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160 lines
4.5 KiB
Text
160 lines
4.5 KiB
Text
purpose
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-------
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A proposal for proper syncing and clocking of a pipeline.
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Requirements
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------------
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- elements should be able to get the time and wait for a specific
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time
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- some elements should be able to control and adjust the clock.
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(clock master)
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- the application should be able to provide another clock.
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Clocks
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------
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A clock extends the abstract GstClock class.
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gst_clock_get_time should always report an equal or increasing
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value with each succesive call.
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Clock providers
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---------------
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Clock providers call gst_element_provides_clock (element, clock)
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in their _init function. This will set some element flags.
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This also means that a clock provider cannot stop being a clock
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provider.
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Clock providers will update the clock at specific intervals.
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get_resolution/set_resolution can be used to control the resolution.
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When a clock provider is not going to update the clock anymore
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it should make sure elements still blocked on the clock get
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unblocked at the right time. This can be done by converting all
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blocking waits to a select call. All further waits on the clock
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should be estimated (using gettimeofday for example)
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Clock receivers
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---------------
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An element needing a clock must implement the element receive_clock method.
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It has to use the clock received in this function or if the
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clock == NULL it should not do any waiting at all.
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Clocks and the scheduler
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------------------------
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The scheduler knows about the clocks, else a clock provider and receiver
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in the same scheduler could cause a deadlock.
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Only one clock provider is allowed per scheduler. multiple clock
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receivers are allowed per scheduler.
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Specifying clocks
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-----------------
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on every bin with a scheduler, gst_bin_use_clock (bin, clock) can be used
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to force the use of this specific clock for all the elements in this bin.
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gst_bin_use_clock (bin, NULL) to disable all clocking for this bin.
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gst_bin_auto_clock (bin) to use the default algorithm to find a clock.
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bins that get a use clock set a flag and store the clock, they also call
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set_clock on all of their children. When other children are added, they
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all get the stored clock.
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Clock distribution
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------------------
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clock providers and receivers are collected in the bins on element
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add/remove, recursing bins if needed.
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the toplevel bin with a scheduler selects the global clock and calls
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set_clock on itself.
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bins dispatch the set_clock to to all of the reveivers. Bins with
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a scheduler and another use_clock do nothing.
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Bins with a scheduler also notify the scheduler of the clock.
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Clock usage
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-----------
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when an element wants to wait for a specific time, if calls
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gst_element_clock_wait (elements, clock, time). The call is dispatched
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to the scheduler of the element which can use the owner field
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of the clock to check if the elements are in the same scheduler.
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For elements waiting for a clock provided by an element in another
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scheduler there is no problem.
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When provider and receiver are in the same scheduler, a deadlock can
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occur since the scheduler will block on the wait without being able to
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schedule the provider again to update the clock.
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A solution would be to call the async notify of the clock and schedule
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some other element (the provider?). We probably need an event based
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scheduler for this. When the async event arrives we can reschedule
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the receiver and continue.
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The current scheduler will assert on this condition for now.
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Changing clocks
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---------------
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The clock can only be changed when the bin is in the PAUSED state.
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State Changes
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-------------
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When the pipeline is PAUSED, the clock is stopped with
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gst_clock_enable (clock, FALSE). The clock should unblock all
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waiting elements ASAP and return GST_CLOCK_STOP in the wait.
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Elements waiting for a clock and receiving the STOP should
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process the last buffer ASAP and break out of their loop.
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When the pipeline is brought to the READY state, the clock is
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set to 0.
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When the clock is enabled again, it should start counting from where
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it was last disabled.
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automatic Clock selection
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-------------------------
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Select a random clock from the Src elements.
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if no Src elements exist with a clock, select a random clock from the
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Sink elements.
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else use a default System Clock.
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Src elements with a clock are prefered because they usualy provide
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live audio/video.
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Issues
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------
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ossrc ! osssink can cause clock drift if osssink doesn't process the bytes
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at the same rate osssrc provides them (different hardware). Things will
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stutter and cracle if this is the case. QoS and a resampler could solve
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this.
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