alsaaudio: reduce playback latency

Change the buffer_get_free pcm_ops function to report the free
ALSA playback buffer. The generic buffer becomes a temporary
buffer and is empty after a call to audio_run_out().

Signed-off-by: Volker Rümelin <vr_qemu@t-online.de>
Acked-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20220923183640.8314-4-vr_qemu@t-online.de>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

1 files changed, 37 insertions, 1 deletions

diff --git a/audio/alsaaudio.c b/audio/alsaaudio.c
index 4a61378..7a2a94c 100644
--- a/audio/alsaaudio.c
+++ b/audio/alsaaudio.c
@@ -602,6 +602,42 @@ static int alsa_open(bool in, struct alsa_params_req *req,
     return -1;
 }
 
+static size_t alsa_buffer_get_free(HWVoiceOut *hw)
+{
+    ALSAVoiceOut *alsa = (ALSAVoiceOut *)hw;
+    snd_pcm_sframes_t avail;
+    size_t alsa_free, generic_free, generic_in_use;
+
+    avail = snd_pcm_avail_update(alsa->handle);
+    if (avail < 0) {
+        if (avail == -EPIPE) {
+            if (!alsa_recover(alsa->handle)) {
+                avail = snd_pcm_avail_update(alsa->handle);
+            }
+        }
+        if (avail < 0) {
+            alsa_logerr(avail,
+                        "Could not obtain number of available frames\n");
+            avail = 0;
+        }
+    }
+
+    alsa_free = avail * hw->info.bytes_per_frame;
+    generic_free = audio_generic_buffer_get_free(hw);
+    generic_in_use = hw->samples * hw->info.bytes_per_frame - generic_free;
+    if (generic_in_use) {
+        /*
+         * This code can only be reached in the unlikely case that
+         * snd_pcm_avail_update() returned a larger number of frames
+         * than snd_pcm_writei() could write. Make sure that all
+         * remaining bytes in the generic buffer can be written.
+         */
+        alsa_free = alsa_free > generic_in_use ? alsa_free - generic_in_use : 0;
+    }
+
+    return alsa_free;
+}
+
 static size_t alsa_write(HWVoiceOut *hw, void *buf, size_t len)
 {
     ALSAVoiceOut *alsa = (ALSAVoiceOut *) hw;
@@ -916,7 +952,7 @@ static struct audio_pcm_ops alsa_pcm_ops = {
     .init_out = alsa_init_out,
     .fini_out = alsa_fini_out,
     .write    = alsa_write,
-    .buffer_get_free = audio_generic_buffer_get_free,
+    .buffer_get_free = alsa_buffer_get_free,
     .run_buffer_out = audio_generic_run_buffer_out,
     .enable_out = alsa_enable_out,