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406 lines
16 KiB
Text
406 lines
16 KiB
Text
States
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------
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Both elements and pads can be in different states. The states of the pads are
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linked to the state of the element so the design of the states is mainly
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focused around the element states.
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An element can be in 4 states. NULL, READY, PAUSED and PLAYING. When an element
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is initially instantiated, it is in the NULL state.
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State definitions
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~~~~~~~~~~~~~~~~~
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- NULL: This is the initial state of an element.
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- READY: The element should be prepared to go to PAUSED.
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- PAUSED: The element should be ready to accept and process data. Sink
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elements however only accept one buffer and then block.
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- PLAYING: The same as PAUSED except for live sources and sinks. Sinks accept
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and rendering data. Live sources produce data.
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We call the sequence NULL->PLAYING an upwards state change and PLAYING->NULL
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a downwards state change.
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State transitions
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~~~~~~~~~~~~~~~~~
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the following state changes are possible:
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NULL -> READY
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- The element must check if the resources it needs are available.
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Device sinks and -sources typically try to probe the device to constrain
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their caps.
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- The element opens the device, this is needed if the previous step requires
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the device to be opened.
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READY -> PAUSED
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- The element pads are activated in order to receive data in PAUSED.
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Streaming threads are started.
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- Some elements might need to return ASYNC and complete the state change
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when they have enough information. It is a requirement for sinks to
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return ASYNC and complete the state change when they receive the first
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buffer or EOS event (preroll). Sinks also block the dataflow when in PAUSED.
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- A pipeline resets the running_time to 0.
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- Live sources return NO_PREROLL and don't generate data.
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PAUSED -> PLAYING
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- Most elements ignore this state change.
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- The pipeline selects a clock and distributes this to all the children
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before setting them to PLAYING. This means that it is only allowed to
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synchronize on the clock in the PLAYING state.
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- The pipeline uses the clock and the running_time to calculate the base_time.
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The base_time is distributed to all children when performing the state
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change.
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- Sink elements stop blocking on the preroll buffer or event and start
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rendering the data.
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- Sinks can post the EOS message in the PLAYING state. It is not allowed to
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post EOS when not in the PLAYING state.
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- While streaming in PAUSED or PLAYING elements can create and remove
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sometimes pads.
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- Live sources start generating data and return SUCCESS.
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PLAYING -> PAUSED
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- Most elements ignore this state change.
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- The pipeline calculates the running_time based on the last selected clock
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and the base_time. It stores this information to continue playback when
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going back to the PLAYING state.
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- Sinks unblock any clock wait calls.
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- When a sink does not have a pending buffer to play, it returns ASYNC from
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this state change and completes the state change when it receives a new
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buffer or an EOS event.
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- Any queued EOS messages are removed since they will be reposted when going
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back to the PLAYING state. The EOS messages are queued in GstBins.
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- Live sources stop generating data and return NO_PREROLL.
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PAUSED -> READY
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- Sinks unblock any waits in the preroll.
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- Elements unblock any waits on devices
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- Chain or get_range functions return FLUSHING.
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- The element pads are deactivated so that streaming becomes impossible and
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all streaming threads are stopped.
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- The sink forgets all negotiated formats
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- Elements remove all sometimes pads
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READY -> NULL
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- Elements close devices
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- Elements reset any internal state.
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State variables
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~~~~~~~~~~~~~~~
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An element has 4 state variables that are protected with the object LOCK:
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- STATE
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- STATE_NEXT
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- STATE_PENDING
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- STATE_RETURN
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The STATE always reflects the current state of the element.
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The STATE_NEXT reflects the next state the element will go to.
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The STATE_PENDING always reflects the required state of the element.
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The STATE_RETURN reflects the last return value of a state change.
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The STATE_NEXT and STATE_PENDING can be VOID_PENDING if the element is in
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the right state.
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An element has a special lock to protect against concurrent invocations of
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_set_state(), called the STATE_LOCK.
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Setting state on elements
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~~~~~~~~~~~~~~~~~~~~~~~~~
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The state of an element can be changed with _element_set_state(). When changing
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the state of an element all intermediate states will also be set on the element
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until the final desired state is set.
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The _set_state() function can return 3 possible values:
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GST_STATE_FAILURE: The state change failed for some reason. The plugin should
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have posted an error message on the bus with information.
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GST_STATE_SUCCESS: The state change is completed successfully.
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GST_STATE_ASYNC: The state change will complete later on. This can happen
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when the element needs a long time to perform the state
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change or for sinks that need to receive the first buffer
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before they can complete the state change (preroll).
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GST_STATE_NO_PREROLL: The state change is completed successfully but the element
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will not be able to produce data in the PAUSED state.
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In the case of an ASYNC state change, it is possible to proceed to the next
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state before the current state change completed, however, the element will only
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get to this next state before completing the previous ASYNC state change.
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After receiving an ASYNC return value, you can use _element_get_state() to poll
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the status of the element. If the polling returns SUCCESS, the element completed
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the state change to the last requested state with _set_state().
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When setting the state of an element, the STATE_PENDING is set to the required
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state. Then the state change function of the element is called and the result of
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that function is used to update the STATE and STATE_RETURN fields, STATE_NEXT,
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STATE_PENDING and STATE_RETURN fields. If the function returned ASYNC, this result
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is immediately returned to the caller.
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Getting state of elements
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~~~~~~~~~~~~~~~~~~~~~~~~~
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The _get_state() function takes 3 arguments, two pointers that will hold the
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current and pending state and one GstClockTime that holds a timeout value. The
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function returns a GstElementStateReturn.
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- If the element returned SUCCESS to the previous _set_state() function, this
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function will return the last state set on the element and VOID_PENDING in
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the pending state value. The function returns GST_STATE_SUCCESS.
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- If the element returned NO_PREROLL to the previous _set_state() function, this
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function will return the last state set on the element and VOID_PENDING in
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the pending state value. The function returns GST_STATE_NO_PREROLL.
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- If the element returned FAILURE to the previous _set_state() call, this
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funciton will return FAILURE with the state set to the current state of
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the element and the pending state set to the value used in the last call
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of _set_state().
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- If the element returned ASYNC to the previous _set_state() call, this function
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will wait for the element to complete its state change up to the amount of time
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specified in the GstClockTime.
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* If the element does not complete the state change in the specified amount of
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time, this function will return ASYNC with the state set to the current state
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and the pending state set to the pending state.
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* If the element completes the state change within the specified timeout, this
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function returns the updated state and VOID_PENDING as the pending state.
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* If the element aborts the ASYNC state change due to an error within the
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specified timeout, this function returns FAILURE with the state set to last
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successful state and pending set to the last attempt. The element should
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also post an error message on the bus with more information about the problem.
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States in GstBin
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~~~~~~~~~~~~~~~~
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A GstBin manages the state of its children. It does this by propagating the state
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changes performed on it to all of its children. The _set_state() function on a
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bin will call the _set_state() function on all of its children, that are
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not already in the target state or in a change state to the target state.
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The children are iterated from the sink elements to the source elements. This makes
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sure that when changing the state of an element, the downstream elements are in
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the correct state to process the eventual buffers. In the case of a downwards
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state change, the sink elements will shut down first which makes the upstream
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elements shut down as well since the _push() function returns a GST_FLOW_FLUSHING
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error.
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If all the children return SUCCESS, the function returns SUCCESS as well.
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If one of the children returns FAILURE, the function returns FAILURE as well. In
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this state it is possible that some elements successfully changed state. The
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application can check which elements have a changed state, which were in error
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and which were not affected by iterating the elements and calling _get_state()
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on the elements.
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If after calling the state function on all children, one of the children returned
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ASYNC, the function returns ASYNC as well.
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If after calling the state function on all children, one of the children returned
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NO_PREROLL, the function returns NO_PREROLL as well.
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If both NO_PREROLL and ASYNC children are present, NO_PREROLL is returned.
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The current state of the bin can be retrieved with _get_state().
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If the bin is performing an ASYNC state change, it will automatically update its
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current state fields when it receives state messages from the children.
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Implementing states in elements
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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READY
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^^^^^
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upward state change
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~~~~~~~~~~~~~~~~~~~
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Upward state changes always return ASYNC either if the STATE_PENDING is
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reached or not.
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Element:
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A -> B => SUCCESS
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- commit state
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A -> B => ASYNC
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- no commit state
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- element commits state ASYNC
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A -> B while ASYNC
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- update STATE_PENDING state
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- no commit state
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- no change_state called on element
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Bin:
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A->B: all elements SUCCESS
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- commit state
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A->B: some elements ASYNC
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- no commit state
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- listen for commit messages on bus
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- for each commit message, poll elements, this happens in another
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thread.
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- if no ASYNC elements, commit state, continue state change
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to STATE_PENDING
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downward state change
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~~~~~~~~~~~~~~~~~~~~~
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Downward state changes only return ASYNC if the final state is ASYNC.
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This is to make sure that it's not needed to wait for an element to
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complete the preroll or other ASYNC state changes when one only wants to
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shut down an element.
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Element:
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A -> B => SUCCESS
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- commit state
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A -> B => ASYNC not final state
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- commit state on behalf of element
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A -> B => ASYNC final state
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- element will commit ASYNC
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Bin:
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A -> B -> SUCCESS
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- commit state
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A -> B -> ASYNC not final state
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- commit state on behalf of element, continue state change
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A -> B => ASYNC final state
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- no commit state
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- listen for commit messages on bus
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- for each commit message, poll elements
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- if no ASYNC elements, commit state
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Locking overview (element)
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~~~~~~~~~~~~~~~~~~~~~~~~~~
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* Element commiting SUCCESS
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- STATE_LOCK is taken in set_state
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- change state is called if SUCCESS, commit state is called
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- commit state calls change_state to next state change.
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- if final state is reached, stack unwinds and result is returned to
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set_state and caller.
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set_state(element) change_state (element) commit_state
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STATE_LOCK | |
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|------------------------>| |
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| | (do state change) |
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| | if SUCCESS |
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| |---------------------->|
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| | | post message
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| |<----------------------| if (!final) change_state (next)
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| | | else SIGNAL
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|<------------------------| |
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| SUCCESS
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STATE_UNLOCK
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SUCCESS
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* Element commiting ASYNC
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- STATE_LOCK is taken in set_state
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- change state is called and returns ASYNC
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- ASYNC returned to the caller.
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- element takes LOCK in streaming thread.
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- element calls commit_state in streaming thread.
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- commit state calls change_state to next state change.
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set_state(element) change_state (element) stream_thread commit_state (element)
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STATE_LOCK | | |
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|------------------------>| | |
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| | (start_task) | |
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| | STREAM_LOCK |
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| | |... |
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|<------------------------| | |
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| ASYNC STREAM_UNLOCK |
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STATE_UNLOCK | |
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| .....sync........ STATE_LOCK |
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ASYNC |----------------->|
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| |---> post_message()
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| |---> if (!final) change_state (next)
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| | else SIGNAL
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|<-----------------|
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STATE_UNLOCK
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STREAM_LOCK
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| ...
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STREAM_UNLOCK
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Remarks
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~~~~~~~
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set_state cannot be called from multiple threads at the same time. The STATE_LOCK
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prevents this.
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state variables are protected with the LOCK.
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calling set_state while gst_state is called should unlock the get_state with
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an error. The cookie will do that.
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set_state(element)
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STATE_LOCK
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LOCK
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update current, next, pending state
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cookie++
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UNLOCK
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change_state
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STATE_UNLOCK
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