pulseaudio: setup buffer attrs

Request reasonable buffer sizes from pulseaudio.  Without this
pa_simple_write() can block quite long and lead to dropouts,
especially with guests which use small audio ring buffers.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: malc <av1474@comtv.ru>

audio/paaudio.c

@@ -289,6 +289,7 @@ static int qpa_init_out (HWVoiceOut *hw, struct audsettings *as)
 {
     int error;
     static pa_sample_spec ss;
+    static pa_buffer_attr ba;
     struct audsettings obt_as = *as;
     PAVoiceOut *pa = (PAVoiceOut *) hw;
 
@@ -296,6 +297,15 @@ static int qpa_init_out (HWVoiceOut *hw, struct audsettings *as)
     ss.channels = as->nchannels;
     ss.rate = as->freq;
 
+    /*
+     * qemu audio tick runs at 250 Hz (by default), so processing
+     * data chunks worth 4 ms of sound should be a good fit.
+     */
+    ba.tlength = pa_usec_to_bytes (4 * 1000, &ss);
+    ba.minreq = pa_usec_to_bytes (2 * 1000, &ss);
+    ba.maxlength = -1;
+    ba.prebuf = -1;
+
     obt_as.fmt = pa_to_audfmt (ss.format, &obt_as.endianness);
 
     pa->s = pa_simple_new (
@@ -306,7 +316,7 @@ static int qpa_init_out (HWVoiceOut *hw, struct audsettings *as)
         "pcm.playback",
         &ss,
         NULL,                   /* channel map */
-        NULL,                   /* buffering attributes */
+        &ba,                    /* buffering attributes */
         &error
         );
     if (!pa->s) {