Tweak engine startup, initial silence

This can help when running with very low latency and the
initial process callback is [indirectly] expensive.

E.g. load a heavy session the a RPi4, initial setup can pull
in a lot of data, which blocks the bus.

In particular with the ALSA backend this can lead to poll timeout
which effectively stops the backend.

libs/ardour/audioengine.cc

@@ -728,7 +728,7 @@ AudioEngine::set_session (Session *s)
 	SessionHandlePtr::set_session (s);
 
 	if (_session) {
-		_init_countdown = std::max (8, (int)(_backend->sample_rate () / _backend->buffer_size ()) / 2);
+		_init_countdown = std::max (8, (int)(_backend->sample_rate () / _backend->buffer_size ()) / 4);
 	}
 }
 

libs/backends/alsa/alsa_audiobackend.cc

@@ -2086,7 +2086,6 @@ AlsaAudioBackend::main_process_thread ()
 	double dll_w2 = dll_w1 * dll_w1;
 
 	uint64_t clock1;
-	_pcmi->pcm_start ();
 	int no_proc_errors = 0;
 	const int bailout = 5 * _samplerate / _samples_per_period;
 
@@ -2095,6 +2094,21 @@ AlsaAudioBackend::main_process_thread ()
 
 	const double sr_norm = 1e-6 * (double) _samplerate / (double)_samples_per_period;
 
+	/* warm up */
+	int cnt = std::max (8, (int)(_samplerate / _samples_per_period) / 2);
+	for (int w = 0; w < cnt; ++w) {
+		for (std::vector<AlsaPort*>::const_iterator it = _system_inputs.begin (); it != _system_inputs.end (); ++it) {
+			memset ((*it)->get_buffer (_samples_per_period), 0, _samples_per_period * sizeof (Sample));
+		}
+		if (engine.process_callback (_samples_per_period)) {
+			_active = false;
+			return 0;
+		}
+		Glib::usleep (1000000 * (_samples_per_period / _samplerate ));
+	}
+
+	_pcmi->pcm_start ();
+
 	while (_run) {
 		long nr;
 		bool xrun = false;

libs/backends/alsa/zita-alsa-pcmi.cc

@@ -230,7 +230,7 @@ snd_pcm_sframes_t Alsa_pcmi::pcm_wait (void)
 	{
 		_state = -1;
 		if (!recover ()) {
-		_state = 1;
+			_state = 1;
 		}
 		return 0;
 	}
@@ -239,7 +239,7 @@ snd_pcm_sframes_t Alsa_pcmi::pcm_wait (void)
 	{
 		_state = -1;
 		if (!recover ()) {
-		_state = 1;
+			_state = 1;
 		}
 		return 0;
 	}