alsa: better period_time default value for high sample rates

The default values for buffer_time and period_time were both capped by
the hardware limits on practically all chips.  The result was a
period_time which was half as big as the buffer_time.  On some chips,
this led to lots of underruns when using a high sample rate (192 kHz),
because MPD had very little time to send new samples to ALSA.

A period time which is one fourth of the buffer time turned out to be
much better.  If no period_time is configured, see how much
buffer_time the hardware accepts, and try to configure one fourth of
it as period_time, instead of hard-coding the default period_time
value.

This is yet another attempt to provide a solution which is valid for
all sound chips.  Using the SND_PCM_NONBLOCK flag also seemed to solve
the underruns, but put a lot more CPU load to MPD.

NEWS

@@ -28,6 +28,7 @@ ver 0.15 - (200?/??/??)
   - mvp: check for reopen errors
   - mvp: fixed default device detection
   - pipe: new audio output plugin which runs a command
+  - alsa: better period_time default value for high sample rates
 * commands:
   - "playlistinfo" supports a range now
   - added "sticker database", command "sticker", which allows clients

src/output/alsa_plugin.c

@@ -32,7 +32,6 @@ static const char default_device[] = "default";
 
 enum {
 	MPD_ALSA_BUFFER_TIME_US = 500000,
-	MPD_ALSA_PERIOD_TIME_US = 125000,
 };
 
 #define MPD_ALSA_RETRY_NR 5
@@ -118,8 +117,7 @@ alsa_configure(struct alsa_data *ad, const struct config_param *param)
 
 	ad->buffer_time = config_get_block_unsigned(param, "buffer_time",
 			MPD_ALSA_BUFFER_TIME_US);
-	ad->period_time = config_get_block_unsigned(param, "period_time",
-			MPD_ALSA_PERIOD_TIME_US);
+	ad->period_time = config_get_block_unsigned(param, "period_time", 0);
 
 #ifdef SND_PCM_NO_AUTO_RESAMPLE
 	if (!config_get_block_bool(param, "auto_resample", true))
@@ -313,6 +311,13 @@ configure_hw:
 			goto error;
 	}
 
+	if (period_time_ro == 0 && buffer_time >= 10000) {
+		period_time_ro = period_time = buffer_time / 4;
+
+		g_debug("default period_time = buffer_time/4 = %u/4 = %u",
+			buffer_time, period_time);
+	}
+
 	if (period_time_ro > 0) {
 		period_time = period_time_ro;
 		cmd = "snd_pcm_hw_params_set_period_time_near";