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111 lines
3.9 KiB
Text
111 lines
3.9 KiB
Text
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Scheduling:
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- remove loop/get/chain from GstElement and add a "iterate" method.
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The iterate method is called with the event (or events) that
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triggered it, performs some action, and resets the events (file
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descriptors becoming readable, semaphores, pads becoming readable
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or writable, or a time occurs).
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- Add GstLoopElement, GstChainElement, etc. for compatibility.
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- Remove existing state handling and create 2 states, "playing" and
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"stopped". "playing" means that the iterate() method of the
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element may be called, that is, the element is allowed to move
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buffers, negotiate, etc. "stopped" means that no gstreamer-ish
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things happen to an element, only gobject-ish. A separate
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reset() method will handle the difference between READY and NULL.
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- Add a flag "ready" to GstElement that is under the control of the
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element. If the element is ready to stream, it sets this flag,
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and the entire pipeline starts streaming. (This is basically
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the difference between PAUSED and PLAYING.) For example, osssink
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won't set the ready flag until the device is opened and there is
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a buffer available to write to the device.
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- Scheduling of elements and movement of buffers will be timed by
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clocks.
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Example:
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Pipeline: sinesrc ! osssink
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- The application creates the pipeline and sets it to "playing".
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- The clock is created and set to "paused".
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- sinesrc.iterate() decides to watch for the event "src pad
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negotiation" and sets the available caps on the pad.
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- osssink.iterate() opens device, determines available caps, and
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sets the available caps on the pad. Then it decides to wait for
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"sink pad negotiation".
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- The scheduler realizes that the two elements are waiting for
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negotiation, so it negotiates the link.
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- sinesrc.iterate() sets the "ready" flag (because it needs no more
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preparation to stream) and decides to watch for the event "src
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pad ready to accept buffer".
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- osssink.iterate() decides to watch for the event "sink pad has
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available buffer".
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- The scheduler realizes that sinesrc.srcpad is now ready, so it
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calls sinesrc.iterate()
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- sinesrc.iterate() creates a buffer and pushes it, and decides to
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wait for the same event.
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- The scheduler realizes that osssink.sinkpad now has a buffer, so
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it calls osssink.iterate().
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- osssink.iterate() is now ready to stream, so it sets the "ready"
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flag and waits for "time 0".
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- The pipeline is now completely ready, so the clock may be
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started. A signal is fired to let the application know this
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(and possibly change the default behavior).
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- The clock starts with the time 0. The scheduler realizes this,
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and decides to schedule osssink.
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- osssink.iterate() is called, and writes the buffer to the device.
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This starts the clock counting. (Actually, the buffer could be
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written by the clock code, since presumably the clock is related
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to osssink.) iterate() then waits for "sink pad has available
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buffer".
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We're now basically in streaming mode. A streaming cycle:
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- osssink.iterate() decides the audio output buffer is full enough,
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so it waits for "time X", where X is the time when the output
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buffer will be below some threshold.
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- osssink.iterate() waits for "sink pad has available buffer"
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- sinesrc.iterate() creates and pushes a buffer, then waits for
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"src pad ready".
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Further ideas:
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- osssink can set a hard deadline time, which means that if it is
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not scheduled before that time, you'll get a skip. Skipping
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involves setting osssink to "not ready" and pauses the clock.
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Then the scheduler needs to go through the same process as above
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to start the clock.
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- As a shortcut, osssink can say "I need a buffer on the sinkpad
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at time X". This information can be passed upstream, and be used
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in filters -- filter.sinkpad says "I need a buffer at time X-N",
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where N is the latency of the filter.
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