PL286/kernel-3.18/Documentation/sync.txt

Motivation:

In complicated DMA pipelines such as graphics (multimedia, camera, gpu, display)
a consumer of a buffer needs to know when the producer has finished producing
it.  Likewise the producer needs to know when the consumer is finished with the
buffer so it can reuse it.  A particular buffer may be consumed by multiple
consumers which will retain the buffer for different amounts of time.  In
addition, a consumer may consume multiple buffers atomically.
The sync framework adds an API which allows synchronization between the
producers and consumers in a generic way while also allowing platforms which
have shared hardware synchronization primitives to exploit them.

Goals:
	* provide a generic API for expressing synchronization dependencies
	* allow drivers to exploit hardware synchronization between hardware
	  blocks
	* provide a userspace API that allows a compositor to manage
	  dependencies.
	* provide rich telemetry data to allow debugging slowdowns and stalls of
	   the graphics pipeline.

Objects:
	* sync_timeline
	* sync_pt
	* sync_fence

sync_timeline:

A sync_timeline is an abstract monotonically increasing counter. In general,
each driver/hardware block context will have one of these.  They can be backed
by the appropriate hardware or rely on the generic sw_sync implementation.
Timelines are only ever created through their specific implementations
(i.e. sw_sync.)

sync_pt:

A sync_pt is an abstract value which marks a point on a sync_timeline. Sync_pts
have a single timeline parent.  They have 3 states: active, signaled, and error.
They start in active state and transition, once, to either signaled (when the
timeline counter advances beyond the sync_pt’s value) or error state.

sync_fence:

Sync_fences are the primary primitives used by drivers to coordinate
synchronization of their buffers.  They are a collection of sync_pts which may
or may not have the same timeline parent.  A sync_pt can only exist in one fence
and the fence's list of sync_pts is immutable once created.  Fences can be
waited on synchronously or asynchronously.  Two fences can also be merged to
create a third fence containing a copy of the two fences’ sync_pts.  Fences are
backed by file descriptors to allow userspace to coordinate the display pipeline
dependencies.

Use:

A driver implementing sync support should have a work submission function which:
     * takes a fence argument specifying when to begin work
     * asynchronously queues that work to kick off when the fence is signaled
     * returns a fence to indicate when its work will be done.
     * signals the returned fence once the work is completed.

Consider an imaginary display driver that has the following API:
/*
 * assumes buf is ready to be displayed.
 * blocks until the buffer is on screen.
 */
    void display_buffer(struct dma_buf *buf);

The new API will become:
/*
 * will display buf when fence is signaled.
 * returns immediately with a fence that will signal when buf
 * is no longer displayed.
 */
struct sync_fence* display_buffer(struct dma_buf *buf,
                                 struct sync_fence *fence);