From a806f95904cdb825c7b9d0cda95c8944907fb226 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Volker=20R=C3=BCmelin?= <vr_qemu@t-online.de>
Date: Tue, 1 Mar 2022 20:13:03 +0100
Subject: [PATCH] audio: copy playback stream in sequential order
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Change the code to copy the playback stream in sequential order.
The advantage can be seen in the next patches where the stream
copy operation effectively becomes a write through operation.

The following diagram shows the average buffer fill level and
the stream copy sequence. ### represents a timer_period sized
chunk. The rest of the buffer sizes are not to scale.

With current code:
         |--------| |#####111| |---#####|
          sw->buf    mix_buf    backend buffer
  1. clip
         |--------| |---#####| |111##222|
          sw->buf    mix_buf    backend buffer
  2. write to audio device
  333 -> |--------| |---#####| |---111##| -> 222
          sw->buf    mix_buf    backend buffer
  3a. sw device write
         |-----333| |---#####| |---111##|
          sw->buf    mix_buf    backend buffer
  3b. resample and mix
         |--------| |333#####| |---111##|
          sw->buf    mix_buf    backend buffer

With this patch:
  111 -> |--------| |---#####| |---#####|
          sw->buf    mix_buf    backend buffer
  1a: sw device write
         |-----111| |---#####| |---#####|
          sw->buf    mix_buf    backend buffer
  1b. resample and mix
         |--------| |111##222| |---#####|
          sw->buf    mix_buf    backend buffer
  2. clip
         |--------| |---111##| |222##333|
          sw->buf    mix_buf    backend buffer
  3. write to audio device
         |--------| |---111##| |---222##| -> 333
          sw->buf    mix_buf    backend buffer

The effective total playback buffer size is reduced by
timer_period.

Signed-off-by: Volker Rümelin <vr_qemu@t-online.de>
Message-Id: <20220301191311.26695-7-vr_qemu@t-online.de>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
---
 audio/audio.c | 24 +++++++++---------------
 1 file changed, 9 insertions(+), 15 deletions(-)

diff --git a/audio/audio.c b/audio/audio.c
index 35437986d9..9e2d7fb209 100644
--- a/audio/audio.c
+++ b/audio/audio.c
@@ -1134,6 +1134,15 @@ static void audio_run_out (AudioState *s)
         size_t played, live, prev_rpos, free;
         int nb_live;
 
+        for (sw = hw->sw_head.lh_first; sw; sw = sw->entries.le_next) {
+            if (sw->active) {
+                free = audio_get_free(sw);
+                if (free > 0) {
+                    sw->callback.fn(sw->callback.opaque, free);
+                }
+            }
+        }
+
         live = audio_pcm_hw_get_live_out (hw, &nb_live);
         if (!nb_live) {
             live = 0;
@@ -1162,14 +1171,6 @@ static void audio_run_out (AudioState *s)
         }
 
         if (!live) {
-            for (sw = hw->sw_head.lh_first; sw; sw = sw->entries.le_next) {
-                if (sw->active) {
-                    free = audio_get_free (sw);
-                    if (free > 0) {
-                        sw->callback.fn (sw->callback.opaque, free);
-                    }
-                }
-            }
             if (hw->pcm_ops->run_buffer_out) {
                 hw->pcm_ops->run_buffer_out(hw);
             }
@@ -1210,13 +1211,6 @@ static void audio_run_out (AudioState *s)
             if (!sw->total_hw_samples_mixed) {
                 sw->empty = 1;
             }
-
-            if (sw->active) {
-                free = audio_get_free (sw);
-                if (free > 0) {
-                    sw->callback.fn (sw->callback.opaque, free);
-                }
-            }
         }
     }
 }