mirror of
https://gitlab.freedesktop.org/gstreamer/gstreamer.git
synced 2024-12-21 07:46:38 +00:00
362 lines
15 KiB
Text
362 lines
15 KiB
Text
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, events or queries 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,
|
|
GstPadProbeType 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_PAD_PROBE_TYPE_INVALID = 0,
|
|
|
|
/* flags to control blocking */
|
|
GST_PAD_PROBE_TYPE_IDLE = (1 << 0),
|
|
GST_PAD_PROBE_TYPE_BLOCK = (1 << 1),
|
|
|
|
/* flags to select datatypes */
|
|
GST_PAD_PROBE_TYPE_BUFFER = (1 << 4),
|
|
GST_PAD_PROBE_TYPE_BUFFER_LIST = (1 << 5),
|
|
GST_PAD_PROBE_TYPE_EVENT_DOWNSTREAM = (1 << 6),
|
|
GST_PAD_PROBE_TYPE_EVENT_UPSTREAM = (1 << 7),
|
|
GST_PAD_PROBE_TYPE_EVENT_FLUSH = (1 << 8),
|
|
GST_PAD_PROBE_TYPE_QUERY_DOWNSTREAM = (1 << 9),
|
|
GST_PAD_PROBE_TYPE_QUERY_UPSTREAM = (1 << 10),
|
|
|
|
/* flags to select scheduling mode */
|
|
GST_PAD_PROBE_TYPE_PUSH = (1 << 12),
|
|
GST_PAD_PROBE_TYPE_PULL = (1 << 13),
|
|
|
|
} GstPadProbeType;
|
|
|
|
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:
|
|
|
|
GstPadProbeReturn (*GstPadProbeCallback) (GstPad *pad, GstPadProbeInfo *info,
|
|
gpointer user_data);
|
|
|
|
A probe info structure is passed as an argument and its type is guaranteed
|
|
to match the mask that was used to register the callback. The data item in the
|
|
info 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_PAD_PROBE_DROP,
|
|
GST_PAD_PROBE_OK,
|
|
GST_PAD_PROBE_REMOVE,
|
|
GST_PAD_PROBE_PASS,
|
|
} GstPadProbeReturn;
|
|
|
|
GST_PAD_PROBE_OK is the normal return value. DROP will drop the item that is
|
|
currently being probed. GST_PAD_PROBE_REMOVE the currently executing probe from the
|
|
list of probes.
|
|
|
|
GST_PAD_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 is useful to perform dynamic linking, it allows to wait for for
|
|
a safe moment when an unlink/link operation can be done. Since the probe is a
|
|
blocking probe, 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_PAD_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
|
|
-------------
|
|
|
|
Push probes have the GST_PAD_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 or event 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 pad.
|
|
|
|
Before the peerpad chain or event function is called, the peer pad block and
|
|
data probes are called.
|
|
|
|
Finally, the IDLE probe is called on the pad after the data was sent to the
|
|
peer pad.
|
|
|
|
The push dataflow probe behavior is the same for buffers and bidirectional events.
|
|
|
|
|
|
pad peerpad
|
|
| |
|
|
gst_pad_push() / | |
|
|
gst_pad_push_event() | |
|
|
-------------------->O |
|
|
O |
|
|
flushing? O |
|
|
FLUSHING O |
|
|
< - - - - - - O |
|
|
O-> do BLOCK probes |
|
|
O |
|
|
O-> do DATA probes |
|
|
no peer? O |
|
|
NOT_LINKED O |
|
|
< - - - - - - O |
|
|
O gst_pad_chain() / |
|
|
O gst_pad_send_event() |
|
|
O------------------------------>O
|
|
O flushing? O
|
|
O FLUSHING O
|
|
O< - - - - - - - - - - - - - - -O
|
|
O O-> do BLOCK probes
|
|
O O
|
|
O O-> do DATA probes
|
|
O O
|
|
O O---> chainfunc /
|
|
O O eventfunc
|
|
O< - - - - - - - - - - - - - - -O
|
|
O |
|
|
O-> do IDLE probes |
|
|
O |
|
|
< - - - - - - O |
|
|
| |
|
|
|
|
|
|
Pull dataflow
|
|
-------------
|
|
|
|
Pull probes have the GST_PAD_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. It also
|
|
allows the callback to set a data item in the probe info.
|
|
|
|
After the blocking probe and 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. The IDLE
|
|
callback is called without a data item so that it will also be called when there
|
|
was an error.
|
|
|
|
If 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 FLUSHING
|
|
| 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 FLUSHING O
|
|
O- - - - - - - - - - - - - - - >O
|
|
do BLOCK probes <-O O
|
|
O O
|
|
getrangefunc <---O O
|
|
O flow error? O
|
|
O- - - - - - - - - - - - - - - >O
|
|
O O
|
|
do DATA probes <-O O
|
|
O- - - - - - - - - - - - - - - >O
|
|
| O
|
|
| do IDLE probes <-O
|
|
| O flow error?
|
|
| O - - - - - - - - - - >
|
|
| O
|
|
| do DATA probes <-O
|
|
| O - - - - - - - - - - >
|
|
| |
|
|
|
|
|
|
Queries
|
|
-------
|
|
|
|
Query probes have the GST_PAD_PROBE_TYPE_QUERY_* flag set in the callbacks.
|
|
|
|
|
|
pad peerpad
|
|
| |
|
|
gst_pad_peer_query() | |
|
|
-------------------->O |
|
|
O |
|
|
O-> do BLOCK probes |
|
|
O |
|
|
O-> do QUERY | PUSH probes |
|
|
no peer? O |
|
|
FALSE O |
|
|
< - - - - - - O |
|
|
O gst_pad_query() |
|
|
O------------------------------>O
|
|
O O-> do BLOCK probes
|
|
O O
|
|
O O-> do QUERY | PUSH probes
|
|
O O
|
|
O O---> queryfunc
|
|
O error O
|
|
<- - - - - - - - - - - - - - - - - - - - - - -O
|
|
O O
|
|
O O-> do QUERY | PULL probes
|
|
O< - - - - - - - - - - - - - - -O
|
|
O |
|
|
O-> do QUERY | PULL probes |
|
|
O |
|
|
< - - - - - - O |
|
|
| |
|
|
|
|
For queries, the PUSH ProbeType is set when the query is traveling to the object
|
|
that will answer the query and the PULL type is set when the query contains the
|
|
answer.
|
|
|
|
Use-cases
|
|
---------
|
|
|
|
Prerolling a partial pipeline
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
.---------. .---------. .----------.
|
|
| filesrc | | demuxer | .-----. | decoder1 |
|
|
| src -> sink src1 ->|queue|-> sink src
|
|
'---------' | | '-----' '----------' X
|
|
| | .----------.
|
|
| | .-----. | decoder2 |
|
|
| src2 ->|queue|-> sink src
|
|
'---------' '-----' '----------' X
|
|
|
|
|
|
The purpose is to create the pipeline dynamically up to the
|
|
decoders but not yet connect them to a sink and without losing
|
|
any data.
|
|
|
|
To do this, the source pads of the decoders is blocked so that no
|
|
events or buffers can escape and we don't interrupt the stream.
|
|
|
|
When all of the dynamic pad are created (no-more-pads emitted by the
|
|
branching point, ie, the demuxer or the queues filled) and the pads
|
|
are blocked (blocked callback received) the pipeline is completely
|
|
prerolled.
|
|
|
|
It should then be possible to perform the following actions on the
|
|
prerolled pipeline:
|
|
|
|
- query duration/position
|
|
- perform a flushing seek to preroll a new position
|
|
- connect other elements and unblock the blocked pads.
|
|
|
|
|
|
dynamically switching an element in a PLAYING pipeline
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
|
|
.----------. .----------. .----------.
|
|
| element1 | | element2 | | element3 |
|
|
... src -> sink src -> sink ...
|
|
'----------' '----------' '----------'
|
|
.----------.
|
|
| element4 |
|
|
sink src
|
|
'----------'
|
|
|
|
The purpose is to replace element2 with element4 in the PLAYING
|
|
pipeline.
|
|
|
|
1) block element1 src pad.
|
|
2) inside the block callback nothing is flowing between
|
|
element1 and element2 and nothing will flow until unblocked.
|
|
3) unlink element1 and element2
|
|
4) optional step: make sure data is flushed out of element2:
|
|
4a) pad event probe on element2 src
|
|
4b) send EOS to element2, this makes sure that element2 flushes
|
|
out the last bits of data it holds.
|
|
4c) wait for EOS to appear in the probe, drop the EOS.
|
|
4d) remove the EOS pad event probe.
|
|
5) unlink element2 and element3
|
|
5a) optionally element2 can now be set to NULL and/or removed from the
|
|
pipeline.
|
|
6) link element4 and element3
|
|
7) link element1 and element4
|
|
8) make sure element4 is in the same state as the rest of the elements. The
|
|
element should at least be PAUSED.
|
|
9) unblock element1 src
|
|
|
|
The same flow can be used to replace an element in a PAUSED pipeline. Of
|
|
course in a PAUSED pipeline there might not be dataflow so the block might
|
|
not immediately happen.
|