audio/audio: fix video playback slowdown with spiceaudio

This patch allows the audio backends get_buffer_out() functions
to drop audio data and mitigates a bug reported on the qemu-devel
mailing list.

https://lists.nongnu.org/archive/html/qemu-devel/2020-09/msg03832.html

The new rules for the variables buf and size returned by
get_buffer_out() are:
size == 0: Downstream playback buffer is full. Retry later.
size > 0, buf != NULL: Copy size bytes to buf for playback.
size > 0, buf == NULL: Drop size bytes.

The audio playback rate with spiceaudio for the no audio case is
too fast, but that's what we had before commit fb35c2cec5
"audio/dsound: fix invalid parameters error". The complete fix
comes with the next patch.

Reported-by: Qi Zhou <atmgnd@outlook.com>
Signed-off-by: Volker Rümelin <vr_qemu@t-online.de>
Message-id: 20200920171729.15861-2-vr_qemu@t-online.de
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

diff --git a/audio/audio.c b/audio/audio.c
index 1a68cfaafb9fee282b9dd187ad2c249e3107f087..7b660dd0c4d0568386c414561a13a625feaeca1b 100644 (file)
--- a/audio/audio.c
+++ b/audio/audio.c
@@ -1091,12 +1091,15 @@ static size_t audio_pcm_hw_run_out(HWVoiceOut *hw, size_t live)
     while (live) {
         size_t size, decr, proc;
         void *buf = hw->pcm_ops->get_buffer_out(hw, &size);
-        if (!buf || size == 0) {
+
+        if (size == 0) {
             break;
         }
 
         decr = MIN(size / hw->info.bytes_per_frame, live);
-        audio_pcm_hw_clip_out(hw, buf, decr);
+        if (buf) {
+            audio_pcm_hw_clip_out(hw, buf, decr);
+        }
         proc = hw->pcm_ops->put_buffer_out(hw, buf,
                                            decr * hw->info.bytes_per_frame) /
             hw->info.bytes_per_frame;