mirror of
https://gitlab.freedesktop.org/gstreamer/gstreamer.git
synced 2024-11-29 21:21:12 +00:00
94e66c5da0
Original commit message from CVS: * docs/design/draft-latency.txt: Slight redesign to allow for dynamic latency adjustments. * docs/design/part-negotiation.txt: Fix some typos.
268 lines
11 KiB
Text
268 lines
11 KiB
Text
Negotiation
|
|
-----------
|
|
|
|
Capabilities negotiation is the process of deciding on an adequate
|
|
format for dataflow within a GStreamer pipeline. Ideally, negotiation
|
|
(also known as "capsnego") transfers information from those parts of the
|
|
pipeline that have information to those parts of the pipeline that are
|
|
flexible, constrained by those parts of the pipeline that are not
|
|
flexible.
|
|
|
|
GStreamer's two scheduling modes, push mode and pull mode, lend
|
|
themselves to different mechanisms to achieve this goal. As it is more
|
|
common we describe push mode negotiation first.
|
|
|
|
Push-mode negotiation
|
|
---------------------
|
|
|
|
Push-mode negotiation happens when elements want to push buffers and
|
|
need to decide on the format. This is called downstream negotiation
|
|
because the upstream element decides the format for the downstream
|
|
element. This is the most common case.
|
|
|
|
Negotiation can also happen when a downstream element wants to receive
|
|
another data format from an upstream element. This is called upstream
|
|
negotiation.
|
|
|
|
The basics of negotiation are as follows:
|
|
|
|
- GstCaps (see part-caps.txt) are refcounted before they
|
|
are attached to a buffer to describe the contents of the buffer.
|
|
It is possible to add a NULL caps to a buffer, this means that the
|
|
buffer type did not change relative to the previous buffer. If no
|
|
previous buffer was received by a downstream element, it is free to
|
|
discard the buffer.
|
|
|
|
- Before receiving a buffer, an element must check if the datatype of
|
|
the buffer has changed. The element should reconfigure itself to the
|
|
new format before processing the buffer data. If the data type on
|
|
the buffer is not acceptable, the element should refuse the buffer by
|
|
returning an appropriate return value from the chain function.
|
|
The core will automatically call the set_caps function for this purpose
|
|
when it is installed on the sink or source pad.
|
|
|
|
- When requesting a buffer from a bufferpool, the prefered type should
|
|
be passed to the buffer allocation function. After receiving a buffer
|
|
from a bufferpool, the datatype should be checked again.
|
|
|
|
- A bufferpool allocation function should try to allocate a buffer of the
|
|
prefered type. If there is a good reason to choose another type, the
|
|
alloc function should see if that other type is accepted by the other
|
|
element, then allocate a buffer of that type and attach the type to the
|
|
buffer before returning it.
|
|
|
|
|
|
The general flow for a source pad starting the negotiation.
|
|
|
|
src sink
|
|
| |
|
|
| accepts? |
|
|
type A |---------------->|
|
|
| yes |
|
|
|<----------------|
|
|
| |
|
|
get buffer | alloc_buf |
|
|
from pool |---------------->|
|
|
with type A | | Create buffer of type A.
|
|
| |
|
|
check type |<----------------|
|
|
and use A | |
|
|
| push |
|
|
push buffer |---------------->| Receive type A, reconfigure to
|
|
with new type| | process type A.
|
|
| |
|
|
|
|
One possible implementation in pseudo code:
|
|
|
|
[element wants to create a buffer]
|
|
if not format
|
|
# see what the peer can do
|
|
peercaps = gst_pad_peer_get_caps (srcpad)
|
|
# see what we can do
|
|
ourcaps = gst_pad_get_caps (srcpad)
|
|
|
|
# get common formats
|
|
candidates = gst_caps_intersect (peercaps, ourcaps)
|
|
|
|
foreach candidate in candidates
|
|
# make sure the caps is fixed
|
|
fixedcaps = gst_pad_fixate_caps (srcpad, candidate)
|
|
|
|
# see if the peer accepts it
|
|
if gst_pad_peer_accept_caps (srcpad, fixedcaps)
|
|
# store the caps as the negotiated caps, this will
|
|
# call the setcaps function on the pad
|
|
gst_pad_set_caps (srcpad, fixedcaps)
|
|
break
|
|
endif
|
|
done
|
|
endif
|
|
|
|
# if the type is different, the buffer will have different caps from
|
|
# the src pad -- setcaps will get called on the pad_push
|
|
buffer = gst_pad_alloc_buffer (srcpad, 0, size, GST_PAD_CAPS (fixedcaps));
|
|
if buffer
|
|
[fill buffer and push]
|
|
elseif
|
|
[no buffer, either no peer or no acceptable format found]
|
|
endif
|
|
|
|
|
|
The general flow for a sink pad starting a renegotiation.
|
|
|
|
src sink
|
|
| |
|
|
| accepts? |
|
|
|<----------------| type B
|
|
| yes |
|
|
|---------------->|
|
|
| |
|
|
get buffer | alloc_buf |
|
|
from pool |---------------->|
|
|
with type A | | Create buffer of new type B.
|
|
| |
|
|
check type |<----------------|
|
|
and | |
|
|
reconfigure | |
|
|
| push |
|
|
push buffer |---------------->| Receive type B, reconfigure to
|
|
with new type| | process type B.
|
|
| |
|
|
|
|
|
|
|
|
Use case:
|
|
|
|
|
|
videotestsrc ! xvimagesink
|
|
|
|
1) Who decides what format to use?
|
|
- src pad always decides, by convention. sinkpad can suggest a format
|
|
by putting it high in the getcaps function GstCaps.
|
|
- since the src decides, it can always choose something that it can do,
|
|
so this step can only fail if the sinkpad stated it could accept
|
|
something while later on it couldn't.
|
|
|
|
2) When does negotiation happen?
|
|
- before srcpad does a push, it figures out a type as stated in 1), then
|
|
it calls the pad alloc function with the type. The sinkpad has to
|
|
create a buffer of that type, src fills the buffer and sends it to sink.
|
|
- since the sink stated in 1) it could accept the type, it will be able to
|
|
create a buffer of the type and handle it.
|
|
- sink checks media type of buffer and configures itself for this type.
|
|
|
|
3) How can sink request another format?
|
|
- sink asks if new format is possible for the source.
|
|
- sink returns buffer with new type in allocfunction.
|
|
- src receives buffer with new type, reconfigures and pushes.
|
|
- sink can always select something it can create and handle since it takes
|
|
the initiative. src should be able to handle the new type since it said
|
|
it could accept it.
|
|
|
|
videotestsrc ! queue ! xvimagesink
|
|
|
|
- queue implements an allocfunction, proxying all calls to its srcpad peer.
|
|
- queue proxies all accept and getcaps to the other peer pad.
|
|
- queue contains buffers with different types.
|
|
|
|
|
|
Pull-mode negotiation
|
|
---------------------
|
|
|
|
Rationale
|
|
.........
|
|
|
|
A pipeline in pull mode has different negotiation needs than one
|
|
activated in push mode. Push mode is optimized for two use cases:
|
|
|
|
* Playback of media files, in which the demuxers and the decoders are
|
|
the points from which format information should disseminate to the
|
|
rest of the pipeline; and
|
|
|
|
* Recording from live sources, in which users are accustomed to putting
|
|
a capsfilter directly after the source element; thus the caps
|
|
information flow proceeds from the user, through the potential caps
|
|
of the source, to the sinks of the pipeline.
|
|
|
|
In contrast, pull mode has other typical use cases:
|
|
|
|
* Playback from a lossy source, such as RTP, in which more knowledge
|
|
about the latency of the pipeline can increase quality; or
|
|
|
|
* Audio synthesis, in which audio APIs are tuned to producing only the
|
|
necessary number of samples, typically driven by a hardware interrupt
|
|
to fill a DMA buffer or a Jack[0] port buffer.
|
|
|
|
* Low-latency effects processing, whereby filters should be applied as
|
|
data is transferred from a ring buffer to a sink instead of
|
|
beforehand. For example, instead of using the internal alsasink
|
|
ringbuffer thread in push-mode wavsrc ! volume ! alsasink, placing
|
|
the volume inside the sound card writer thread via wavsrc !
|
|
audioringbuffer ! volume ! alsasink.
|
|
|
|
[0] http://jackit.sf.net
|
|
|
|
The problem with pull mode is that the sink has to know the format in
|
|
order to know how many bytes to pull via gst_pad_pull_range(). This
|
|
means that before pulling, the sink must initiate negotation to decide
|
|
on a format.
|
|
|
|
Recalling the principles of capsnego, whereby information must flow from
|
|
those that have it to those that do not, we see that the two named use
|
|
cases have different negotiation requirements:
|
|
|
|
* RTP and low-latency playback are both like the normal playback case,
|
|
in which information flows downstream.
|
|
|
|
* In audio synthesis, the part of the pipeline that has the most
|
|
information is the sink, constrained by the capabilities of the graph
|
|
that feeds it. However the caps are not completely specified; at some
|
|
point the user has to intervene to choose the sample rate, at least.
|
|
This can be done externally to gstreamer, as in the jack elements, or
|
|
internally via a capsfilter, as is customary with live sources.
|
|
|
|
Given that sinks potentially need the input of sources, as in the RTP
|
|
case and at least as a filter in the synthesis case, there must be a
|
|
negotiation phase before the pull thread is activated. Also, given the
|
|
low latency offered by pull mode, we want to avoid capsnego from within
|
|
the pulling thread, in case it causes us to miss our scheduling
|
|
deadlines.
|
|
|
|
The time to do capsnego, then, is after activate_pull() has succeeded,
|
|
but before the sink has spawned the pulling thread. Because of the
|
|
latency concerns just mentioned, capsnego does not occur in the pulling
|
|
thread.
|
|
|
|
|
|
Mechanism
|
|
.........
|
|
|
|
The sink initiates the negotiation process by intersecting the results
|
|
of gst_pad_get_caps() on its sink pad and its peer src pad. This is the
|
|
operation performed by gst_pad_get_allowed_caps(). In the simple
|
|
passthrough case, the peer pad's getcaps() function should return the
|
|
intersection of calling get_allowed_caps() on all of its sink pads. In
|
|
this way the sink element knows the capabilities of the entire pipeline.
|
|
|
|
The sink element then fixates the resulting caps, if necessary,
|
|
resulting in the flow caps. It notifies the pipeline of the caps by
|
|
calling gst_pad_set_caps() on its sink pad. Sink pads should proxy the
|
|
setcaps() to their peer src pads. In the simple passthrough case, src
|
|
pads' setcaps() functions proxy the setcaps() to all of their sink pads,
|
|
which then set_caps() on their peers, and so the entire pipeline becomes
|
|
configured before dataflow has started. All pads have fixed caps.
|
|
|
|
If the sink element could not set caps on its sink pad, it should post
|
|
an error message on the bus indicating that negotiation was not
|
|
possible.
|
|
|
|
In this way all pads are negotiated before data starts flowing, so all
|
|
getrange() requests have a defined meaning for the number of bytes being
|
|
pulled.
|
|
|
|
During dataflow, gst_pad_pull_range() checks the caps on the pulled
|
|
buffer. If they are different from the sink pad's caps, it will return
|
|
GST_FLOW_NOT_NEGOTIATED. Because of the low-latency requirements,
|
|
changing caps in an activate pull-mode pipeline is not supported, as it
|
|
might require e.g. the sound card to reconfigure its hardware buffers,
|
|
and start capsnego again.
|