From f3dcbd15545b25ef2034dff0fb6097e224a27c9d Mon Sep 17 00:00:00 2001 From: Erik Gilling Date: Fri, 14 Sep 2012 14:36:34 -0700 Subject: [PATCH] sync: add Documentation/sync.txt Change-Id: Ic7f3a6d4622083be607b82ddd8d676609225bb8f Signed-off-by: Erik Gilling --- Documentation/sync.txt | 75 ++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 75 insertions(+) create mode 100644 Documentation/sync.txt diff --git a/Documentation/sync.txt b/Documentation/sync.txt new file mode 100644 index 000000000000..a2d05e7fa193 --- /dev/null +++ b/Documentation/sync.txt @@ -0,0 +1,75 @@ +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); -- 2.11.0