--- /dev/null
+Probes
+------
+
+ Probes are callbacks that can be installed by the application and will notify
+ the application about the states of the dataflow.
+
+
+Requirements
+------------
+
+Applications should be able to monitor and control the dataflow on pads. We
+identify the following types:
+
+ - be notified when the pad is/becomes idle and make sure the pad stays idle.
+ This is essential to be able to implement dynamic relinking of elements
+ without breaking the dataflow.
+
+ - be notified when data or events are pushed or sent on a pad. It should also
+ be possible to inspect and modify the data.
+
+ - be able to drop, pass and block on data based on the result of the callback.
+
+ - be able to drop, pass data on blocking pads based on methods performed by
+ the application thread.
+
+
+Overview
+--------
+
+ The function gst_pad_add_probe() is used to add a probe to a pad. It accepts a
+ probe type mask and a callback.
+
+ gulong gst_pad_add_probe (GstPad *pad,
+ GstProbeType mask,
+ GstPadProbeCallback callback,
+ gpointer user_data,
+ GDestroyNotify destroy_data);
+
+ The function returns a gulong that uniquely identifies the probe and that can
+ be used to remove the probe with gst_pad_remove_probe():
+
+ void gst_pad_remove_probe (GstPad *pad, gulong id);
+
+ The mask parameter is a bitwise or of the following flags:
+
+ typedef enum
+ {
+ GST_PROBE_TYPE_INVALID = 0,
+
+ /* flags to control blocking */
+ GST_PROBE_TYPE_IDLE = (1 << 0),
+ GST_PROBE_TYPE_BLOCK = (1 << 1),
+
+ /* flags to select datatypes */
+ GST_PROBE_TYPE_BUFFER = (1 << 2),
+ GST_PROBE_TYPE_BUFFER_LIST = (1 << 3),
+ GST_PROBE_TYPE_EVENT = (1 << 4),
+
+ /* flags to select scheduling mode */
+ GST_PROBE_TYPE_PUSH = (1 << 5),
+ GST_PROBE_TYPE_PULL = (1 << 6),
+ } GstProbeType;
+
+ When adding a probe with the IDLE or BLOCK flag, the probe will become a
+ blocking probe (see below). Otherwise the probe will be a DATA probe.
+
+ The datatype and scheduling selector flags are used to select what kind of
+ datatypes and scheduling modes should be allowed in the callback.
+
+ The blocking flags must match the triggered probe exactly.
+
+ The probe callback is defined as:
+
+ GstProbeReturn (*GstPadProbeCallback) (GstPad *pad, GstProbeType type,
+ gpointer type_data,
+ gpointer user_data);
+
+ The executing probe type is passed as an argument and is guaranteed to match
+ the mask that was used to register the callback. type_data contains type
+ specific data, which is usually the data item that is blocked or NULL when
+ no data item is present.
+
+ The probe can return any of the following return values:
+
+ typedef enum
+ {
+ GST_PROBE_DROP,
+ GST_PROBE_OK,
+ GST_PROBE_REMOVE,
+ GST_PROBE_PASS,
+ } GstProbeReturn;
+
+ GST_PROBE_OK is the normal return value. DROP will drop the item that is
+ currently being probed. GST_PROBE_REMOVE the currently executing probe from the
+ list of probes.
+
+ GST_PROBE_PASS is relevant for blocking probes and will temporarily unblock the
+ pad and let the item trough, it will then block again on the next item.
+
+
+Blocking probes
+---------------
+
+ Blocking probes are probes with BLOCK or IDLE flags set. They will always
+ block the dataflow and trigger the callback according to the following rules:
+
+ When the IDLE flag is set, the probe callback is called as soon as no data is
+ flowing over the pad. If at the time of probe registration, the pad is idle,
+ the callback will be called immediately from the current thread. Otherwise,
+ the callback will be called as soon as the pad becomes idle in the streaming
+ thread.
+
+ The IDLE probe in useful to perform dynamic linking, it allows to wait for for
+ a safe moment when an unlink/link operation can be done. Since the event is a
+ blocking event, it will also make sure that the pad stays idle until the probe
+ is removed.
+
+ When the BLOCK flag is set, the probe callback will be called when new data
+ arrives on the pad and right before the pad goes into the blocking state. This
+ callback is thus only called when there is new data on the pad.
+
+ The blocking probe is removed with gst_pad_remove_probe() or when the probe
+ callback return GST_PROBE_REMOVE. In both cases, and if this was the last
+ blocking probe on the pad, the pad is unblocked and dataflow can continue.
+
+
+Non-Blocking probes
+--------------------
+
+ Non-blocking probes or DATA probes are probes triggered when data is flowing
+ over the pad. The are called after the blocking probes are run and always with
+ data.
+
+
+Push dataflow
+-------------
+
+All probes have the GST_PROBE_TYPE_PUSH flag set in the callbacks.
+
+In push based scheduling, the blocking probe is called first with the DATA item.
+Then the data probes are called before the peer pad chain function is called.
+
+The data probes are called before the peer pad is checked. This allows for
+linking the pad in either the BLOCK or DATA probes on the srcpad.
+
+Before the sinkpad chain function is called, the data probes are called.
+
+Finally, the IDLE probe is called on the srcpad after the data was sent to the
+peer pad.
+
+
+ srcpad sinkpad
+ | |
+ gst_pad_push() | |
+ -------------->O |
+ O |
+ flushing? O |
+ WRONG_STATE O |
+ < - - - - - - O |
+ O-> do BLOCK probes |
+ O |
+ O-> do DATA probes |
+ no peer? O |
+ NOT_LINKED O |
+ < - - - - - - O |
+ O gst_pad_chain() |
+ O------------------------------>O
+ O flushing? O
+ O WRONG_STATE O
+ O< - - - - - - - - - - - - - - -O
+ O O-> do DATA probes
+ O O
+ O O---> chainfunc
+ O< - - - - - - - - - - - - - - -O
+ O |
+ O-> do IDLE probes |
+ O |
+ < - - - - - - O |
+ | |
+
+
+Pull dataflow
+-------------
+
+All probes have the GST_PROBE_TYPE_PULL flag set in the callbacks.
+
+The gst_pad_pull_range() call will first trigger the BLOCK probes without a DATA
+item. This allows the pad to be linked before the peer pad is resolved.
+
+After the getrange function is called on the peer pad and there is a data item,
+the DATA probes are called.
+
+When control returns to the sinkpad, the IDLE callbacks are called.
+
+It there is a valid DATA item, the DATA probes are called for the item.
+
+
+ srcpad sinkpad
+ | |
+ | | gst_pad_pull_range()
+ | O<---------------------
+ | O
+ | O flushing?
+ | O WRONG_STATE
+ | O - - - - - - - - - - >
+ | do BLOCK probes <-O
+ | O no peer?
+ | O NOT_LINKED
+ | O - - - - - - - - - - >
+ | gst_pad_get_range() O
+ O<------------------------------O
+ O O
+ O flushing? O
+ O WRONG_STATE O
+ O- - - - - - - - - - - - - - - >O
+ getrangefunc <---O O
+ O flow error? O
+ O- - - - - - - - - - - - - - - >O
+ O O
+ dp DATA probes <-O O
+ O- - - - - - - - - - - - - - - >O
+ | O
+ | do IDLE probes <-O
+ | O flow error?
+ | O - - - - - - - - - - >
+ | O
+ | do DATA probes <-O
+ | O - - - - - - - - - - >
+ | |
+
projects / platform / upstream / gstreamer.git / commitdiff