mirror of
https://gitlab.freedesktop.org/gstreamer/gstreamer.git
synced 2024-12-24 01:00:37 +00:00
design: states: section breakdown, markup, readability and grammar fixes
This commit is contained in:
Reynaldo H. Verdejo Pinochet
parent
5d7f7c112c
commit
3e315b6b2e
1 changed files with 97 additions and 101 deletions
|
|
@ -4,84 +4,84 @@ Both elements and pads can be in different states. The states of the
|
|||
pads are linked to the state of the element so the design of the states
|
||||
is mainly focused around the element states.
|
||||
|
||||
An element can be in 4 states. NULL, READY, PAUSED and PLAYING. When an
|
||||
An element can be in 4 states. `NULL`, `READY`, `PAUSED` and `PLAYING`. When an
|
||||
element is initially instantiated, it is in the NULL state.
|
||||
|
||||
## State definitions
|
||||
|
||||
- NULL: This is the initial state of an element.
|
||||
- `NULL`: This is the initial state of an element.
|
||||
|
||||
- READY: The element should be prepared to go to PAUSED.
|
||||
- `READY`: The element should be prepared to go to PAUSED.
|
||||
|
||||
- PAUSED: The element should be ready to accept and process data. Sink
|
||||
elements however only accept one buffer and then block.
|
||||
- `PAUSED`: The element should be ready to accept and process data. Sink
|
||||
elements, however, only accept one buffer and then block.
|
||||
|
||||
- PLAYING: The same as PAUSED except for live sources and sinks. Sinks
|
||||
- `PLAYING`: The same as PAUSED except for live sources and sinks. Sinks
|
||||
accept and render data. Live sources produce data.
|
||||
|
||||
We call the sequence NULL→PLAYING an upwards state change and
|
||||
PLAYING→NULL a downwards state change.
|
||||
We call the sequence `NULL→PLAYING` an upwards state change and
|
||||
`PLAYING→NULL` a downwards state change.
|
||||
|
||||
## State transitions
|
||||
|
||||
the following state changes are possible:
|
||||
|
||||
* *NULL -> READY*:
|
||||
* `NULL -> READY`:
|
||||
- The element must check if the resources it needs are available.
|
||||
Device sinks and sources typically try to probe the device to constrain
|
||||
their caps.
|
||||
- The element opens the device, this is needed if the previous step requires
|
||||
the device to be opened.
|
||||
|
||||
* *READY -> PAUSED*:
|
||||
- The element pads are activated in order to receive data in PAUSED.
|
||||
* `READY -> PAUSED`:
|
||||
- The element pads are activated in order to receive data in `PAUSED`.
|
||||
Streaming threads are started.
|
||||
- Some elements might need to return `ASYNC` and complete the state change
|
||||
when they have enough information. It is a requirement for sinks to
|
||||
return `ASYNC` and complete the state change when they receive the first
|
||||
buffer or EOS event (preroll). Sinks also block the dataflow when in PAUSED.
|
||||
- A pipeline resets the running_time to 0.
|
||||
- Live sources return NO_PREROLL and don't generate data.
|
||||
- A pipeline resets the `running_time` to 0.
|
||||
- Live sources return `NO_PREROLL` and don't generate data.
|
||||
|
||||
* *PAUSED -> PLAYING*:
|
||||
* `PAUSED -> PLAYING`:
|
||||
- Most elements ignore this state change.
|
||||
- The pipeline selects a clock and distributes this to all the children
|
||||
before setting them to PLAYING. This means that it is only allowed to
|
||||
synchronize on the clock in the PLAYING state.
|
||||
- The pipeline uses the clock and the running_time to calculate the base_time.
|
||||
The base_time is distributed to all children when performing the state
|
||||
change.
|
||||
before setting them to `PLAYING`. This means that it is only allowed to
|
||||
synchronize on the clock in the `PLAYING` state.
|
||||
- The pipeline uses the clock and the `running_time` to calculate the
|
||||
`base_time`. This `base_time` is distributed to all children when
|
||||
performing the state change.
|
||||
- Sink elements stop blocking on the preroll buffer or event and start
|
||||
rendering the data.
|
||||
- Sinks can post the EOS message in the PLAYING state. It is not allowed to
|
||||
post EOS when not in the PLAYING state.
|
||||
- While streaming in PAUSED or PLAYING elements can create and remove
|
||||
- Sinks can post the EOS message in the `PLAYING` state. It is not allowed to
|
||||
post EOS when not in the `PLAYING` state.
|
||||
- While streaming in `PAUSED` or `PLAYING` elements can create and remove
|
||||
sometimes pads.
|
||||
- Live sources start generating data and return SUCCESS.
|
||||
- Live sources start generating data and return `SUCCESS`.
|
||||
|
||||
* *PLAYING -> PAUSED*:
|
||||
* `PLAYING -> PAUSED`:
|
||||
- Most elements ignore this state change.
|
||||
- The pipeline calculates the running_time based on the last selected clock
|
||||
and the base_time. It stores this information to continue playback when
|
||||
going back to the PLAYING state.
|
||||
- The pipeline calculates the `running_time` based on the last selected clock
|
||||
and the `base_time`. It stores this information to continue playback when
|
||||
going back to the `PLAYING` state.
|
||||
- Sinks unblock any clock wait calls.
|
||||
- When a sink does not have a pending buffer to play, it returns `ASYNC` from
|
||||
this state change and completes the state change when it receives a new
|
||||
buffer or an EOS event.
|
||||
- Any queued EOS messages are removed since they will be reposted when going
|
||||
back to the PLAYING state. The EOS messages are queued in GstBins.
|
||||
- Live sources stop generating data and return NO_PREROLL.
|
||||
back to the PLAYING state. The EOS messages are queued in `GstBins`.
|
||||
- Live sources stop generating data and return `NO_PREROLL`.
|
||||
|
||||
* *PAUSED -> READY*:
|
||||
* `PAUSED -> READY`:
|
||||
- Sinks unblock any waits in the preroll.
|
||||
- Elements unblock any waits on devices
|
||||
- Chain or get_range functions return FLUSHING.
|
||||
- Chain or `get_range()` functions return `FLUSHING`.
|
||||
- The element pads are deactivated so that streaming becomes impossible and
|
||||
all streaming threads are stopped.
|
||||
- The sink forgets all negotiated formats
|
||||
- Elements remove all sometimes pads
|
||||
|
||||
* *READY -> NULL*:
|
||||
* `READY -> NULL`:
|
||||
- Elements close devices
|
||||
- Elements reset any internal state.
|
||||
|
||||
|
|
@ -89,82 +89,81 @@ the following state changes are possible:
|
|||
|
||||
An element has 4 state variables that are protected with the object LOCK:
|
||||
|
||||
- *STATE*
|
||||
- *STATE_NEXT*
|
||||
- *STATE_PENDING*
|
||||
- *STATE_RETURN*
|
||||
- `STATE`
|
||||
- `STATE_NEXT`
|
||||
- `STATE_PENDING`
|
||||
- `STATE_RETURN`
|
||||
|
||||
The STATE always reflects the current state of the element. The
|
||||
STATE\_NEXT reflects the next state the element will go to. The
|
||||
STATE\_PENDING always reflects the required state of the element. The
|
||||
STATE\_RETURN reflects the last return value of a state change.
|
||||
The `STATE` always reflects the current state of the element. The
|
||||
`STATE_NEXT` reflects the next state the element will go to. The
|
||||
`STATE_PENDING` always reflects the required state of the element. The
|
||||
`STATE_RETURN` reflects the last return value of a state change.
|
||||
|
||||
The STATE\_NEXT and STATE\_PENDING can be VOID\_PENDING if the element
|
||||
The `STATE_NEXT` and `STATE_PENDING` can be `VOID_PENDING` if the element
|
||||
is in the right state.
|
||||
|
||||
An element has a special lock to protect against concurrent invocations
|
||||
of set\_state(), called the STATE\_LOCK.
|
||||
of `set_state()`, called the `STATE_LOCK`.
|
||||
|
||||
## Setting state on elements
|
||||
|
||||
The state of an element can be changed with \_element\_set\_state().
|
||||
The state of an element can be changed with `_element_set_state()`.
|
||||
When changing the state of an element all intermediate states will also
|
||||
be set on the element until the final desired state is set.
|
||||
|
||||
The `set\_state()` function can return 3 possible values:
|
||||
The `set_state()` function can return 3 possible values:
|
||||
|
||||
* *GST_STATE_FAILURE*: The state change failed for some reason. The plugin should
|
||||
have posted an error message on the bus with information.
|
||||
* `GST_STATE_FAILURE`: The state change failed for some reason. The plugin should have posted an error message on the bus with information.
|
||||
|
||||
* *GST_STATE_SUCCESS*: The state change is completed successfully.
|
||||
* `GST_STATE_SUCCESS`: The state change is completed successfully.
|
||||
|
||||
* *GST_STATE_ASYNC*: The state change will complete later on. This can happen
|
||||
* `GST_STATE_ASYNC`: The state change will complete later on. This can happen
|
||||
when the element needs a long time to perform the state change or for sinks
|
||||
that need to receive the first buffer before they can complete the state change
|
||||
(preroll).
|
||||
|
||||
* *GST_STATE_NO_PREROLL*: The state change is completed successfully but the
|
||||
element will not be able to produce data in the PAUSED state.
|
||||
* `GST_STATE_NO_PREROLL`: The state change is completed successfully but the
|
||||
element will not be able to produce data in the `PAUSED` state.
|
||||
|
||||
In the case of an `ASYNC` state change, it is possible to proceed to the
|
||||
next state before the current state change completed, however, the
|
||||
next state before the current state change completes, however, the
|
||||
element will only get to this next state before completing the previous
|
||||
`ASYNC` state change. After receiving an `ASYNC` return value, you can use
|
||||
`element\_get\_state()` to poll the status of the element. If the
|
||||
`element_get_state()` to poll the status of the element. If the
|
||||
polling returns `SUCCESS`, the element completed the state change to the
|
||||
last requested state with `set\_state()`.
|
||||
last requested state with `set_state()`.
|
||||
|
||||
When setting the state of an element, the STATE\_PENDING is set to the
|
||||
When setting the state of an element, the `STATE_PENDING` is set to the
|
||||
required state. Then the state change function of the element is called
|
||||
and the result of that function is used to update the STATE and
|
||||
STATE\_RETURN fields, STATE\_NEXT, STATE\_PENDING and STATE\_RETURN
|
||||
and the result of that function is used to update the `STATE` and
|
||||
`STATE_RETURN` fields, `STATE_NEXT`, `STATE_PENDING` and `STATE_RETURN`
|
||||
fields. If the function returned `ASYNC`, this result is immediately
|
||||
returned to the caller.
|
||||
|
||||
## Getting state of elements
|
||||
## Getting the state of elements
|
||||
|
||||
The get\_state() function takes 3 arguments, two pointers that will
|
||||
hold the current and pending state and one GstClockTime that holds a
|
||||
timeout value. The function returns a GstElementStateReturn.
|
||||
The `get_state()` function takes 3 arguments, two pointers that will
|
||||
hold the current and pending state and one `GstClockTime` that holds a
|
||||
timeout value. The function returns a `GstElementStateReturn`.
|
||||
|
||||
- If the element returned `SUCCESS` to the previous \_set\_state()
|
||||
- If the element returned `SUCCESS` to the previous `_set_state()`
|
||||
function, this function will return the last state set on the
|
||||
element and VOID\_PENDING in the pending state value. The function
|
||||
returns GST\_STATE\_SUCCESS.
|
||||
element and `VOID_PENDING` in the pending state value. The function
|
||||
returns `GST_STATE_SUCCESS`.
|
||||
|
||||
- If the element returned NO\_PREROLL to the previous \_set\_state()
|
||||
- If the element returned `NO_PREROLL` to the previous `_set_state()`
|
||||
function, this function will return the last state set on the
|
||||
element and VOID\_PENDING in the pending state value. The function
|
||||
returns GST\_STATE\_NO\_PREROLL.
|
||||
element and `VOID_PENDING` in the pending state value. The function
|
||||
returns `GST_STATE_NO_PREROLL`.
|
||||
|
||||
- If the element returned FAILURE to the previous \_set\_state() call,
|
||||
this function will return FAILURE with the state set to the current
|
||||
- If the element returned `FAILURE` to the previous `_set_state()` call,
|
||||
this function will return `FAILURE` with the state set to the current
|
||||
state of the element and the pending state set to the value used in
|
||||
the last call of \_set\_state().
|
||||
the last call of `_set_state()`.
|
||||
|
||||
- If the element returned `ASYNC` to the previous \_set\_state() call,
|
||||
- If the element returned `ASYNC` to the previous `_set_state()` call,
|
||||
this function will wait for the element to complete its state change
|
||||
up to the amount of time specified in the GstClockTime.
|
||||
up to the amount of time specified in the `GstClockTime`.
|
||||
|
||||
- If the element does not complete the state change in the
|
||||
specified amount of time, this function will return `ASYNC` with
|
||||
|
|
@ -173,19 +172,19 @@ timeout value. The function returns a GstElementStateReturn.
|
|||
|
||||
- If the element completes the state change within the specified
|
||||
timeout, this function returns the updated state and
|
||||
VOID\_PENDING as the pending state.
|
||||
`VOID_PENDING` as the pending state.
|
||||
|
||||
- If the element aborts the `ASYNC` state change due to an error
|
||||
within the specified timeout, this function returns FAILURE with
|
||||
within the specified timeout, this function returns `FAILURE` with
|
||||
the state set to last successful state and pending set to the
|
||||
last attempt. The element should also post an error message on
|
||||
the bus with more information about the problem.
|
||||
|
||||
## States in GstBin
|
||||
|
||||
A GstBin manages the state of its children. It does this by propagating
|
||||
A `GstBin` manages the state of its children. It does this by propagating
|
||||
the state changes performed on it to all of its children. The
|
||||
\_set\_state() function on a bin will call the \_set\_state() function
|
||||
`_set_state()` function on a bin will call the `_set_state()` function
|
||||
on all of its children, that are not already in the target state or in a
|
||||
change state to the target state.
|
||||
|
||||
|
|
@ -194,27 +193,27 @@ This makes sure that when changing the state of an element, the
|
|||
downstream elements are in the correct state to process the eventual
|
||||
buffers. In the case of a downwards state change, the sink elements will
|
||||
shut down first which makes the upstream elements shut down as well
|
||||
since the \_push() function returns a GST\_FLOW\_FLUSHING error.
|
||||
since the `_push()` function returns a `GST_FLOW_FLUSHING` error.
|
||||
|
||||
If all the children return `SUCCESS`, the function returns `SUCCESS` as
|
||||
well.
|
||||
|
||||
If one of the children returns FAILURE, the function returns FAILURE as
|
||||
If one of the children returns `FAILURE`, the function returns `FAILURE` as
|
||||
well. In this state it is possible that some elements successfully
|
||||
changed state. The application can check which elements have a changed
|
||||
state, which were in error and which were not affected by iterating the
|
||||
elements and calling \_get\_state() on the elements.
|
||||
elements and calling `_get_state()` on the elements.
|
||||
|
||||
If after calling the state function on all children, one of the children
|
||||
returned `ASYNC`, the function returns `ASYNC` as well.
|
||||
|
||||
If after calling the state function on all children, one of the children
|
||||
returned NO\_PREROLL, the function returns NO\_PREROLL as well.
|
||||
returned `NO_PREROLL`, the function returns `NO_PREROLL` as well.
|
||||
|
||||
If both NO\_PREROLL and `ASYNC` children are present, NO\_PREROLL is
|
||||
If both `NO_PREROLL` and `ASYNC` children are present, `NO_PREROLL` is
|
||||
returned.
|
||||
|
||||
The current state of the bin can be retrieved with \_get\_state().
|
||||
The current state of the bin can be retrieved with `_get_state()`.
|
||||
|
||||
If the bin is performing an `ASYNC` state change, it will automatically
|
||||
update its current state fields when it receives state messages from the
|
||||
|
|
@ -222,11 +221,9 @@ children.
|
|||
|
||||
## Implementing states in elements
|
||||
|
||||
### READY
|
||||
### Upward state change
|
||||
|
||||
## upward state change
|
||||
|
||||
Upward state changes always return `ASYNC` either if the STATE\_PENDING is
|
||||
Upward state changes always return `ASYNC` either if the `STATE_PENDING` is
|
||||
reached or not.
|
||||
|
||||
Element:
|
||||
|
|
@ -239,9 +236,9 @@ Element:
|
|||
- element commits state `ASYNC`
|
||||
|
||||
* A -> B while `ASYNC`
|
||||
- update STATE_PENDING state
|
||||
- update `STATE_PENDING` state
|
||||
- no commit state
|
||||
- no change_state called on element
|
||||
- no `change_state()` called on element
|
||||
|
||||
Bin:
|
||||
|
||||
|
|
@ -254,11 +251,11 @@ Bin:
|
|||
- for each commit message, poll elements, this happens in another
|
||||
thread.
|
||||
- if no `ASYNC` elements, commit state, continue state change
|
||||
to STATE_PENDING
|
||||
to `STATE_PENDING`
|
||||
|
||||
## downward state change
|
||||
### Downward state change
|
||||
|
||||
Downward state changes only return `ASYNC` if the final state is ASYNC.
|
||||
Downward state changes only return `ASYNC` if the final state is `ASYNC`.
|
||||
This is to make sure that it’s not needed to wait for an element to
|
||||
complete the preroll or other `ASYNC` state changes when one only wants to
|
||||
shut down an element.
|
||||
|
|
@ -292,15 +289,14 @@ A -> B => `ASYNC` final state
|
|||
|
||||
- Element committing `SUCCESS`
|
||||
|
||||
- STATE\_LOCK is taken in set\_state
|
||||
- `STATE_LOCK` is taken in `set_state()`
|
||||
|
||||
- change state is called if `SUCCESS`, commit state is called
|
||||
|
||||
- commit state calls change\_state to next state change.
|
||||
- commit state calls `change_state()` to next state change.
|
||||
|
||||
- if final state is reached, stack unwinds and result is returned
|
||||
to set\_state and
|
||||
caller.
|
||||
to `set_state()` and caller.
|
||||
|
||||
```
|
||||
set_state(element) change_state (element) commit_state
|
||||
|
|
@ -334,7 +330,7 @@ STATE_UNLOCK
|
|||
|
||||
- Element committing `ASYNC`
|
||||
|
||||
- STATE\_LOCK is taken in set\_state
|
||||
- `STATE_LOCK` is taken in `set_state()`
|
||||
|
||||
- change state is called and returns `ASYNC`
|
||||
|
||||
|
|
@ -342,9 +338,9 @@ STATE_UNLOCK
|
|||
|
||||
- element takes LOCK in streaming thread.
|
||||
|
||||
- element calls commit\_state in streaming thread.
|
||||
- element calls `commit_state` in streaming thread.
|
||||
|
||||
- commit state calls change\_state to next state
|
||||
- commit state calls `change_state()` to next state
|
||||
change.
|
||||
|
||||
```
|
||||
|
|
@ -380,13 +376,13 @@ STATE_UNLOCK | |
|
|||
|
||||
## Remarks
|
||||
|
||||
set\_state cannot be called from multiple threads at the same time. The
|
||||
STATE\_LOCK prevents this.
|
||||
`set_state()` cannot be called from multiple threads at the same time. The
|
||||
`STATE_LOCK` prevents this.
|
||||
|
||||
State variables are protected with the LOCK.
|
||||
|
||||
Calling set\_state while gst\_state is called should unlock the
|
||||
get\_state with an error. The cookie will do that.
|
||||
Calling `set_state()` while `get_state()` is called should unlock the
|
||||
`get_state()` with an error. The cookie will do that.
|
||||
|
||||
``` c
|
||||
set_state(element)
|
||||
|
|
|
|||
Loading…
Reference in a new issue