paaudio: avoid to clip samples multiple times

The pulseaudio backend currently converts, clips and copies audio
playback samples in the mixing-engine sample buffer multiple
times.

In qpa_get_buffer_out() the function pa_stream_begin_write()
returns a rather large buffer and this allows audio_pcm_hw_run_out()
in audio/audio.c to copy all samples in the mixing-engine buffer
to the pulse audio buffer. Immediately after copying, qpa_write()
notices with a call to pa_stream_writable_size() that pulse audio
only needs a smaller part of the copied samples and ignores the
rest. This copy and ignore process happens several times for each
audio sample.

To fix this behaviour, call pa_stream_writable_size() in
qpa_get_buffer_out() to limit the number of samples
audio_pcm_hw_run_out() will convert. With this change the
pulseaudio pcm_ops functions put_buffer_out and write are no
longer identical and a separate qpa_put_buffer_out is needed.

Signed-off-by: Volker Rümelin <vr_qemu@t-online.de>
Message-id: 9315afe5-5958-c0b4-ea1e-14769511a9d5@t-online.de
Message-Id: <20210110100239.27588-13-vr_qemu@t-online.de>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

audio/paaudio.c

@@ -207,6 +207,7 @@ static void *qpa_get_buffer_out(HWVoiceOut *hw, size_t *size)
     PAVoiceOut *p = (PAVoiceOut *) hw;
     PAConnection *c = p->g->conn;
     void *ret;
+    size_t l;
     int r;
 
     pa_threaded_mainloop_lock(c->mainloop);
@@ -214,12 +215,19 @@ static void *qpa_get_buffer_out(HWVoiceOut *hw, size_t *size)
     CHECK_DEAD_GOTO(c, p->stream, unlock_and_fail,
                     "pa_threaded_mainloop_lock failed\n");
 
+    l = pa_stream_writable_size(p->stream);
+    CHECK_SUCCESS_GOTO(c, l != (size_t) -1, unlock_and_fail,
+                       "pa_stream_writable_size 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");
 
     pa_threaded_mainloop_unlock(c->mainloop);
+    if (*size > l) {
+        *size = l;
+    }
     return ret;
 
 unlock_and_fail:
@@ -228,6 +236,28 @@ unlock_and_fail:
     return NULL;
 }
 
+static size_t qpa_put_buffer_out(HWVoiceOut *hw, void *data, size_t length)
+{
+    PAVoiceOut *p = (PAVoiceOut *)hw;
+    PAConnection *c = p->g->conn;
+    int r;
+
+    pa_threaded_mainloop_lock(c->mainloop);
+
+    CHECK_DEAD_GOTO(c, p->stream, unlock_and_fail,
+                    "pa_threaded_mainloop_lock failed\n");
+
+    r = pa_stream_write(p->stream, data, length, NULL, 0LL, PA_SEEK_RELATIVE);
+    CHECK_SUCCESS_GOTO(c, r >= 0, unlock_and_fail, "pa_stream_write failed\n");
+
+    pa_threaded_mainloop_unlock(c->mainloop);
+    return length;
+
+unlock_and_fail:
+    pa_threaded_mainloop_unlock(c->mainloop);
+    return 0;
+}
+
 static size_t qpa_write(HWVoiceOut *hw, void *data, size_t length)
 {
     PAVoiceOut *p = (PAVoiceOut *) hw;
@@ -861,7 +891,7 @@ static struct audio_pcm_ops qpa_pcm_ops = {
     .fini_out = qpa_fini_out,
     .write    = qpa_write,
     .get_buffer_out = qpa_get_buffer_out,
-    .put_buffer_out = qpa_write, /* pa handles it */
+    .put_buffer_out = qpa_put_buffer_out,
     .volume_out = qpa_volume_out,
 
     .init_in  = qpa_init_in,