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 4 state variables that are protected with the object LOCK:
94 The STATE always reflects the current state of the element.
95 The STATE_NEXT reflects the next state the element will go to.
96 The STATE_PENDING always reflects the required state of the element.
97 The STATE_RETURN reflects the last return value of a state change.
99 The STATE_NEXT and STATE_PENDING can be VOID_PENDING if the element is in
102 An element has a special lock to protect against concurrent invocations of
103 _set_state(), called the STATE_LOCK.
106 Setting state on elements
107 -------------------------
109 The state of an element can be changed with _element_set_state(). When chaning
110 the state of an element all intermediate states will also be set on the element
111 until the final desired state is set.
113 The _set_state() function can return 3 possible values:
115 GST_STATE_FAILURE: The state change failed for some reason. The plugin should
116 have posted an error message on the bus with information.
118 GST_STATE_SUCCESS: The state change is completed successfully.
120 GST_STATE_ASYNC: The state change will complete later on. This can happen
121 When the element needs a long time to perform the state
122 change or for sinks that need to receive the first buffer
123 before they can complete the state change (preroll).
125 GST_STATE_NO_PREROLL: The state change is completed successfully but the element
126 will not be able to produce data in the PAUSED state.
128 In the case of an async state change, it is possible to proceed to the next
129 state before the current state change completed, however, the element will only
130 get to this next state before completing the previous ASYNC state change.
131 After receiving an ASYNC return value, you can use _element_get_state() to poll
132 the status of the element. If the polling returns SUCCESS, the element completed
133 the state change to the last requested state with _set_state().
135 When setting the state of an element, the STATE_PENDING is set to the required
136 state. Then the state change function of the element is called and the result of
137 that function is used to update the STATE and STATE_RETURN fields, STATE_NEXT,
138 STATE_PENDING and STATE_RETURN fields. If the function returned ASYNC, this result
139 is immediatly returned to the caller.
142 Getting state of elements
143 -------------------------
145 The _get_state() function takes 3 arguments, two pointers that will hold the
146 current and pending state and one GstClockTime that holds a timeout value. The
147 function returns a GstElementStateReturn.
149 - If the element returned SUCCESS to the previous _set_state() function, this
150 function will return the last state set on the element and VOID_PENDING in
151 the pending state value. The function returns GST_STATE_SUCCESS.
153 - If the element returned NO_PREROLL to the previous _set_state() function, this
154 function will return the last state set on the element and VOID_PENDING in
155 the pending state value. The function returns GST_STATE_NO_PREROLL.
157 - If the element returned FAILURE to the previous _set_state() call, this
158 funciton will return FAILURE with the state set to the current state of
159 the element and the pending state set to the value used in the last call
162 - If the element returned ASYNC to the previous _set_state() call, this function
163 will wait for the element to complete its state change up to the amount of time
164 specified in the GstClockTime.
166 * If the element does not complete the state change in the specified amount of
167 time, this function will return ASYNC with the state set to the current state
168 and the pending state set to the pending state.
170 * If the element completes the state change within the specified timeout, this
171 function returns the updated state and VOID_PENDING as the pending state.
173 * If the element aborts the ASYNC state change due to an error within the
174 specified timeout, this function returns FAILURE with the state set to last
175 successfull state and pending set to the last attempt. The element should
176 also post an error message on the bus with more information about the problem.
182 A GstBin manages the state of its children. It does this by propagating the state
183 changes performed on it to all of its children. The _set_state() function on a
184 bin will call the _set_state() function on all of its children.
186 The children are iterated from the sink elements to the source elements. This makes
187 sure that when changing the state of an element, the downstream elements are in
188 the correct state to process the eventual buffers. In the case of a downwards
189 state change, the sink elements will shut down first which makes the upstream
190 elements shut down as well since the _push() function returns a GST_FLOW_WRONG_STATE
193 If all the children return SUCCESS, the function returns SUCCESS as well.
195 If one of the children returns FAILURE, the function returns FAILURE as well. In
196 this state it is possible that some elements successfuly changed state. The
197 application can check which elements have a changed state, which were in error
198 and which were not affected by iterating the elements and calling _get_state()
201 If after calling the state function on all children, one of the children returned
202 ASYNC, the function returns ASYNC as well.
204 If after calling the state function on all children, one of the children returned
205 NO_PREROLL, the function returns NO_PREROLL as well.
207 The current state of the bin can be retrieved with _get_state().
209 If the bin is performing an ASYNC state change, it will automatically update its
210 current state fields when it receives state messages from the children.
213 Implementing states in elements
214 -------------------------------
224 Upward state changes always return ASYNC either if the STATE_PENDING is
234 - element commits state ASYNC
237 - update STATE_PENDING state
239 - no change_state called on element
243 A->B: all elements SUCCESS
246 A->B: some elements ASYNC
248 - listen for commit messages on bus
249 - for each commit message, poll elements
250 - if no ASYNC elements, commit state, continue state change
253 downward state change
254 ----------------------
256 Downward state changes only return ASYNC if the final state is ASYNC.
257 This is to make sure that it's not needed to wait for an element to
258 complete the preroll or other ASYNC state changes when one only wants to
259 shut down an element.
266 A -> B => ASYNC not final state
267 - commit state on behalf of element
269 A -> B => ASYNC final state
270 - element will commit ASYNC
277 A -> B -> ASYNC not final state
278 - commit state on behalf of element, continue state change
280 A -> B => ASYNC final state
282 - listen for commit messages on bus
283 - for each commit message, poll elements
284 - if no ASYNC elements, commit state
287 Locking overview (element)
288 --------------------------
290 * Element commiting SUCCESS
292 - STATE_LOCK is taken in set_state
293 - change state is called if SUCCESS, commit state is called
294 - commit state calls change_state to next state change.
295 - if final state is reached, stack unwinds and result is returned to
296 set_state and caller.
299 set_state(element) change_state (element) commit_state
305 |------------------------>| |
308 | | (do state change) |
312 | |---------------------->|
315 | |<----------------------| if (!final) change_state (next)
320 |<------------------------| |
329 * Element commiting ASYNC
331 - STATE_LOCK is taken in set_state
332 - change state is called and returns ASYNC
333 - ASYNC returned to the caller.
334 - element takes STATE_LOCK in streaming thread.
335 - element calls commit_state in streaming thread.
336 - commit state calls change_state to next state change.
339 set_state(element) change_state (element) stream_thread commit_state (element)
345 |------------------------>| | |
352 |<------------------------| | |
353 | ASYNC STREAM_UNLOCK |
355 | .....sync........ STATE_LOCK |
356 ASYNC |----------------->|
358 | |---> post_message()
359 | |---> if (!final) change_state (next)
368 *********************************************
369 *********************************************
371 set_state cannot be called from multiple threads at the same time. The STATE_LOCK
374 state variables are protected with the LOCK.
376 calling set_state while gst_state is called should unlock the get_state with
377 an error. The cookie will do that.
385 update current, next, pending state