4 Both elements and pads can be in different states. The states of the pads are
5 linked to the state of the element so the design of the states is mainly
6 focused around the element states.
8 An element can be in 4 states. NULL, READY, PAUSED and PLAYING. When an element
9 is initially instantiated, it is in the NULL state.
15 - NULL: This is the initial state of an element.
16 - READY: The element should be prepared to go to PAUSED.
17 - PAUSED: The element should be ready to accept and process data. Sink
18 elements however only accept one buffer and then block.
19 - PLAYING: The same as PAUSED except for sinks, who are now accepting
22 We call the sequence NULL->PLAYING an upwards state change and PLAYING->NULL
23 a downwards state change.
29 the following state changes are possible:
32 - The element must check if the resources it needs are available.
33 Audiosinks typically try to probe the device.
36 - The element opens the device and prepares itself for PLAYING.
37 - the element pads are activated in order to receive data in PAUSED.
38 streaming threads are started.
39 - some elements might need to return ASYNC and complete the state change
40 when they have enough information. It is a requirement for sinks to
41 return ASYNC and complete the state change when they receive the first
42 buffer or EOS event (prerol). Sinks also block the dataflow when in PAUSED.
43 - a pipeline resets the stream time to 0.
44 - live sources return NO_PREROLL and don't generate data.
47 - most elements ignore this state change.
48 - The pipeline selects a clock and distributes this to all the children
49 before setting them to PLAYING. This means that it is only alowed to
50 synchronize on the clock in the PLAYING state.
51 - The pipeline uses the clock and the stream time to calculate the base time.
52 The base time is distributed to all children when performing the state
54 - sink elements stop blocking on the preroll buffer or event and start
56 - sinks can post the EOS message in the PLAYING state. It is not allowed to
57 post EOS when not in the PLAYING state.
58 - while streaming in PAUSED or PLAYING elements can create and remove
60 - live sources start generating data and return SUCCESS.
63 - most elements ignore this state change.
64 - The pipeline calculates the stream time based on the last selected clock
65 and the base time. It stores this information to continue playback when
66 going back to the PLAYING state.
67 - sinks unblock any clock wait calls.
68 - when a sink did not have a pending buffer to play, it returns ASYNC from
69 this state change and complete the state change when they receive a new buffer
71 - any queued EOS messages are removed since they will be reposted when going
72 back to the PLAYING state. The EOS messages are queued in GstBins.
73 - live sources stop generating data and return NO_PREROLL.
76 - sinks unblock any waits in the preroll.
77 - elements unblock any waits on devices
78 - chain or get_range functions return WRONG_STATE.
79 - the element pads are deactivated so that streaming becomes impossible and
80 all streaming threads are stopped.
83 - element removes any dynamically created pads
89 An element has 4 state variables that are protected with the object LOCK:
96 The STATE always reflects the current state of the element.
97 The STATE_NEXT reflects the next state the element will go to.
98 The STATE_PENDING always reflects the required state of the element.
99 The STATE_RETURN reflects the last return value of a state change.
101 The STATE_NEXT and STATE_PENDING can be VOID_PENDING if the element is in
104 An element has a special lock to protect against concurrent invocations of
105 _set_state(), called the STATE_LOCK.
108 Setting state on elements
109 -------------------------
111 The state of an element can be changed with _element_set_state(). When chaning
112 the state of an element all intermediate states will also be set on the element
113 until the final desired state is set.
115 The _set_state() function can return 3 possible values:
117 GST_STATE_FAILURE: The state change failed for some reason. The plugin should
118 have posted an error message on the bus with information.
120 GST_STATE_SUCCESS: The state change is completed successfully.
122 GST_STATE_ASYNC: The state change will complete later on. This can happen
123 When the element needs a long time to perform the state
124 change or for sinks that need to receive the first buffer
125 before they can complete the state change (preroll).
127 GST_STATE_NO_PREROLL: The state change is completed successfully but the element
128 will not be able to produce data in the PAUSED state.
130 In the case of an ASYNC state change, it is possible to proceed to the next
131 state before the current state change completed, however, the element will only
132 get to this next state before completing the previous ASYNC state change.
133 After receiving an ASYNC return value, you can use _element_get_state() to poll
134 the status of the element. If the polling returns SUCCESS, the element completed
135 the state change to the last requested state with _set_state().
137 When setting the state of an element, the STATE_PENDING is set to the required
138 state. Then the state change function of the element is called and the result of
139 that function is used to update the STATE and STATE_RETURN fields, STATE_NEXT,
140 STATE_PENDING and STATE_RETURN fields. If the function returned ASYNC, this result
141 is immediatly returned to the caller.
144 Getting state of elements
145 -------------------------
147 The _get_state() function takes 3 arguments, two pointers that will hold the
148 current and pending state and one GstClockTime that holds a timeout value. The
149 function returns a GstElementStateReturn.
151 - If the element returned SUCCESS to the previous _set_state() function, this
152 function will return the last state set on the element and VOID_PENDING in
153 the pending state value. The function returns GST_STATE_SUCCESS.
155 - If the element returned NO_PREROLL to the previous _set_state() function, this
156 function will return the last state set on the element and VOID_PENDING in
157 the pending state value. The function returns GST_STATE_NO_PREROLL.
159 - If the element returned FAILURE to the previous _set_state() call, this
160 funciton will return FAILURE with the state set to the current state of
161 the element and the pending state set to the value used in the last call
164 - If the element returned ASYNC to the previous _set_state() call, this function
165 will wait for the element to complete its state change up to the amount of time
166 specified in the GstClockTime.
168 * If the element does not complete the state change in the specified amount of
169 time, this function will return ASYNC with the state set to the current state
170 and the pending state set to the pending state.
172 * If the element completes the state change within the specified timeout, this
173 function returns the updated state and VOID_PENDING as the pending state.
175 * If the element aborts the ASYNC state change due to an error within the
176 specified timeout, this function returns FAILURE with the state set to last
177 successfull state and pending set to the last attempt. The element should
178 also post an error message on the bus with more information about the problem.
184 A GstBin manages the state of its children. It does this by propagating the state
185 changes performed on it to all of its children. The _set_state() function on a
186 bin will call the _set_state() function on all of its children.
188 The children are iterated from the sink elements to the source elements. This makes
189 sure that when changing the state of an element, the downstream elements are in
190 the correct state to process the eventual buffers. In the case of a downwards
191 state change, the sink elements will shut down first which makes the upstream
192 elements shut down as well since the _push() function returns a GST_FLOW_WRONG_STATE
195 If all the children return SUCCESS, the function returns SUCCESS as well.
197 If one of the children returns FAILURE, the function returns FAILURE as well. In
198 this state it is possible that some elements successfuly changed state. The
199 application can check which elements have a changed state, which were in error
200 and which were not affected by iterating the elements and calling _get_state()
203 If after calling the state function on all children, one of the children returned
204 ASYNC, the function returns ASYNC as well.
206 If after calling the state function on all children, one of the children returned
207 NO_PREROLL, the function returns NO_PREROLL as well.
209 If both NO_PREROLL and ASYNC children are present, NO_PREROLL is returned.
211 The current state of the bin can be retrieved with _get_state().
213 If the bin is performing an ASYNC state change, it will automatically update its
214 current state fields when it receives state messages from the children.
217 Implementing states in elements
218 -------------------------------
228 Upward state changes always return ASYNC either if the STATE_PENDING is
238 - element commits state ASYNC
241 - update STATE_PENDING state
243 - no change_state called on element
247 A->B: all elements SUCCESS
250 A->B: some elements ASYNC
252 - listen for commit messages on bus
253 - for each commit message, poll elements, this happens in another
255 - if no ASYNC elements, commit state, continue state change
258 downward state change
259 ----------------------
261 Downward state changes only return ASYNC if the final state is ASYNC.
262 This is to make sure that it's not needed to wait for an element to
263 complete the preroll or other ASYNC state changes when one only wants to
264 shut down an element.
271 A -> B => ASYNC not final state
272 - commit state on behalf of element
274 A -> B => ASYNC final state
275 - element will commit ASYNC
282 A -> B -> ASYNC not final state
283 - commit state on behalf of element, continue state change
285 A -> B => ASYNC final state
287 - listen for commit messages on bus
288 - for each commit message, poll elements
289 - if no ASYNC elements, commit state
292 Locking overview (element)
293 --------------------------
295 * Element commiting SUCCESS
297 - STATE_LOCK is taken in set_state
298 - change state is called if SUCCESS, commit state is called
299 - commit state calls change_state to next state change.
300 - if final state is reached, stack unwinds and result is returned to
301 set_state and caller.
304 set_state(element) change_state (element) commit_state
310 |------------------------>| |
313 | | (do state change) |
317 | |---------------------->|
320 | |<----------------------| if (!final) change_state (next)
325 |<------------------------| |
334 * Element commiting ASYNC
336 - STATE_LOCK is taken in set_state
337 - change state is called and returns ASYNC
338 - ASYNC returned to the caller.
339 - element takes LOCK in streaming thread.
340 - element calls commit_state in streaming thread.
341 - commit state calls change_state to next state change.
344 set_state(element) change_state (element) stream_thread commit_state (element)
350 |------------------------>| | |
357 |<------------------------| | |
358 | ASYNC STREAM_UNLOCK |
360 | .....sync........ STATE_LOCK |
361 ASYNC |----------------->|
363 | |---> post_message()
364 | |---> if (!final) change_state (next)
373 *********************************************
374 *********************************************
376 set_state cannot be called from multiple threads at the same time. The STATE_LOCK
379 state variables are protected with the LOCK.
381 calling set_state while gst_state is called should unlock the get_state with
382 an error. The cookie will do that.
390 update current, next, pending state