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This protocol allows clients to request explicit synchronization for buffers. It is tied to the Linux DRM synchronization object framework.
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.
With 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, with explicit synchronization, DRM synchronization object timeline points mark when the asynchronous operations are complete. When submitting a buffer, the client provides a timeline point which will be waited on before the compositor accesses the buffer, and another timeline point that the compositor will signal when it no longer needs to access the buffer contents for the purposes of the surface commit.
Linux DRM synchronization objects are documented at: https://dri.freedesktop.org/docs/drm/gpu/drm-mm.html#drm-sync-objects
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