Adding upstream version 0.3.65.upstream/0.3.65upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

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+/** \page page_tutorial4 Tutorial - Part 4: Playing A Tone
+
+\ref page_tutorial3 | \ref page_tutorial "Index" | \ref page_tutorial5
+
+In this tutorial we show how to use a stream to play a tone.
+
+Let's take a look at the code before we break it down:
+
+\snippet tutorial4.c code
+
+Save as tutorial4.c and compile with:
+
+    gcc -Wall tutorial4.c -o tutorial4 -lm $(pkg-config --cflags --libs libpipewire-0.3)
+
+We start with the usual boilerplate, `pw_init()` and a `pw_main_loop_new()`.
+We're going to store our objects in a structure so that we can pass them
+around in callbacks later.
+
+\code{.c}
+struct data {
+	struct pw_main_loop *loop;
+	struct pw_stream *stream;
+	double accumulator;
+};
+
+int main(int argc, char *argv[])
+{
+	struct data data = { 0, };
+
+	pw_init(&argc, &argv);
+
+	data.loop = pw_main_loop_new(NULL);
+\endcode
+
+Next we create a stream object. It takes the mainloop as first argument and
+a stream name as the second. Next we provide some properties for the stream
+and a callback + data.
+
+\code{.c}
+	data.stream = pw_stream_new_simple(
+			pw_main_loop_get_loop(data.loop),
+			"audio-src",
+			pw_properties_new(
+				PW_KEY_MEDIA_TYPE, "Audio",
+				PW_KEY_MEDIA_CATEGORY, "Playback",
+				PW_KEY_MEDIA_ROLE, "Music",
+				NULL),
+			&stream_events,
+			&data);
+\endcode
+
+We are using `pw_stream_new_simple()` but there is also a `pw_stream_new()` that
+takes an existing `struct pw_core` as the first argument and that requires you
+to add the event handle manually, for more control. The `pw_stream_new_simple()`
+is, as the name implies, easier to use because it creates  a `struct pw_context`
+and `struct pw_core` automatically.
+
+In the properties we need to give as much information about the stream as we
+can so that the session manager can make good decisions about how and where
+to route this stream. There are three important properties to configure:
+
+- `PW_KEY_MEDIA_TYPE`: The media type; like Audio, Video, MIDI.
+- `PW_KEY_MEDIA_CATEGORY`: The category; like Playback, Capture, Duplex, Monitor.
+- `PW_KEY_MEDIA_ROLE`: The media role; like Movie, Music, Camera, Screen,
+  Communication, Game, Notification, DSP, Production, Accessibility, Test.
+
+The properties are owned by the stream and freed when the stream is destroyed
+later.
+
+This is the event structure that we use to listen for events:
+
+\code{.c}
+static const struct pw_stream_events stream_events = {
+	PW_VERSION_STREAM_EVENTS,
+	.process = on_process,
+};
+\endcode
+
+We are for the moment only interested now in the `process` event. This event
+is called whenever we need to produce more data. We'll see how that function
+is implemented but first we need to setup the format of the stream:
+
+\code{.c}
+	const struct spa_pod *params[1];
+	uint8_t buffer[1024];
+	struct spa_pod_builder b = SPA_POD_BUILDER_INIT(buffer, sizeof(buffer));
+
+#define DEFAULT_RATE		44100
+#define DEFAULT_CHANNELS	2
+
+	params[0] = spa_format_audio_raw_build(&b, SPA_PARAM_EnumFormat,
+			&SPA_AUDIO_INFO_RAW_INIT(
+				.format = SPA_AUDIO_FORMAT_S16,
+				.channels = DEFAULT_CHANNELS,
+				.rate = DEFAULT_RATE ));
+\endcode
+
+This is using a `struct spa_pod_builder` to make a `struct spa_pod *` object
+in the buffer array on the stack. The parameter is of type `SPA_PARAM_EnumFormat`
+which means that it enumerates the possible formats for this stream. We have
+only one, a Signed 16 bit stereo format at 44.1KHz.
+
+We use `spa_format_audio_raw_build()` which is a helper function to make the param
+with the builder. See \ref page_spa_pod for more information about how to
+make these POD objects.
+
+Now we're ready to connect the stream and run the main loop:
+
+\code{.c}
+	pw_stream_connect(data.stream,
+			  PW_DIRECTION_OUTPUT,
+			  PW_ID_ANY,
+			  PW_STREAM_FLAG_AUTOCONNECT |
+			  PW_STREAM_FLAG_MAP_BUFFERS |
+			  PW_STREAM_FLAG_RT_PROCESS,
+			  params, 1);
+
+	pw_main_loop_run(data.loop);
+\endcode
+
+To connect we specify that we have a `PW_DIRECTION_OUTPUT` stream. The third argument
+is always `PW_ID_ANY`. Next we set some flags:
+
+- `PW_STREAM_FLAG_AUTOCONNECT`: Automatically connect this stream. This instructs
+  the session manager to link us to some consumer.
+- `PW_STREAM_FLAG_MAP_BUFFERS`:	mmap the buffers for us so we can access the
+  memory. If you don't set these flags you have either work with the fd or mmap 
+  yourself.
+- `PW_STREAM_FLAG_RT_PROCESS`: Run the process function in the realtime thread.
+  Only use this if the process function only uses functions that are realtime 
+  safe, this means no allocation or file access or any locking.
+
+And last we pass the extra parameters for our stream. Here we only have the
+allowed formats (`SPA_PARAM_EnumFormat`).
+
+Running the mainloop will then start processing and will result in our
+`process` callback to be called. Let's have a look at that function now.
+
+The main program flow of the process function is:
+
+- `pw_stream_dequeue_buffer()` to obtain a buffer to write into.
+- Get pointers in buffer memory to write to.
+- Write data into buffer.
+- Adjust buffer with number of written bytes, offset, stride.
+- `pw_stream_queue_buffer()` to queue the buffer for playback.
+
+\snippet tutorial4.c on_process
+
+Check out the docs for \ref page_spa_buffer for more information
+about how to work with buffers.
+
+Try to change the number of channels, samplerate or format; the stream
+will automatically convert to the format on the server.
+
+
+\ref page_tutorial3 | \ref page_tutorial "Index" | \ref page_tutorial5
+
+*/