paaudio: reduce effective playback buffer size

Add the buffer_get_free pcm_ops function to reduce the effective
playback buffer size. All intermediate audio playback buffers
become temporary buffers.

Signed-off-by: Volker Rümelin <vr_qemu@t-online.de>
Message-Id: <20220301191311.26695-11-vr_qemu@t-online.de>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

audio/paaudio.c

@@ -201,13 +201,11 @@ static size_t qpa_read(HWVoiceIn *hw, void *data, size_t length)
     return 0;
 }
 
-static void *qpa_get_buffer_out(HWVoiceOut *hw, size_t *size)
+static size_t qpa_buffer_get_free(HWVoiceOut *hw)
 {
-    PAVoiceOut *p = (PAVoiceOut *) hw;
+    PAVoiceOut *p = (PAVoiceOut *)hw;
     PAConnection *c = p->g->conn;
-    void *ret;
     size_t l;
-    int r;
 
     pa_threaded_mainloop_lock(c->mainloop);
 
@@ -216,24 +214,40 @@ static void *qpa_get_buffer_out(HWVoiceOut *hw, size_t *size)
     if (pa_stream_get_state(p->stream) != PA_STREAM_READY) {
         /* wait for stream to become ready */
         l = 0;
-        ret = NULL;
         goto unlock;
     }
 
     l = pa_stream_writable_size(p->stream);
     CHECK_SUCCESS_GOTO(c, l != (size_t) -1, unlock_and_fail,
                        "pa_stream_writable_size failed\n");
 
+unlock:
+    pa_threaded_mainloop_unlock(c->mainloop);
+    return l;
+
+unlock_and_fail:
+    pa_threaded_mainloop_unlock(c->mainloop);
+    return 0;
+}
+
+static void *qpa_get_buffer_out(HWVoiceOut *hw, size_t *size)
+{
+    PAVoiceOut *p = (PAVoiceOut *)hw;
+    PAConnection *c = p->g->conn;
+    void *ret;
+    int r;
+
+    pa_threaded_mainloop_lock(c->mainloop);
+
+    CHECK_DEAD_GOTO(c, p->stream, unlock_and_fail,
+                    "pa_threaded_mainloop_lock failed\n");
+
     *size = -1;
     r = pa_stream_begin_write(p->stream, &ret, size);
     CHECK_SUCCESS_GOTO(c, r >= 0, unlock_and_fail,
                        "pa_stream_begin_write failed\n");
 
-unlock:
     pa_threaded_mainloop_unlock(c->mainloop);
-    if (*size > l) {
-        *size = l;
-    }
     return ret;
 
 unlock_and_fail:
@@ -901,6 +915,7 @@ static struct audio_pcm_ops qpa_pcm_ops = {
     .init_out = qpa_init_out,
     .fini_out = qpa_fini_out,
     .write    = qpa_write,
+    .buffer_get_free = qpa_buffer_get_free,
     .get_buffer_out = qpa_get_buffer_out,
     .put_buffer_out = qpa_put_buffer_out,
     .volume_out = qpa_volume_out,