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 21, April 23, 2013
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 <khrplatform.h>
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 <type> parameter of eglCreateSyncKHR, and returned
in <value> when eglGetSyncAttribKHR is called with <attribute>
EGL_SYNC_TYPE_KHR:
EGL_SYNC_REUSABLE_KHR 0x30FA
Accepted by the <attribute> parameter of eglGetSyncAttribKHR:
EGL_SYNC_TYPE_KHR 0x30F7
EGL_SYNC_STATUS_KHR 0x30F1
Accepted by the <mode> parameter of eglSignalSyncKHR and returned in
<value> when eglGetSyncAttribKHR is called with <attribute>
EGL_SYNC_STATUS_KHR:
EGL_SIGNALED_KHR 0x30F2
EGL_UNSIGNALED_KHR 0x30F3
Accepted in the <flags> parameter of eglClientWaitSyncKHR:
EGL_SYNC_FLUSH_COMMANDS_BIT_KHR 0x0001
Accepted in the <timeout> 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, <sync objects> 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: <signaled> and
<unsignaled>. 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 <attributes> 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.
<Reusable 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 <type> associated with the
specified display <dpy>, and returns a handle to the new object.
<attrib_list> 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 <type> is EGL_SYNC_REUSABLE_KHR, a reusable sync object is
created. In this case <attrib_list> 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 <dpy> is not the name of a valid, initialized EGLDisplay,
EGL_NO_SYNC_KHR is returned and an EGL_BAD_DISPLAY error is
generated.
* If <attrib_list> is neither NULL nor empty (containing only
EGL_NONE), EGL_NO_SYNC_KHR is returned and an EGL_BAD_ATTRIBUTE
error is generated.
* If <type> 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 <sync> is
signaled, or until <timeout> nanoseconds have passed.
More than one eglClientWaitSyncKHR may be outstanding on the same
<sync> at any given time. When there are multiple threads blocked on
the same <sync> 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 <timeout> is zero, then eglClientWaitSyncKHR simply
tests the current status of <sync>. If the value of <timeout> is the
special value EGL_FOREVER_KHR, then eglClientWaitSyncKHR does not
time out. For all other values, <timeout> 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 <sync>
was signaled. A return value of EGL_CONDITION_SATISFIED_KHR
indicates that <sync> was signaled before the timeout expired, which
includes the case when <sync> 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
<flags>, and <sync> 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 <sync>. 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 <sync> is not a valid sync object for <dpy>, EGL_FALSE is
returned and an EGL_BAD_PARAMETER error is generated.
* If <dpy> does not match the EGLDisplay passed to
eglCreateSyncKHR when <sync> was created, the behaviour is
undefined.
The command
EGLBoolean eglSignalSyncKHR(
EGLDisplay dpy,
EGLSyncKHR sync,
EGLenum mode);
signals or unsignals the reusable sync object <sync> by changing its
status to <mode>, which must be one of the values in table 3.bb. If
as a result of calling eglSignalSyncKHR the status of <sync>
transitions from unsignaled to signaled, any eglClientWaitSyncKHR
commands blocking on <sync> will unblock.
Assuming no errors are generated, EGL_TRUE is returned.
Mode Effect
------------------ -------------
EGL_SIGNALED_KHR Set the status of <sync> to signaled
EGL_UNSIGNALED_KHR Set the status of <sync> to unsignaled
Table 3.bb Modes Accepted by eglSignalSyncKHR Command
Errors
------
* If <sync> is not a valid sync object for <dpy>, EGL_FALSE is
returned and an EGL_BAD_PARAMETER error is generated.
* If the type of <sync> is not EGL_SYNC_REUSABLE_KHR, EGL_FALSE is
returned and an EGL_BAD_MATCH error is generated.
* If <dpy> does not match the EGLDisplay passed to
eglCreateSyncKHR when <sync> 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 <sync>. Legal values
for <attribute> 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 <value>.
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 <sync> is not a valid sync object for <dpy>, EGL_FALSE is
returned and an EGL_BAD_PARAMETER error is generated.
* If <dpy> does not match the display passed to eglCreateSyncKHR
when <sync> was created, the behaviour is undefined.
* If <attribute> is not one of the attributes in table 3.cc,
EGL_FALSE is returned and an EGL_BAD_ATTRIBUTE error is
generated.
* If <attribute> is not supported for the type of sync object
passed in <sync>, 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 <sync> when
eglDestroySyncKHR is called, they will be woken up, as if <sync>
were signaled.
If no errors are generated, EGL_TRUE is returned, and <sync> will no
longer be the handle of a valid sync object.
Errors
------
* If <sync> is not a valid sync object for <dpy>, EGL_FALSE is
returned and an EGL_BAD_PARAMETER error is generated.
* If <dpy> does not match the display passed to eglCreateSyncKHR
when <sync> 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
#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 <dpy> 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
