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docs: update activation design docs

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This commit is contained in:
Wim Taymans 2012-09-28 11:18:11 +02:00
parent ef012f8e58
commit 3009f73b14
1 changed files with 22 additions and 21 deletions
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43
docs/design/part-activation.txt
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@ -27,49 +27,50 @@ function of the pad.
Because the core does not know in which mode to activate a pad (PUSH or
PULL), it delegates that choice to a method on the pad, activate(). The
activate() function of a pad should choose whether to operate in PUSH or
PULL mode. Once the choice is made, it should call one of the two
mode-specific activation functions, activate_push() or activate_pull().
The default activate() function will call activate_push(), as it is the
default mechanism for data flow. A sink pad that supports either mode of
operation might call activate_pull() if calling check_get_range()
returns TRUE, and activate_push() otherwise.
PULL mode. Once the choice is made, it should call activate_mode()
with the selected activation mode.
The default activate() function will call activate_mode() with
#GST_PAD_MODE_PUSH, as it is the default mechanism for data flow.
A sink pad that supports either mode of operation might call
activate_mode(PULL) if the SCHEDULING query upstream contains the
#GST_PAD_MODE_PULL scheduling mode, and activate_mode(PUSH) otherwise.
Consider the case fakesrc ! fakesink, where fakesink is configured to
operate in PULL mode. State changes in the pipeline will start with
fakesink, which is the most downstream element. The core will call
activate() on fakesink's sink pad. For fakesink to go into PULL mode, it
needs to implement a custom activate() function that will call
activate_pull() on its sink pad (because the default is to use PUSH
mode). activate_pull() is then responsible for starting the task that
pulls from fakesrc:src. Clearly, fakesrc needs to be notified that
fakesrc is about to pull on its src pad, even though the pipeline has
not yet changed fakesrc's state. For this reason, activate_pull() must
first call activate_pull() on fakesink:sink's peer before starting
fakesink's task.
activate_mode(PULL) on its sink pad (because the default is to
use PUSH mode). activate_mode(PULL) is then responsible for starting
the task that pulls from fakesrc:src. Clearly, fakesrc needs to be
notified that fakesrc is about to pull on its src pad, even though the
pipeline has not yet changed fakesrc's state. For this reason,
GStreamer will first call call activate_mode(PULL) on fakesink:sink's
peer before calling activate_mode(PULL) on fakesink:sinks.
In short, upstream elements operating in PULL mode must be ready to
produce data in READY, after having activate_pull() called on their
source pad. Also, a call to activate_pull() needs to propagate through
produce data in READY, after having activate_mode(PULL) called on their
source pad. Also, a call to activate_mode(PULL) needs to propagate through
the pipeline to every pad that a gst_pad_pull() will reach. In the case
fakesrc ! identity ! fakesink, calling activate_pull() on identity's
fakesrc ! identity ! fakesink, calling activate_mode(PULL) on identity's
source pad would need to activate its sink pad in pull mode as well,
which should propagate all the way to fakesrc.
If, on the other hand, fakesrc ! fakesink is operating in PUSH mode, the
activation sequence is different. First, activate() on fakesink:sink
calls activate_push() on fakesink:sink. Then fakesrc's pads are
calls activate_mode(PUSH) on fakesink:sink. Then fakesrc's pads are
activated: sources first, then sinks (of which fakesrc has none).
fakesrc:src's activation function is then called.
Note that it does not make sense to set an activation function on a
source pad. The peer of a source pad is downstream, meaning it should
have been activated first. If it was activated in PULL mode, the
source pad should have already had activate_pull() called on it, and
source pad should have already had activate_mode(PULL) called on it, and
thus needs no further activation. Otherwise it should be in PUSH mode,
which is the choice of the default activation function.
So, in the PUSH case, the default activation function chooses PUSH mode,
which calls activate_push(), which will then start a task on the source
which calls activate_mode(PUSH), which will then start a task on the source
pad and begin pushing. In this way PUSH scheduling is a bit easier,
because it follows the order of state changes in a pipeline. fakesink is
already in PAUSED with an active sink pad by the time fakesrc starts
@ -81,8 +82,8 @@ Deactivation
Pad deactivation occurs when its parent goes into the READY state or when the
pad is deactivated explicitly by the application or element.
gst_pad_set_active() is called with a FALSE argument, which then calls
activate_push() or activate_pull() with a FALSE argument, depending on the
activation mode of the pad.
activate_mode(PUSH) or activate_mode(PULL) with a FALSE argument, depending
on the current activation mode of the pad.
Mode switching
~~~~~~~~~~~~~~