This commit adds a state `hw_paused` for pull-based AO.
`driver->set_paused(false)` is only called if `hw_paused` is true.
`hw_paused` is cleared after `ao_reset`, so `set_paused` will
not be called after a reset; instead, `driver->start()` will
be called, which properly starts the AO.
A figure from pipewire documentation:
```
stream time domain graph time domain
/-----------------------\/-----------------------------\
queue +-+ +-+ +-----------+ +--------+
----> | | | |->| converter | -> graph -> | kernel | -> speaker
<---- +-+ +-+ +-----------+ +--------+
dequeue buffers \-------------------/\--------/
graph internal
latency latency
\--------/\-------------/\-----------------------------/
queued buffered delay
```
We calculate `end_time` in the following steps:
1. get current timestamp in mpv
```
int64_t end_time = mp_time_ns();
```
2. add duration of samples to enqueue
```
end_time += MP_TIME_S_TO_NS(nframes) / ao->samplerate;
```
3. add delay of the pipewire graph
```
end_time += MP_TIME_S_TO_NS(time.delay) * time.rate.num / time.rate.denom;
```
4. add duration of queued and buffered samples.
```
end_time += MP_TIME_S_TO_NS(time.queued) / ao->samplerate;
end_time += MP_TIME_S_TO_NS(time.buffered) / ao->samplerate;
```
New in this commit. `time.queued` is usually zero as `SPA_PARAM_BUFFERS_buffers`
is default to 1; however it is not always.
`time.buffered` is non-zero if there is a resampler involved.
5. add elapsed duration from when `time` is captured
```
end_time -= pw_stream_get_nsec(p->stream) - time.now;
```
New in this commit. `time` is captured at `time.now`.
From then, time has passed so we need to exclude the elapsed time,
by calculating the diff of `pw_stream_get_nsec()` and `time.now`.
`hotplug_cb` was registered only in `hotplug_init()`.
This commit make it registered in `init()` as well,
so that the ao can listen for latency change
in playback.
`buf` contains a `struct spa_data` for each channel.
Therefore the number of channels does not matter to calculate the frame capacity of one `struct spa_data`.
In practice this shouldn't make a difference as `b->requested` would reduce nframes even more.
The device latency may change during hotplugging.
This commit updates p->hw_latency_ns each time
hotplug_cb is called so that it can reflect
updated device latency.
Misaki Kasumi