From 3009f73b147bf912c43de2616bbe8d996538bab7 Mon Sep 17 00:00:00 2001 From: Wim Taymans Date: Fri, 28 Sep 2012 11:18:11 +0200 Subject: [PATCH] docs: update activation design docs --- docs/design/part-activation.txt | 43 +++++++++++++++++---------------- 1 file changed, 22 insertions(+), 21 deletions(-) diff --git a/docs/design/part-activation.txt b/docs/design/part-activation.txt index 47f033af92..bd020e9363 100644 --- a/docs/design/part-activation.txt +++ b/docs/design/part-activation.txt @@ -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 ~~~~~~~~~~~~~~