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93 lines
4.5 KiB
Text
93 lines
4.5 KiB
Text
Pad (de)activation
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------------------
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Activation
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~~~~~~~~~~
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When changing states, a bin will set the state on all of its children in
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sink-to-source order. As elements undergo the READY->PAUSED transition,
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their pads are activated so as to prepare for data flow. Some pads will
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start tasks to drive the data flow.
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An element activates its pads from sourcepads to sinkpads. This to make
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sure that when the sinkpads are activated and ready to accept data, the
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sourcepads are already active to pass the data downstream.
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Pads can be activated in one of two modes, PUSH and PULL. PUSH pads are
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the normal case, where the source pad in a link sends data to the sink
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pad via gst_pad_push(). PULL pads instead have sink pads request data
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from the source pads via gst_pad_pull_range().
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To activate a pad, the core will call gst_pad_set_active() with a TRUE
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argument, indicating that the pad should be active. If the pad is
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already active, be it in a PUSH or PULL mode, gst_pad_set_active() will
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return without doing anything. Otherwise it will call the activation
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function of the pad.
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Because the core does not know in which mode to activate a pad (PUSH or
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PULL), it delegates that choice to a method on the pad, activate(). The
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activate() function of a pad should choose whether to operate in PUSH or
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PULL mode. Once the choice is made, it should call activate_mode()
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with the selected activation mode.
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The default activate() function will call activate_mode() with
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#GST_PAD_MODE_PUSH, as it is the default mechanism for data flow.
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A sink pad that supports either mode of operation might call
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activate_mode(PULL) if the SCHEDULING query upstream contains the
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#GST_PAD_MODE_PULL scheduling mode, and activate_mode(PUSH) otherwise.
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Consider the case fakesrc ! fakesink, where fakesink is configured to
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operate in PULL mode. State changes in the pipeline will start with
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fakesink, which is the most downstream element. The core will call
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activate() on fakesink's sink pad. For fakesink to go into PULL mode, it
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needs to implement a custom activate() function that will call
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activate_mode(PULL) on its sink pad (because the default is to
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use PUSH mode). activate_mode(PULL) is then responsible for starting
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the task that pulls from fakesrc:src. Clearly, fakesrc needs to be
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notified that fakesrc is about to pull on its src pad, even though the
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pipeline has not yet changed fakesrc's state. For this reason,
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GStreamer will first call call activate_mode(PULL) on fakesink:sink's
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peer before calling activate_mode(PULL) on fakesink:sinks.
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In short, upstream elements operating in PULL mode must be ready to
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produce data in READY, after having activate_mode(PULL) called on their
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source pad. Also, a call to activate_mode(PULL) needs to propagate through
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the pipeline to every pad that a gst_pad_pull() will reach. In the case
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fakesrc ! identity ! fakesink, calling activate_mode(PULL) on identity's
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source pad would need to activate its sink pad in pull mode as well,
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which should propagate all the way to fakesrc.
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If, on the other hand, fakesrc ! fakesink is operating in PUSH mode, the
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activation sequence is different. First, activate() on fakesink:sink
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calls activate_mode(PUSH) on fakesink:sink. Then fakesrc's pads are
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activated: sources first, then sinks (of which fakesrc has none).
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fakesrc:src's activation function is then called.
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Note that it does not make sense to set an activation function on a
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source pad. The peer of a source pad is downstream, meaning it should
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have been activated first. If it was activated in PULL mode, the
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source pad should have already had activate_mode(PULL) called on it, and
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thus needs no further activation. Otherwise it should be in PUSH mode,
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which is the choice of the default activation function.
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So, in the PUSH case, the default activation function chooses PUSH mode,
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which calls activate_mode(PUSH), which will then start a task on the source
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pad and begin pushing. In this way PUSH scheduling is a bit easier,
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because it follows the order of state changes in a pipeline. fakesink is
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already in PAUSED with an active sink pad by the time fakesrc starts
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pushing data.
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Deactivation
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~~~~~~~~~~~~
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Pad deactivation occurs when its parent goes into the READY state or when the
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pad is deactivated explicitly by the application or element.
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gst_pad_set_active() is called with a FALSE argument, which then calls
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activate_mode(PUSH) or activate_mode(PULL) with a FALSE argument, depending
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on the current activation mode of the pad.
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Mode switching
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~~~~~~~~~~~~~~
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Changing from push to pull modes needs a bit of thought. This is actually
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possible and implemented but not yet documented here.
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