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docs/design/part-negotiation.txt: Update with, um, one way that pull-mode negotiation might work?
Original commit message from CVS: 2007-01-10 Andy Wingo <wingo@pobox.com> * docs/design/part-negotiation.txt: Update with, um, one way that pull-mode negotiation might work? * gst/gstpad.h: * gst/gstpad.c (gst_pad_get_allowed_caps): Remove the restriction that the pad must be a src pad; makes sense to call it the other way in pull mode, and the logic is symmetric anyway.
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Andy Wingo
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4 changed files with 118 additions and 22 deletions
10
ChangeLog
10
ChangeLog
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@ -1,3 +1,13 @@
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2007-01-10 Andy Wingo <wingo@pobox.com>
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* docs/design/part-negotiation.txt: Update with, um, one way that
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pull-mode negotiation might work?
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* gst/gstpad.h:
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* gst/gstpad.c (gst_pad_get_allowed_caps): Remove the restriction
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that the pad must be a src pad; makes sense to call it the other
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way in pull mode, and the logic is symmetric anyway.
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2007-01-10 Tim-Philipp Müller <tim at centricular dot net>
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* plugins/elements/gstfilesink.c:
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@ -1,10 +1,24 @@
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Negotiation
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-----------
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Negotiation happens when elements want to push buffers and need to decide
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on the format. This is called downstream negotiation because the upstream
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element decides the format for the downstream element. This is the most
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common case.
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Capabilities negotiation is the process of deciding on an adequate
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format for dataflow within a GStreamer pipeline. Ideally, negotiation
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(also known as "capsnego") transfers information from those parts of the
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pipeline that have information to those parts of the pipeline that are
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flexible, constrained by those parts of the pipeline that are not
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flexible.
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GStreamer's two scheduling modes, push mode and pull mode, lend
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themselves to different mechanisms to acheive this goal. As it is more
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common we describe push mode negotiation first.
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Push-mode negotiation
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---------------------
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Pussh-mode negotiation happens when elements want to push buffers and
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need to decide on the format. This is called downstream negotiation
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because the upstream element decides the format for the downstream
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element. This is the most common case.
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Negotiation can also happen when a downstream element wants to receive
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another data format from an upstream element. This is called upstream
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@ -152,4 +166,75 @@ videotestsrc ! queue ! xvimagesink
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- queue contains buffers with different types.
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Pull-mode negotiation
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---------------------
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A pipeline in pull mode has different negotiation needs than one
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activated in push mode. Push mode is optimized for two use cases:
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* Playback of media files, in which the demuxers and the decoders are
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the points from which format information should disseminate to the
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rest of the pipeline; and
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* Recording from live sources, in which users are accustomed to putting
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a capsfilter directly after the source element; thus the caps
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information flow proceeds from the user, through the potential caps
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of the source, to the sinks of the pipeline.
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In contrast, pull mode has other typical use cases:
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* Playback from a lossy source, such as RTP, in which more knowledge
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about the latency of the pipeline can increase quality; or
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* Audio synthesis, in which audio APIs are tuned to producing only the
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necessary number of samples, typically driven by a hardware interrupt
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to fill a DMA buffer or a Jack[0] port buffer.
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* Low-latency effects processing, whereby filters should be applied as
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data is transferred from a ring buffer to a sink instead of
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beforehand. For example, instead of using the internal alsasink
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ringbuffer thread in push-mode wavsrc ! volume ! alsasink, placing
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the volume inside the sound card writer thread via wavsrc !
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audioringbuffer ! volume ! alsasink.
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[0] http://jackit.sf.net
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The problem with push mode is that the sink has to know the format in
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order to know how many bytes to pull via gst_pad_pull_range(). This
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means that before pulling, the sink must initiate negotation to decide
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on a format.
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Recalling the principles of capsnego, whereby information must flow from
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those that have it to those that do not, we see that the two named use
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cases have different negotiation requirements:
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* RTP and low-latency playback are both like the normal playback case,
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in which information flows downstream.
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* In audio synthesis, the part of the pipeline that has the most
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information is the sink, constrained by the capabilities of the graph
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that feeds it. However the caps are not completely specified; at some
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point the user has to intervene to choose the sample rate, at least.
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This can be done externally to gstreamer, as in the jack elements, or
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internally via a capsnego, as is customary with live sources.
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Given that sinks potentially need the input of sources, as in the RTP
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case and at least as a filter in the synthesis case, there must be a
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negotiation phase before the pull thread is activated.
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[ok at this point i'm a bit at a loss about how it should go]
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This negotiation phase is initiated by the sink, after it succeeds in
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calling gst_pad_activate_pull(), but before it spawns a thread to start
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pulling. The sink will call gst_pad_get_allowed_caps() on the its sink
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pad and fixate if necessary to determine the flow caps. It then calls
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gst_pad_set_caps on its sink pad's peer, to configure the upstream
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elements.
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A typical element will implement a setcaps() function on its src pads
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that proxies the setcaps() to all peers of its sink pads. This way, when
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getrange() is called on a pad, it knows what format it is being asked to
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produce.
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If the sink element could not set caps on its peer(s), it should post an
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error message on the bus indicating that negotiation was not possible.
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35
gst/gstpad.c
35
gst/gstpad.c
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@ -2475,40 +2475,41 @@ gst_pad_get_peer (GstPad * pad)
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/**
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* gst_pad_get_allowed_caps:
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* @srcpad: a #GstPad, it must a a source pad.
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* @pad: a #GstPad.
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*
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* Gets the capabilities of the allowed media types that can flow through
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* @srcpad and its peer. The pad must be a source pad.
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* The caller must free the resulting caps.
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* @pad and its peer.
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*
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* Returns: the allowed #GstCaps of the pad link. Free the caps when
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* you no longer need it. This function returns NULL when the @srcpad has no
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* peer.
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* The allowed capabilities is calculated as the intersection of the results of
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* calling gst_pad_get_caps() on @pad and its peer. The caller owns a reference
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* on the resulting caps.
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*
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* Returns: the allowed #GstCaps of the pad link. Unref the caps when you no
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* longer need it. This function returns NULL when @pad has no peer.
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*
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* MT safe.
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*/
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GstCaps *
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gst_pad_get_allowed_caps (GstPad * srcpad)
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gst_pad_get_allowed_caps (GstPad * pad)
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{
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GstCaps *mycaps;
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GstCaps *caps;
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GstCaps *peercaps;
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GstPad *peer;
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g_return_val_if_fail (GST_IS_PAD (srcpad), NULL);
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g_return_val_if_fail (GST_PAD_IS_SRC (srcpad), NULL);
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g_return_val_if_fail (GST_IS_PAD (pad), NULL);
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GST_OBJECT_LOCK (srcpad);
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GST_OBJECT_LOCK (pad);
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peer = GST_PAD_PEER (srcpad);
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peer = GST_PAD_PEER (pad);
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if (G_UNLIKELY (peer == NULL))
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goto no_peer;
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GST_CAT_DEBUG_OBJECT (GST_CAT_PROPERTIES, srcpad, "getting allowed caps");
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GST_CAT_DEBUG_OBJECT (GST_CAT_PROPERTIES, pad, "getting allowed caps");
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gst_object_ref (peer);
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GST_OBJECT_UNLOCK (srcpad);
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mycaps = gst_pad_get_caps (srcpad);
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GST_OBJECT_UNLOCK (pad);
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mycaps = gst_pad_get_caps (pad);
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peercaps = gst_pad_get_caps (peer);
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gst_object_unref (peer);
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@ -2517,15 +2518,15 @@ gst_pad_get_allowed_caps (GstPad * srcpad)
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gst_caps_unref (peercaps);
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gst_caps_unref (mycaps);
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GST_CAT_DEBUG_OBJECT (GST_CAT_CAPS, srcpad, "allowed caps %" GST_PTR_FORMAT,
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GST_CAT_DEBUG_OBJECT (GST_CAT_CAPS, pad, "allowed caps %" GST_PTR_FORMAT,
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caps);
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return caps;
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no_peer:
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{
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GST_CAT_DEBUG_OBJECT (GST_CAT_PROPERTIES, srcpad, "no peer");
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GST_OBJECT_UNLOCK (srcpad);
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GST_CAT_DEBUG_OBJECT (GST_CAT_PROPERTIES, pad, "no peer");
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GST_OBJECT_UNLOCK (pad);
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return NULL;
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}
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@ -825,7 +825,7 @@ GstCaps * gst_pad_peer_get_caps (GstPad * pad);
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gboolean gst_pad_peer_accept_caps (GstPad * pad, GstCaps *caps);
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/* capsnego for connected pads */
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GstCaps * gst_pad_get_allowed_caps (GstPad * srcpad);
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GstCaps * gst_pad_get_allowed_caps (GstPad * pad);
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GstCaps * gst_pad_get_negotiated_caps (GstPad * pad);
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/* data passing functions to peer */
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