Linux explicit synchronization (dma-fence)

Protocol for providing explicit synchronization

This global is a factory interface, allowing clients to request explicit synchronization for buffers on a per-surface basis.

See zwp_linux_surface_synchronization_v1 for more information.

This interface is derived from Chromium's zcr_linux_explicit_synchronization_v1.

Warning! The protocol described in this file is experimental and backward incompatible changes may be made. Backward compatible changes may be added together with the corresponding interface version bump. Backward incompatible changes are done by bumping the version number in the protocol and interface names and resetting the interface version. Once the protocol is to be declared stable, the 'z' prefix and the version number in the protocol and interface names are removed and the interface version number is reset.

destroy()
Destroy explicit synchronization factory object

Destroy this explicit synchronization factory object. Other objects, including zwp_linux_surface_synchronization_v1 objects created by this factory, shall not be affected by this request.

get_synchronization(id: new_id<zwp_linux_surface_synchronization_v1>, surface: object<wl_surface>)
Argument
Type
Description
idnew_id<zwp_linux_surface_synchronization_v1>
the new synchronization interface id
surfaceobject<wl_surface>
the surface
Extend surface interface for explicit synchronization

Instantiate an interface extension for the given wl_surface to provide explicit synchronization.

If the given wl_surface already has an explicit synchronization object associated, the synchronization_exists protocol error is raised.

Graphics APIs, like EGL or Vulkan, that manage the buffer queue and commits of a wl_surface themselves, are likely to be using this extension internally. If a client is using such an API for a wl_surface, it should not directly use this extension on that surface, to avoid raising a synchronization_exists protocol error.

Argument
Value
Description
synchronization_exists0
the surface already has a synchronization object associated

Per-surface explicit synchronization support

This object implements per-surface explicit synchronization.

Synchronization refers to co-ordination of pipelined operations performed on buffers. Most GPU clients will schedule an asynchronous operation to render to the buffer, then immediately send the buffer to the compositor to be attached to a surface.

In implicit synchronization, ensuring that the rendering operation is complete before the compositor displays the buffer is an implementation detail handled by either the kernel or userspace graphics driver.

By contrast, in explicit synchronization, dma_fence objects mark when the asynchronous operations are complete. When submitting a buffer, the client provides an acquire fence which will be waited on before the compositor accesses the buffer. The Wayland server, through a zwp_linux_buffer_release_v1 object, will inform the client with an event which may be accompanied by a release fence, when the compositor will no longer access the buffer contents due to the specific commit that requested the release event.

Each surface can be associated with only one object of this interface at any time.

In version 1 of this interface, explicit synchronization is only guaranteed to be supported for buffers created with any version of the wp_linux_dmabuf buffer factory. Version 2 additionally guarantees explicit synchronization support for opaque EGL buffers, which is a type of platform specific buffers described in the EGL_WL_bind_wayland_display extension. Compositors are free to support explicit synchronization for additional buffer types.

destroy()
Destroy synchronization object

Destroy this explicit synchronization object.

Any fence set by this object with set_acquire_fence since the last commit will be discarded by the server. Any fences set by this object before the last commit are not affected.

zwp_linux_buffer_release_v1 objects created by this object are not affected by this request.

set_acquire_fence(fd: fd)
Argument
Type
Description
fdfd
acquire fence fd
Set the acquire fence

Set the acquire fence that must be signaled before the compositor may sample from the buffer attached with wl_surface.attach. The fence is a dma_fence kernel object.

The acquire fence is double-buffered state, and will be applied on the next wl_surface.commit request for the associated surface. Thus, it applies only to the buffer that is attached to the surface at commit time.

If the provided fd is not a valid dma_fence fd, then an INVALID_FENCE error is raised.

If a fence has already been attached during the same commit cycle, a DUPLICATE_FENCE error is raised.

If the associated wl_surface was destroyed, a NO_SURFACE error is raised.

If at surface commit time the attached buffer does not support explicit synchronization, an UNSUPPORTED_BUFFER error is raised.

If at surface commit time there is no buffer attached, a NO_BUFFER error is raised.

get_release(release: new_id<zwp_linux_buffer_release_v1>)
Argument
Type
Description
releasenew_id<zwp_linux_buffer_release_v1>
new zwp_linux_buffer_release_v1 object
Release fence for last-attached buffer

Create a listener for the release of the buffer attached by the client with wl_surface.attach. See zwp_linux_buffer_release_v1 documentation for more information.

The release object is double-buffered state, and will be associated with the buffer that is attached to the surface at wl_surface.commit time.

If a zwp_linux_buffer_release_v1 object has already been requested for the surface in the same commit cycle, a DUPLICATE_RELEASE error is raised.

If the associated wl_surface was destroyed, a NO_SURFACE error is raised.

If at surface commit time there is no buffer attached, a NO_BUFFER error is raised.

Argument
Value
Description
invalid_fence0
the fence specified by the client could not be imported
duplicate_fence1
multiple fences added for a single surface commit
duplicate_release2
multiple releases added for a single surface commit
no_surface3
the associated wl_surface was destroyed
unsupported_buffer4
the buffer does not support explicit synchronization
no_buffer5
no buffer was attached

Buffer release explicit synchronization

This object is instantiated in response to a zwp_linux_surface_synchronization_v1.get_release request.

It provides an alternative to wl_buffer.release events, providing a unique release from a single wl_surface.commit request. The release event also supports explicit synchronization, providing a fence FD for the client to synchronize against.

Exactly one event, either a fenced_release or an immediate_release, will be emitted for the wl_surface.commit request. The compositor can choose release by release which event it uses.

This event does not replace wl_buffer.release events; servers are still required to send those events.

Once a buffer release object has delivered a 'fenced_release' or an 'immediate_release' event it is automatically destroyed.

fenced_release(fence: fd)
Argument
Type
Description
fencefd
fence for last operation on buffer
Release buffer with fence

Sent when the compositor has finalised its usage of the associated buffer for the relevant commit, providing a dma_fence which will be signaled when all operations by the compositor on that buffer for that commit have finished.

Once the fence has signaled, and assuming the associated buffer is not pending release from other wl_surface.commit requests, no additional explicit or implicit synchronization is required to safely reuse or destroy the buffer.

This event destroys the zwp_linux_buffer_release_v1 object.

immediate_release()
Release buffer immediately

Sent when the compositor has finalised its usage of the associated buffer for the relevant commit, and either performed no operations using it, or has a guarantee that all its operations on that buffer for that commit have finished.

Once this event is received, and assuming the associated buffer is not pending release from other wl_surface.commit requests, no additional explicit or implicit synchronization is required to safely reuse or destroy the buffer.

This event destroys the zwp_linux_buffer_release_v1 object.


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