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 - sinks return ASYNC from this state change and complete the state change
69 when they receive a buffer or an EOS event.
70 - any queued EOS messages are removed since they will be reposted when going
71 back to the PLAYING state.
72 - live sources stop generating data and return NO_PREROLL.
75 - sinks unblock any waits in the preroll.
76 - elements unblock any waits on devices
77 - the element pads are deactivated so that streaming becomes impossible and
78 all streaming threads are stopped.
81 - element removes any dynamically created pads
87 An element has a special lock to manage the state changes. This lock is called
90 The STATE_LOCK protects 3 element variables:
97 The STATE always reflects the current state of the element.
98 The STATE_NEXT reflects the next state the element will go to.
99 The STATE_PENDING always reflects the required state of the element.
100 The STATE_RETURN reflects the last return value of a state change.
102 The STATE_NEXT and STATE_PENDING can be VOID_PENDING if the element is in
105 Setting state on elements
106 -------------------------
108 The state of an element can be changed with _element_set_state(). When chaning
109 the state of an element all intermediate states will also be set on the element
110 until the final desired state is set.
112 The _set_state() function can return 3 possible values:
114 GST_STATE_FAILURE: The state change failed for some reason. The plugin should
115 have posted an error message on the bus with information.
117 GST_STATE_SUCCESS: The state change is completed successfully.
119 GST_STATE_ASYNC: The state change will complete later on. This can happen
120 When the element needs a long time to perform the state
121 change or for sinks that need to receive the first buffer
122 before they can complete the state change (preroll).
124 GST_STATE_NO_PREROLL: The state change is completed successfully but the element
125 will not be able to produce data in the PAUSED state.
127 In the case of an async state change, it is possible to proceed to the next
128 state before the current state change completed, however, the element will only
129 get to this next state before completing the previous ASYNC state change.
130 After receiving an ASYNC return value, you can use _element_get_state() to poll
131 the status of the element. If the polling returns SUCCESS, the element completed
132 the state change to the last requested state with _set_state().
134 When setting the state of an element, the STATE_PENDING is set to the required
135 state. Then the state change function of the element is called and the result of
136 that function is used to update the STATE and STATE_RETURN fields, STATE_NEXT,
137 STATE_PENDING and STATE_RETURN fields. If the function returned ASYNC, this result
138 is immediatly returned to the caller.
141 Getting state of elements
142 -------------------------
144 The _get_state() function takes 3 arguments, two pointers that will hold the
145 current and pending state and one GTimeVal that holds a timeout value. The
146 function returns a GstElementStateReturn.
148 - If the element returned SUCCESS to the previous _set_state() function, this
149 function will return the last state set on the element and VOID_PENDING in
150 the pending state value. The function returns GST_STATE_SUCCESS.
152 - If the element returned NO_PREROLL to the previous _set_state() function, this
153 function will return the last state set on the element and VOID_PENDING in
154 the pending state value. The function returns GST_STATE_NO_PREROLL.
156 - If the element returned FAILURE to the previous _set_state() call, this
157 funciton will return FAILURE with the state set to the current state of
158 the element and the pending state set to the value used in the last call
161 - If the element returned ASYNC to the previous _set_state() call, this function
162 will wait for the element to complete its state change up to the amount of time
163 specified in the GTimeVal.
165 * If the element does not complete the state change in the specified amount of
166 time, this function will return ASYNC with the state set to the current state
167 and the pending state set to the pending state.
169 * If the element completes the state change within the specified timeout, this
170 function returns the updated state and VOID_PENDING as the pending state.
172 * If the element aborts the ASYNC state change due to an error within the
173 specified timeout, this function returns FAILURE with the state set to last
174 successfull state and pending set to the last attempt. The element should
175 also post an error message on the bus with more information about the problem.
181 A GstBin manages the state of its children. It does this by propagating the state
182 changes performed on it to all of its children. The _set_state() function on a
183 bin will call the _set_state() function on all of its children.
185 The children are iterated from the sink elements to the source elements. This makes
186 sure that when changing the state of an element, the downstream elements are in
187 the correct state to process the eventual buffers. In the case of a downwards
188 state change, the sink elements will shut down first which makes the upstream
189 elements shut down as well since the _push() function returns a GST_FLOW_WRONG_STATE
192 If all the children return SUCCESS, the function returns SUCCESS as well.
194 If one of the children returns FAILURE, the function returns FAILURE as well. In
195 this state it is possible that some elements successfuly changed state. The
196 application can check which elements have a changed state, which were in error
197 and which were not affected by iterating the elements and calling _get_state()
200 If after calling the state function on all children, one of the children returned
201 ASYNC, the function returns ASYNC as well.
203 If after calling the state function on all children, one of the children returned
204 NO_PREROLL, the function returns NO_PREROLL as well.
206 The current state of the bin can be retrieved with _get_state().
208 If the bin is performing an ASYNC state change, it will automatically update its
209 current state fields when it receives state messages from the children.
212 Implementing states in elements
213 -------------------------------
223 Upward state changes always return ASYNC either if the STATE_PENDING is
233 - element commits state ASYNC
236 - update STATE_PENDING state
238 - no change_state called on element
242 A->B: all elements SUCCESS
245 A->B: some elements ASYNC
247 - listen for commit messages on bus
248 - for each commit message, poll elements
249 - if no ASYNC elements, commit state, continue state change
252 downward state change
253 ----------------------
255 Downward state changes only return ASYNC if the final state is ASYNC.
256 This is to make sure that it's not needed to wait for an element to
257 complete the preroll or other ASYNC state changes when one only wants to
258 shut down an element.
265 A -> B => ASYNC not final state
266 - commit state on behalf of element
268 A -> B => ASYNC final state
269 - element will commit ASYNC
276 A -> B -> ASYNC not final state
277 - commit state on behalf of element, continue state change
279 A -> B => ASYNC final state
281 - listen for commit messages on bus
282 - for each commit message, poll elements
283 - if no ASYNC elements, commit state
286 Locking overview (element)
287 --------------------------
289 * Element commiting SUCCESS
291 - STATE_LOCK is taken in set_state
292 - change state is called if SUCCESS, commit state is called
293 - commit state calls change_state to next state change.
294 - if final state is reached, stack unwinds and result is returned to
295 set_state and caller.
298 set_state(element) change_state (element) commit_state
304 |------------------------>| |
307 | | (do state change) |
311 | |---------------------->|
314 | |<----------------------| if (!final) change_state (next)
319 |<------------------------| |
328 * Element commiting ASYNC
330 - STATE_LOCK is taken in set_state
331 - change state is called and returns ASYNC
332 - ASYNC returned to the caller.
333 - element takes STATE_LOCK in streaming thread.
334 - element calls commit_state in streaming thread.
335 - commit state calls change_state to next state change.
338 set_state(element) change_state (element) stream_thread commit_state (element)
344 |------------------------>| | |
351 |<------------------------| | |
352 | ASYNC STREAM_UNLOCK |
354 | .....sync........ STATE_LOCK |
355 ASYNC |----------------->|
357 | |---> post_message()
358 | |---> if (!final) change_state (next)