provide an efficient means of serializing client and native API operations within a thread, are provided to enable synchronization of client API operations between threads and/or between API contexts. Sync objects may be tested or waited upon by application threads. Sync objects have a status with two possible states: and . Initially, sync objects are unsignaled. EGL may be asked to wait for a sync object to become signaled, or a sync object's status may be queried. Depending on the type of a sync object, its status may be changed either by an external event, or by explicitly signaling and unsignaling the sync. Sync objects are associated with an EGLDisplay when they are created, and have defining additional aspects of the sync object. All sync objects include attributes for their type and their status. Additional attributes are discussed below for different types of sync objects. are created in the unsignaled state, and may be signaled and/or unsignaled repeatedly. Every transition of a reusable sync object's status from unsignaled to signaled will release any threads waiting on that sync object. The command EGLSyncKHR eglCreateSyncKHR( EGLDisplay dpy, EGLenum type, const EGLint *attrib_list); creates a sync object of the specified associated with the specified display , and returns a handle to the new object. is an attribute-value list specifying other attributes of the sync object, terminated by an attribute entry EGL_NONE. Attributes not specified in the list will be assigned their default values. If is EGL_SYNC_REUSABLE_KHR, a reusable sync object is created. In this case must be NULL or empty (containing only EGL_NONE). Attributes of the reusable sync object are set as follows: Attribute Name Initial Attribute Value(s) --------------- -------------------------- EGL_SYNC_TYPE_KHR EGL_SYNC_REUSABLE_KHR EGL_SYNC_STATUS_KHR EGL_UNSIGNALED_KHR Errors ------ * If is not the name of a valid, initialized EGLDisplay, EGL_NO_SYNC_KHR is returned and an EGL_BAD_DISPLAY error is generated. * If is neither NULL nor empty (containing only EGL_NONE), EGL_NO_SYNC_KHR is returned and an EGL_BAD_ATTRIBUTE error is generated. * If is not a supported type of sync object, EGL_NO_SYNC_KHR is returned and an EGL_BAD_ATTRIBUTE error is generated. The command EGLint eglClientWaitSyncKHR( EGLDisplay dpy, EGLSyncKHR sync, EGLint flags, EGLTimeKHR timeout); blocks the calling thread until the specified sync object is signaled, or until nanoseconds have passed. More than one eglClientWaitSyncKHR may be outstanding on the same at any given time. When there are multiple threads blocked on the same and the sync object is signaled, all such threads are released, but the order in which they are released is not defined. If the value of is zero, then eglClientWaitSyncKHR simply tests the current status of . If the value of is the special value EGL_FOREVER_KHR, then eglClientWaitSyncKHR does not time out. For all other values, is adjusted to the closest value allowed by the implementation-dependent timeout accuracy, which may be substantially longer than one nanosecond. eglClientWaitSyncKHR returns one of three status values describing the reason for returning. A return value of EGL_TIMEOUT_EXPIRED_KHR indicates that the specified timeout period expired before was signaled, or if is zero, indicates that is not signaled. A return value of EGL_CONDITION_SATISFIED_KHR indicates that was signaled before the timeout expired, which includes the case when was already signaled when eglClientWaitSyncKHR was called. If an error occurs then an error is generated and EGL_FALSE is returned. If the EGL_SYNC_FLUSH_COMMANDS_BIT_KHR bit is set in , and is unsignaled when eglClientWaitSyncKHR is called, then the equivalent of Flush() will be performed for the current API context (i.e., the context returned by eglGetCurrentContext()) before blocking on . If no context is current for the bound API, the EGL_SYNC_FLUSH_COMMANDS_BIT_KHR bit is ignored. If a sync object is destroyed while an eglClientWaitSyncKHR is blocking on that object, eglClientWaitSyncKHR will unblock and return immediately, just as if the sync object had been signaled prior to being destroyed. Errors ------ * If is not a valid sync object for , EGL_FALSE is returned and an EGL_BAD_PARAMETER error is generated. * If does not match the EGLDisplay passed to eglCreateSyncKHR when was created, the behaviour is undefined. The command EGLBoolean eglSignalSyncKHR( EGLDisplay dpy, EGLSyncKHR sync, EGLenum mode); signals or unsignals the reusable sync object by changing its status to , which must be one of the values in table 3.bb. If as a result of calling eglSignalSyncKHR the status of transitions from unsignaled to signaled, any eglClientWaitSyncKHR commands blocking on will unblock. Assuming no errors are generated, EGL_TRUE is returned. Mode Effect ------------------ ------------- EGL_SIGNALED_KHR Set the status of to signaled EGL_UNSIGNALED_KHR Set the status of to unsignaled Table 3.bb Modes Accepted by eglSignalSyncKHR Command Errors ------ * If is not a valid sync object for , EGL_FALSE is returned and an EGL_BAD_PARAMETER error is generated. * If the type of is not EGL_SYNC_REUSABLE_KHR, EGL_FALSE is returned and an EGL_BAD_MATCH error is generated. * If does not match the EGLDisplay passed to eglCreateSyncKHR when was created, the behaviour is undefined. The command EGLBoolean eglGetSyncAttribKHR( EGLDisplay dpy, EGLSyncKHR sync, EGLint attribute, EGLint *value); is used to query attributes of the sync object . Legal values for depend on the type of sync object, as shown in table 3.cc. Assuming no errors are generated, EGL_TRUE is returned and the value of the queried attribute is returned in . Attribute Description Supported Sync Objects ----------------- ----------------------- ---------------------- EGL_SYNC_TYPE_KHR Type of the sync object All EGL_SYNC_STATUS_KHR Status of the sync object All Table 3.cc Attributes Accepted by eglGetSyncAttribKHR Command Errors ------ * If is not a valid sync object for , EGL_FALSE is returned and an EGL_BAD_PARAMETER error is generated. * If does not match the display passed to eglCreateSyncKHR when was created, the behaviour is undefined. * If is not one of the attributes in table 3.cc, EGL_FALSE is returned and an EGL_BAD_ATTRIBUTE error is generated. * If is not supported for the type of sync object passed in , EGL_FALSE is returned and an EGL_BAD_MATCH error is generated. If any error occurs, <*value> is not modified. The command EGLBoolean eglDestroySyncKHR( EGLDisplay dpy, EGLSyncKHR sync); is used to destroy an existing sync object. If any eglClientWaitSyncKHR commands are blocking on when eglDestroySyncKHR is called, they will be woken up, as if were signaled. If no errors are generated, EGL_TRUE is returned, and will no longer be the handle of a valid sync object. Errors ------ * If is not a valid sync object for , EGL_FALSE is returned and an EGL_BAD_PARAMETER error is generated. * If does not match the display passed to eglCreateSyncKHR when was created, the behaviour is undefined." Issues Note about the Issues --------------------- The wording for this extension was originally written as a single extension defining two types of sync object; a "reusable sync object" and a "fence sync object". That extension was split to produce standalone extensions for each type of sync object, and references to the other type removed from the specification language. This issues list has been simplied to remove references to fence sync objects but is otherwise very similar to the EGL_KHR_fence_sync extension issues list. 1. Explain the key choices made in this extension. RESPONSE: This extension has been written to enable adoption to be as wide as possible, and to behave as similarly as possible to synchronization primitives available in desktop OpenGL (e.g., NV_fence, ARB_sync). In the interest of enabling widespread adoption, this extension (following the ARB_sync model) has foregone the inclusion of synchronization primitives and synchronization tests which may be performed entirely inside client API command streams, instead performing synchronization tests (eglClientWaitSyncKHR) inside the application & host CPU. In the interest of maintaining similarity with previous synchronization primitives, this extension attempts to copy the ARB_sync specification wherever possible (both functionally and stylistically), only making changes where needed to operate inside EGL (rather than a client API context) and match EGL naming conventions. 2. Why place this behavior in EGL, rather than in the client APIs? RESPONSE: Ultimately, synchronization between multiple asynchronous client API contexts (potentially executing in different threads) is a problem which affects or will affect all EGL client APIs. Rather than creating separate synchronization primitives in each of the client APIs (and then wrapping them in an EGL container), in the interest of developer simplicity & consistency this behavior is being placed inside EGL. 3. What does this extension provide that can not be accomplished with the existing, more efficient eglWaitClient and eglWaitNative API functions? RESPONSE: eglWaitClient and eglWaitNative may be implemented in extremely lightweight manners, in some cases not blocking the calling thread at all; however, they can not be used to synchronize between client API contexts and native APIs executing in separate threads (or simply between client API contexts executing in separate threads), such as between a thread with an active OpenGL context and a second thread performing video decode. 4. [REMOVED - found in the fence_sync extension.] 5. Should integration with native platform synchronization objects be included in this extension, or reserved for future (platform-specific) extensions? RESOLVED: Integration with native platform synchronization objects should not be part of this extension, but can be added as future layered extensions if needed. These layered extensions can be platform-specific, or perhaps OpenKODE based. Originally, this extension included the ability to create native platform synchronization objects from EGLSync objects. This feature was removed for a few reasons: i) The proposed mechanism suggested mapping EGLSync objects to pthread conditional variables on platforms with pthread support. However, pthread conditional variables require an associated mutex and there was no mechanism to relay this associated mutex to the application. ii) On certain platforms support for converting to native platform synchronization objects adds great complexity to the implementation. iii) Now that OpenKODE is more mature, it would be better to allow conversion from EGLSyncKHR objects to OpenKODE synchronization primitives rather than platform-specific ones. We suggest that this functionality, if needed, be added as a layered extension instead of being included here. This way, EGL_KHR_sync remains minimal and easy to implement on a variety of platforms. 6. Please provide a more detailed description of how eglClientWaitSyncKHR behaves. RESOLVED: eglClientWaitSyncKHR blocks until the status of the sync object transitions to the signaled state. Sync object status is either signaled or unsignaled. More detailed rules describing signalling follow (these may need to be imbedded into the actual spec language): * A reusable sync object has two possible status values: signaled or unsignaled. * When created, the status of the sync object is unsignaled by default. * A reusable sync can be set to signaled or unsignaled status using eglSignalSyncKHR. * A wait function called on a sync object in the unsignaled state will block. It unblocks (note, not "returns to the application") when the sync object transitions to the signaled state. * A wait function called on a sync object in the signaled state will return immediately. 7. Should the 'flags' argument to eglClientWaitSyncKHR be EGLint or EGLuint? RESOLVED: EGLint, setting a precedent for explicit bitmask types in EGL going forward. We don't have an EGLuint type and it is overkill for this purposes when other bitmasks (surface type and api type) are already using EGLint attribute fields. 8. Can multiple WaitSyncs be placed on the same sync object? RESOLVED: Yes. This has been allowed all along but we now state it more clearly in the spec language. However, there is some concern that this is hard to implement and of limited use, and we might remove this capability before approving the extension. One way to do this while allowing multiple waiters at some future point is to expose it through the API to developers as either a sync attribute allowing multiple waits (default not allowing it), or a parameter to WaitSync, which initially must be something like EGL_SINGLE_WAIT_ONLY. 9. Should eglDestroySyncKHR release all WaitSyncs placed on a reusable sync object? RESOLVED: Yes. It is safest to release all threads waiting on a reusable object when the sync object is deleted so that waiting threads do not wait forever. Revision History #22 (Jon Leech, January 31, 2014) - Clarify return value of ClientWaitSyncKHR when called with of zero for an unsignaled (Bug 11576). #21 (Jon Leech, April 23, 2013) - Simplify issues list to remove issues specific to fence sync objects. #20 (Jon Leech, September 8, 2009) - Change status to complete and note approval by the Promoters. Minor formatting changes. #19 (Robert Palmer, July 14, 2009) - Branch wording from draft KHR_sync specification. Remove ability to create "fence sync objects and all tokens/wording specific to them. #18 (Robert Palmer, July 8, 2009) - Issues 8 and 9 declared resolved in EGL meeting 2009-07-08 #17 (Robert Palmer, July 8, 2009) - Update eglDestroySyncKHR to special-case deletion of fence sync objects. This is explained in issue 9. - Corrected EGL_REUSABLE_SYNC_KHR -> EGL_SYNC_REUSABLE_KHR - Define value for EGL_SYNC_REUSABLE_KHR - Fix typo and whitespace #16 (Jon Leech, July 7, 2009) - Update description of new tokens to match changes to the eglCreateSyncKHR entry point in revision 15. #15 (Jon Leech, June 16, 2009) - Define separate one-time fence sync and reusable sync extensions and corresponding extension strings. Remove AUTO_RESET and eglFenceKHR. Rename eglCreateFenceSyncKHR to eglCreateSyncKHR and change initial status of reusable syncs to unsignaled. Clarify which functions apply to which types of sync objects. Update issues list. #14 (Jon Leech, April 29, 2009) - Clarify that all waiters are woken up on signalling a sync. Remove tabs to cleanup some formatting issues. #13 (Acorn Pooley, April 2, 2009) - Renamed GL_OES_egl_sync -> GL_OES_EGL_sync VG_KHR_egl_sync -> VG_KHR_EGL_sync #12 (Jon Leech, April 1, 2009) - Changed sync flags type from EGLuint to EGLint and add issue 7. #11 (Acorn Pooley, February 4, 2009) - add error case to eglGetSyncAttribKHR. - fix year on rev 8-10 (2008->2009) #10 (Acorn Pooley, February 4, 2009) - clarify some error message descriptions #9 (Greg Prisament, January 15, 2009) - Destroy now wakes up all waits (eglClientWaitSyncKHR) - Add EGLDisplay as first parameter to all commands - Split into 3 extension strings, EGL_KHR_sync, GL_OES_egl_sync, VG_KHR_egl_sync, all described in this document. - Add attribute AUTO_RESET_KHR - Time type uses the type from khrplatform.h - Remove EGL_ALREADY_SIGNALLED #8 (Jon Leech, November 11, 2009) - Assign enum values #7 (Acorn Pooley, October 30, 2008) - Fix typos - remove obsolete wording about Native sync objects (see issue 5) - formatting: remove tabs, 80 columns #6 (Acorn Pooley, October 27, 2008) - Corrected 'enum' to 'EGLenum' in prototypes. #5 (Jon Leech, September 9, 2008) - Removed native sync support (eglCreateNativeSyncKHR and EGL_SYNC_NATIVE_SYNC_KHR), and re-flowed spec to fit in 80 columns. #4 (Jon Leech, November 20, 2007) - Corrected 'enum' to 'EGLenum' in prototypes. #3 (Jon Leech, April 5, 2007) - Added draft Status and TBD Number #2 (November 27, 2006) - Changed OES token to KHR