76 lines
3.1 KiB
Text
76 lines
3.1 KiB
Text
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Motivation:
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In complicated DMA pipelines such as graphics (multimedia, camera, gpu, display)
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a consumer of a buffer needs to know when the producer has finished producing
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it. Likewise the producer needs to know when the consumer is finished with the
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buffer so it can reuse it. A particular buffer may be consumed by multiple
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consumers which will retain the buffer for different amounts of time. In
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addition, a consumer may consume multiple buffers atomically.
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The sync framework adds an API which allows synchronization between the
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producers and consumers in a generic way while also allowing platforms which
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have shared hardware synchronization primitives to exploit them.
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Goals:
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* provide a generic API for expressing synchronization dependencies
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* allow drivers to exploit hardware synchronization between hardware
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blocks
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* provide a userspace API that allows a compositor to manage
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dependencies.
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* provide rich telemetry data to allow debugging slowdowns and stalls of
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the graphics pipeline.
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Objects:
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* sync_timeline
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* sync_pt
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* sync_fence
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sync_timeline:
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A sync_timeline is an abstract monotonically increasing counter. In general,
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each driver/hardware block context will have one of these. They can be backed
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by the appropriate hardware or rely on the generic sw_sync implementation.
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Timelines are only ever created through their specific implementations
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(i.e. sw_sync.)
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sync_pt:
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A sync_pt is an abstract value which marks a point on a sync_timeline. Sync_pts
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have a single timeline parent. They have 3 states: active, signaled, and error.
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They start in active state and transition, once, to either signaled (when the
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timeline counter advances beyond the sync_pt’s value) or error state.
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sync_fence:
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Sync_fences are the primary primitives used by drivers to coordinate
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synchronization of their buffers. They are a collection of sync_pts which may
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or may not have the same timeline parent. A sync_pt can only exist in one fence
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and the fence's list of sync_pts is immutable once created. Fences can be
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waited on synchronously or asynchronously. Two fences can also be merged to
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create a third fence containing a copy of the two fences’ sync_pts. Fences are
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backed by file descriptors to allow userspace to coordinate the display pipeline
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dependencies.
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Use:
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A driver implementing sync support should have a work submission function which:
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* takes a fence argument specifying when to begin work
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* asynchronously queues that work to kick off when the fence is signaled
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* returns a fence to indicate when its work will be done.
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* signals the returned fence once the work is completed.
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Consider an imaginary display driver that has the following API:
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/*
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* assumes buf is ready to be displayed.
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* blocks until the buffer is on screen.
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*/
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void display_buffer(struct dma_buf *buf);
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The new API will become:
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/*
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* will display buf when fence is signaled.
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* returns immediately with a fence that will signal when buf
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* is no longer displayed.
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*/
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struct sync_fence* display_buffer(struct dma_buf *buf,
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struct sync_fence *fence);
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