Tweak ALSA nperiod setting, fallback to nearest

This is mainly for RME RayDAT that has a fixed buffersize of 16k:

  dev_name : hw:HDSPMxc2f6c5,0
  channels : 36
  min_rate : 32000
  max_rate : 192000
  min_bufz : 16384
  max_bufz : 16384
  min_nper : 4
  max_nper : 512

However nperiod configuration determines the effective latency
regardless.

This is similar to https://github.com/jackaudio/jack1/blob/master/drivers/alsa/alsa_driver.c#L476-L486

libs/ardouralsautil/deviceparams.cc

@@ -115,8 +115,8 @@ ARDOUR::get_alsa_device_parameters (const char* device_name, const bool play, AL
 		fprintf (stdout, "  channels : %u\n", nfo->max_channels);
 		fprintf (stdout, "  min_rate : %u\n", nfo->min_rate);
 		fprintf (stdout, "  max_rate : %u\n", nfo->max_rate);
-		fprintf (stdout, "  min_psiz : %lu\n", nfo->min_size);
+		fprintf (stdout, "  min_psiz : %lu\n", min_psiz);
-		fprintf (stdout, "  max_psiz : %lu\n", nfo->max_size);
+		fprintf (stdout, "  max_psiz : %lu\n", max_psiz);
 		fprintf (stdout, "  min_bufz : %lu\n", min_bufz);
 		fprintf (stdout, "  max_bufz : %lu\n", max_bufz);
 		fprintf (stdout, "  min_nper : %d\n", nfo->min_nper);

libs/backends/alsa/alsa_audiobackend.cc

@@ -270,25 +270,23 @@ std::vector<uint32_t>
 AlsaAudioBackend::available_period_sizes (const std::string& driver, const std::string& device) const
 {
 	std::vector<uint32_t> ps;
+	ps.push_back (2);
 
 	ALSADeviceInfo* nfo = NULL;
 	if (device == get_standard_device_name(DeviceNone)) {
 		return ps;
 	}
+
 	if (device == _output_audio_device && _output_audio_device_info.valid) {
 		nfo = &_output_audio_device_info;
-	} else {
+		if (nfo->max_nper > 2) {
-		ps.push_back (2);
-		return ps;
-	}
-
-	if (nfo->min_nper == 2) {
-		ps.push_back (2);
-		if (nfo->max_nper >= 3) {
 			ps.push_back (3);
 		}
+		if (nfo->max_nper > 3) {
+			ps.push_back (nfo->min_nper);
+		}
 	} else {
-		ps.push_back (nfo->min_nper);
+		ps.push_back (3);
 	}
 
 	return ps;

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

@@ -486,11 +486,7 @@ void Alsa_pcmi::initialise (const char *play_name, const char *capt_name, const
 		}
 		if (snd_pcm_hw_params_get_periods (_play_hwpar, &nfrag, &dir) || (nfrag != _play_nfrag) || dir)
 		{
-			if (_debug & DEBUG_INIT) fprintf (stderr, "Alsa_pcmi: can't get requested number of periods for playback.\n");
+			if (_debug & DEBUG_INIT) fprintf (stderr, "Alsa_pcmi warning: requested %u periods for playback, using %u.\n", _play_nfrag, nfrag);
-			if ((_debug & FRAG_NEAR) == 0) {
-				_state = -5;
-				return;
-			}
 		}
 
 		snd_pcm_hw_params_get_format (_play_hwpar, &_play_format);
@@ -776,29 +772,24 @@ int Alsa_pcmi::set_hwpar (snd_pcm_t *handle,  snd_pcm_hw_params_t *hwpar, const
 				sname, _fsize);
 		return -4;
 	}
-	if ((_debug & FRAG_NEAR)) {
+
-		unsigned int nf = nfrag;
+	unsigned int nf = nfrag;
-		snd_pcm_hw_params_set_periods_min (handle, hwpar, &nf, NULL);
+	snd_pcm_hw_params_set_periods_min (handle, hwpar, &nf, NULL);
-		if (nf > nfrag) {
+	if (nf < nfrag) {
-			nfrag = nf;
+		nf = nfrag;
-		}
-		if (snd_pcm_hw_params_set_periods_near (handle, hwpar, &nfrag, NULL) < 0) {
-			if (_debug & DEBUG_INIT) fprintf (stderr, "Alsa_pcmi: can't set %s periods to %u.\n",
-					sname, nfrag);
-			return -5;
-		}
-	} else {
-		if (snd_pcm_hw_params_set_periods (handle, hwpar, nfrag, 0) < 0)
-		{
-			if (_debug & DEBUG_INIT) fprintf (stderr, "Alsa_pcmi: can't set %s periods to %u.\n",
-					sname, nfrag);
-			return -5;
-		}
 	}
-	if (snd_pcm_hw_params_set_buffer_size (handle, hwpar, _fsize * nfrag) < 0)
+	if (snd_pcm_hw_params_set_periods_near (handle, hwpar, &nf, NULL) < 0) {
+		if (_debug & DEBUG_INIT) fprintf (stderr, "Alsa_pcmi: can't set %s periods to %u (requested %u).\n",
+				sname, nf, nfrag);
+		return -5;
+	}
+
+	if (_debug & DEBUG_INIT) fprintf (stderr, "Alsa_pcmi: use %d periods for %s ((requested %u).\n", nf, sname, nfrag);
+
+	if (snd_pcm_hw_params_set_buffer_size (handle, hwpar, _fsize * nf) < 0)
 	{
 		if (_debug & DEBUG_INIT) fprintf (stderr, "Alsa_pcmi: can't set %s buffer length to %lu.\n",
-				sname, _fsize * nfrag);
+				sname, _fsize * nf);
 		return -4;
 	}
 	if (snd_pcm_hw_params (handle, hwpar) < 0)

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

@@ -60,8 +60,7 @@ public:
 		DEBUG_DATA = 8,
 		DEBUG_ALL  = 15,
 		FORCE_16B  = 256,
-		FORCE_2CH  = 512,
+		FORCE_2CH  = 512
-		FRAG_NEAR  = 1024
 	};
 
 	void printinfo (void);