ANDROID_native_fence_sync

EGL_ANDROID_native_fence_sync

Jamie Gennis

Jamie Gennis, Google Inc. (jgennis 'at' google.com)

Complete

Version 3, September 4, 2012

EGL Extension #50

Requires EGL 1.1

This extension is written against the wording of the EGL 1.2 Specification

EGL_KHR_fence_sync is required.

This extension enables the creation of EGL fence sync objects that are
associated with a native synchronization fence object that is referenced
using a file descriptor.  These EGL fence sync objects have nearly
identical semantics to those defined by the KHR_fence_sync extension,
except that they have an additional attribute storing the file descriptor
referring to the native fence object.

This extension assumes the existence of a native fence synchronization
object that behaves similarly to an EGL fence sync object.  These native
objects must have a signal status like that of an EGLSyncKHR object that
indicates whether the fence has ever been signaled.  Once signaled the
native object's signal status may not change again.

None.

EGLint eglDupNativeFenceFDANDROID(
                    EGLDisplay dpy,
                    EGLSyncKHR);

Accepted by the <type> parameter of eglCreateSyncKHR, and returned
in <value> when eglGetSyncAttribKHR is called with <attribute>
EGL_SYNC_TYPE_KHR:

EGL_SYNC_NATIVE_FENCE_ANDROID          0x3144

Accepted by the <attrib_list> parameter of eglCreateSyncKHR:

EGL_SYNC_NATIVE_FENCE_FD_ANDROID       0x3145

Accepted by the <attrib_list> parameter of eglCreateSyncKHR, and returned
by eglDupNativeFenceFDANDROID in the event of an error:

EGL_NO_NATIVE_FENCE_FD_ANDROID         -1

Returned in <value> when eglGetSyncAttribKHR is called with <attribute>
EGL_SYNC_CONDITION_KHR:

EGL_SYNC_NATIVE_FENCE_SIGNALED_ANDROID 0x3146

Add the following after the sixth paragraph of Section 3.8.1 (Sync
Objects), added by KHR_fence_sync

"If <type> is EGL_SYNC_NATIVE_FENCE_ANDROID, an EGL native fence sync
object is created. In this case the EGL_SYNC_NATIVE_FENCE_FD_ANDROID
attribute may optionally be specified. If this attribute is specified, it
must be set to either a file descriptor that refers to a native fence
object or to the value EGL_NO_NATIVE_FENCE_FD_ANDROID.

The default values for the EGL native fence sync object attributes are as
follows:

  Attribute Name                     Initial Attribute Value(s)
  ---------------                    --------------------------
  EGL_SYNC_TYPE_KHR                  EGL_SYNC_NATIVE_FENCE_ANDROID
  EGL_SYNC_STATUS_KHR                EGL_UNSIGNALED_KHR
  EGL_SYNC_CONDITION_KHR             EGL_SYNC_PRIOR_COMMANDS_COMPLETE_KHR
  EGL_SYNC_NATIVE_FENCE_FD_ANDROID   EGL_NO_NATIVE_FENCE_FD_ANDROID

If the EGL_SYNC_NATIVE_FENCE_FD_ANDROID attribute is not
EGL_NO_NATIVE_FENCE_FD_ANDROID then the EGL_SYNC_CONDITION_KHR attribute is
set to EGL_SYNC_NATIVE_FENCE_SIGNALED_ANDROID and the EGL_SYNC_STATUS_KHR
attribute is set to reflect the signal status of the native fence object.
Additionally, the EGL implementation assumes ownership of the file
descriptor, so the caller must not use it after calling eglCreateSyncKHR."

Modify Section 3.8.1 (Sync Objects), added by KHR_fence_sync, starting at
the seventh paragraph

"When a fence sync object is created or when an EGL native fence sync
object is created with the EGL_SYNC_NATIVE_FENCE_FD_ANDROID attribute set
to EGL_NO_NATIVE_FENCE_FD_ANDROID, eglCreateSyncKHR also inserts a fence
command into the command stream of the bound client API's current context
(i.e., the context returned by eglGetCurrentContext), and associates it
with the newly created sync object.

After associating the fence command with an EGL native fence sync object,
the next Flush() operation performed by the current client API causes a
new native fence object to be created, and the
EGL_SYNC_NATIVE_FENCE_ANDROID attribute of the EGL native fence object is
set to a file descriptor that refers to the new native fence object. This
new native fence object is signaled when the EGL native fence sync object
is signaled.

When the condition of the sync object is satisfied by the fence command,
the sync is signaled by the associated client API context, causing any
eglClientWaitSyncKHR commands (see below) blocking on <sync> to unblock.
If the sync object is an EGL native fence sync object then the native
fence object is also signaled when the condition is satisfied. The
EGL_SYNC_PRIOR_COMMANDS_COMPLETE_KHR condition is satisfied by completion
of the fence command corresponding to the sync object and all preceding
commands in the associated client API context's command stream. The sync
object will not be signaled until all effects from these commands on the
client API's internal and framebuffer state are fully realized. No other
state is affected by execution of the fence command.

The EGL_SYNC_NATIVE_FENCE_SIGNALED_ANDROID condition is satisfied by the
signaling of the native fence object referred to by the
EGL_SYNC_NATIVE_FENCE_FD_ANDROID attribute. When this happens any
eglClientWaitSyncKHR commands blocking on <sync> unblock."

Modify the list of eglCreateSyncKHR errors in Section 3.8.1 (Sync Objects),
added by KHR_fence_sync

"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 <type> is EGL_SYNC_FENCE_KHR and <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 EGL_SYNC_NATIVE_FENCE_ANDROID and <attrib_list> contains
    an attribute other than EGL_SYNC_NATIVE_FENCE_FD_ANDROID,
    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.
  * If <type> is EGL_SYNC_FENCE_KHR or EGL_SYNC_NATIVE_FENCE_ANDROID and
    no context is current for the bound API (i.e., eglGetCurrentContext
    returns EGL_NO_CONTEXT), EGL_NO_SYNC_KHR is returned and an
    EGL_BAD_MATCH error is generated.
  * If <type> is EGL_SYNC_FENCE_KHR or EGL_SYNC_NATIVE_FENCE_ANDROID and
    <dpy> does not match the EGLDisplay of the currently bound context for
    the currently bound client API (the EGLDisplay returned by
    eglGetCurrentDisplay()) then EGL_NO_SYNC_KHR is returned and an
    EGL_BAD_MATCH error is generated.
  * If <type> is EGL_SYNC_FENCE_KHR or EGL_SYNC_NATIVE_FENCE_ANDROID and
    the currently bound client API does not support the client API
    extension indicating it can place fence commands, then EGL_NO_SYNC_KHR
    is returned and an EGL_BAD_MATCH error is generated."

Modify table 3.cc in Section 3.8.1 (Sync Objects), added by KHR_fence_sync

"
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
EGL_SYNC_CONDITION_KHR             Signaling condition        EGL_SYNC_FENCE_KHR and
                                                              EGL_SYNC_NATIVE_FENCE_ANDROID only
"

Modify the second paragraph description of eglDestroySyncKHR in Section
3.8.1 (Sync Objects), added by KHR_fence_sync

"If no errors are generated, EGL_TRUE is returned, and <sync> will no
longer be the handle of a valid sync object.  Additionally, if <sync> is an
EGL native fence sync object and the EGL_SYNC_NATIVE_FENCE_FD_ANDROID
attribute is not EGL_NO_NATIVE_FENCE_FD_ANDROID then that file descriptor
is closed."

Add the following after the last paragraph of Section 3.8.1 (Sync
Objects), added by KHR_fence_sync

The command

    EGLint eglDupNativeFenceFDANDROID(
                        EGLDisplay dpy,
                        EGLSyncKHR sync);

duplicates the file descriptor stored in the
EGL_SYNC_NATIVE_FENCE_FD_ANDROID attribute of an EGL native fence sync
object and returns the new file descriptor.

Errors
------

  * If <sync> is not a valid sync object for <dpy>,
    EGL_NO_NATIVE_FENCE_FD_ANDROID is returned and an EGL_BAD_PARAMETER
    error is generated.
  * If the EGL_SYNC_NATIVE_FENCE_FD_ANDROID attribute of <sync> is
    EGL_NO_NATIVE_FENCE_FD_ANDROID, EGL_NO_NATIVE_FENCE_FD_ANDROID 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."

1. Should EGLSyncKHR objects that wrap native fence objects use the
EGL_SYNC_FENCE_KHR type?

RESOLVED: A new sync object type will be added.

We don't want to require all EGL fence sync objects to wrap native fence
objects, so we need some way to tell the EGL implementation at sync object
creation whether the sync object should support querying the native fence
FD attribute. We could do this with either a new sync object type or with a
boolean attribute. It might be nice to pick up future signaling conditions
that might be added for fence sync objects, but there may be things that
get added that don't make sense in the context of native fence objects.

2. Who is responsible for dup'ing the native fence file descriptors?

RESOLVED: Whenever a file descriptor is passed into or returned from an
EGL call in this extension, ownership of that file descriptor is
transfered. The recipient of the file descriptor must close it when it is
no longer needed, and the provider of the file descriptor must dup it
before providing it if they require continued use of the native fence.

3. Can the EGL_SYNC_NATIVE_FENCE_FD_ANDROID attribute be queried?

RESOLVED: No.

Returning the file descriptor owned by the EGL implementation would
violate the file descriptor ownership rule described in issue #2. Having
eglGetSyncAttribKHR return a different (dup'd) file descriptor each time
it's called seems wrong, so a new function was added to explicitly dup the
file descriptor.

That said, the attribute is useful both as a way to pass an existing file
descriptor to eglCreateSyncKHR and as a way to describe the subsequent
behavior of EGL native fence sync objects, so it is left as an attribute
for which the value cannot be queried.