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 live sources and sinks. Sinks accept
20 and rendering data. Live sources produce data.
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 Device sinks and -sources typically try to probe the device to constain
35 - The element opens the device, this is needed if the previous step requires
36 the device to be opened.
39 - the element pads are activated in order to receive data in PAUSED.
40 Streaming threads are started.
41 - some elements might need to return ASYNC and complete the state change
42 when they have enough information. It is a requirement for sinks to
43 return ASYNC and complete the state change when they receive the first
44 buffer or EOS event (prerol). Sinks also block the dataflow when in PAUSED.
45 - a pipeline resets the running_time to 0.
46 - live sources return NO_PREROLL and don't generate data.
49 - most elements ignore this state change.
50 - The pipeline selects a clock and distributes this to all the children
51 before setting them to PLAYING. This means that it is only alowed to
52 synchronize on the clock in the PLAYING state.
53 - The pipeline uses the clock and the running_time to calculate the base_time.
54 The base_time is distributed to all children when performing the state
56 - sink elements stop blocking on the preroll buffer or event and start
58 - sinks can post the EOS message in the PLAYING state. It is not allowed to
59 post EOS when not in the PLAYING state.
60 - while streaming in PAUSED or PLAYING elements can create and remove
62 - live sources start generating data and return SUCCESS.
65 - most elements ignore this state change.
66 - The pipeline calculates the running_time based on the last selected clock
67 and the base_time. It stores this information to continue playback when
68 going back to the PLAYING state.
69 - sinks unblock any clock wait calls.
70 - when a sink did not have a pending buffer to play, it returns ASYNC from
71 this state change and complete the state change when they receive a new buffer
73 - any queued EOS messages are removed since they will be reposted when going
74 back to the PLAYING state. The EOS messages are queued in GstBins.
75 - live sources stop generating data and return NO_PREROLL.
78 - sinks unblock any waits in the preroll.
79 - elements unblock any waits on devices
80 - chain or get_range functions return WRONG_STATE.
81 - the element pads are deactivated so that streaming becomes impossible and
82 all streaming threads are stopped.
83 - the sink forgets all negotiated formats
86 - elements close devices
87 - element removes any dynamically created pads
93 An element has 4 state variables that are protected with the object LOCK:
100 The STATE always reflects the current state of the element.
101 The STATE_NEXT reflects the next state the element will go to.
102 The STATE_PENDING always reflects the required state of the element.
103 The STATE_RETURN reflects the last return value of a state change.
105 The STATE_NEXT and STATE_PENDING can be VOID_PENDING if the element is in
108 An element has a special lock to protect against concurrent invocations of
109 _set_state(), called the STATE_LOCK.
112 Setting state on elements
113 -------------------------
115 The state of an element can be changed with _element_set_state(). When chaning
116 the state of an element all intermediate states will also be set on the element
117 until the final desired state is set.
119 The _set_state() function can return 3 possible values:
121 GST_STATE_FAILURE: The state change failed for some reason. The plugin should
122 have posted an error message on the bus with information.
124 GST_STATE_SUCCESS: The state change is completed successfully.
126 GST_STATE_ASYNC: The state change will complete later on. This can happen
127 When the element needs a long time to perform the state
128 change or for sinks that need to receive the first buffer
129 before they can complete the state change (preroll).
131 GST_STATE_NO_PREROLL: The state change is completed successfully but the element
132 will not be able to produce data in the PAUSED state.
134 In the case of an ASYNC state change, it is possible to proceed to the next
135 state before the current state change completed, however, the element will only
136 get to this next state before completing the previous ASYNC state change.
137 After receiving an ASYNC return value, you can use _element_get_state() to poll
138 the status of the element. If the polling returns SUCCESS, the element completed
139 the state change to the last requested state with _set_state().
141 When setting the state of an element, the STATE_PENDING is set to the required
142 state. Then the state change function of the element is called and the result of
143 that function is used to update the STATE and STATE_RETURN fields, STATE_NEXT,
144 STATE_PENDING and STATE_RETURN fields. If the function returned ASYNC, this result
145 is immediatly returned to the caller.
148 Getting state of elements
149 -------------------------
151 The _get_state() function takes 3 arguments, two pointers that will hold the
152 current and pending state and one GstClockTime that holds a timeout value. The
153 function returns a GstElementStateReturn.
155 - If the element returned SUCCESS 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_SUCCESS.
159 - If the element returned NO_PREROLL to the previous _set_state() function, this
160 function will return the last state set on the element and VOID_PENDING in
161 the pending state value. The function returns GST_STATE_NO_PREROLL.
163 - If the element returned FAILURE to the previous _set_state() call, this
164 funciton will return FAILURE with the state set to the current state of
165 the element and the pending state set to the value used in the last call
168 - If the element returned ASYNC to the previous _set_state() call, this function
169 will wait for the element to complete its state change up to the amount of time
170 specified in the GstClockTime.
172 * If the element does not complete the state change in the specified amount of
173 time, this function will return ASYNC with the state set to the current state
174 and the pending state set to the pending state.
176 * If the element completes the state change within the specified timeout, this
177 function returns the updated state and VOID_PENDING as the pending state.
179 * If the element aborts the ASYNC state change due to an error within the
180 specified timeout, this function returns FAILURE with the state set to last
181 successfull state and pending set to the last attempt. The element should
182 also post an error message on the bus with more information about the problem.
188 A GstBin manages the state of its children. It does this by propagating the state
189 changes performed on it to all of its children. The _set_state() function on a
190 bin will call the _set_state() function on all of its children.
192 The children are iterated from the sink elements to the source elements. This makes
193 sure that when changing the state of an element, the downstream elements are in
194 the correct state to process the eventual buffers. In the case of a downwards
195 state change, the sink elements will shut down first which makes the upstream
196 elements shut down as well since the _push() function returns a GST_FLOW_WRONG_STATE
199 If all the children return SUCCESS, the function returns SUCCESS as well.
201 If one of the children returns FAILURE, the function returns FAILURE as well. In
202 this state it is possible that some elements successfuly changed state. The
203 application can check which elements have a changed state, which were in error
204 and which were not affected by iterating the elements and calling _get_state()
207 If after calling the state function on all children, one of the children returned
208 ASYNC, the function returns ASYNC as well.
210 If after calling the state function on all children, one of the children returned
211 NO_PREROLL, the function returns NO_PREROLL as well.
213 If both NO_PREROLL and ASYNC children are present, NO_PREROLL is returned.
215 The current state of the bin can be retrieved with _get_state().
217 If the bin is performing an ASYNC state change, it will automatically update its
218 current state fields when it receives state messages from the children.
221 Implementing states in elements
222 -------------------------------
232 Upward state changes always return ASYNC either if the STATE_PENDING is
242 - element commits state ASYNC
245 - update STATE_PENDING state
247 - no change_state called on element
251 A->B: all elements SUCCESS
254 A->B: some elements ASYNC
256 - listen for commit messages on bus
257 - for each commit message, poll elements, this happens in another
259 - if no ASYNC elements, commit state, continue state change
262 downward state change
263 ----------------------
265 Downward state changes only return ASYNC if the final state is ASYNC.
266 This is to make sure that it's not needed to wait for an element to
267 complete the preroll or other ASYNC state changes when one only wants to
268 shut down an element.
275 A -> B => ASYNC not final state
276 - commit state on behalf of element
278 A -> B => ASYNC final state
279 - element will commit ASYNC
286 A -> B -> ASYNC not final state
287 - commit state on behalf of element, continue state change
289 A -> B => ASYNC final state
291 - listen for commit messages on bus
292 - for each commit message, poll elements
293 - if no ASYNC elements, commit state
296 Locking overview (element)
297 --------------------------
299 * Element commiting SUCCESS
301 - STATE_LOCK is taken in set_state
302 - change state is called if SUCCESS, commit state is called
303 - commit state calls change_state to next state change.
304 - if final state is reached, stack unwinds and result is returned to
305 set_state and caller.
308 set_state(element) change_state (element) commit_state
314 |------------------------>| |
317 | | (do state change) |
321 | |---------------------->|
324 | |<----------------------| if (!final) change_state (next)
329 |<------------------------| |
338 * Element commiting ASYNC
340 - STATE_LOCK is taken in set_state
341 - change state is called and returns ASYNC
342 - ASYNC returned to the caller.
343 - element takes LOCK in streaming thread.
344 - element calls commit_state in streaming thread.
345 - commit state calls change_state to next state change.
348 set_state(element) change_state (element) stream_thread commit_state (element)
354 |------------------------>| | |
361 |<------------------------| | |
362 | ASYNC STREAM_UNLOCK |
364 | .....sync........ STATE_LOCK |
365 ASYNC |----------------->|
367 | |---> post_message()
368 | |---> if (!final) change_state (next)
377 *********************************************
378 *********************************************
380 set_state cannot be called from multiple threads at the same time. The STATE_LOCK
383 state variables are protected with the LOCK.
385 calling set_state while gst_state is called should unlock the get_state with
386 an error. The cookie will do that.
394 update current, next, pending state