audio: handle leftover audio frame from upsampling

Upsampling may leave one remaining audio frame in the input
buffer. The emulated audio playback devices are currently
resposible to write this audio frame again in the next write
cycle. Push that task down to audio_pcm_sw_write.

This is another step towards an audio callback interface that
guarantees that when audio frontends are told they can write
n audio frames, they can actually do so.

Acked-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Acked-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Volker Rümelin <vr_qemu@t-online.de>
Message-Id: <20230224190555.7409-13-vr_qemu@t-online.de>

audio/audio.c

@@ -731,16 +731,21 @@ static size_t audio_pcm_sw_write(SWVoiceOut *sw, void *buf, size_t buf_len)
     hw_free = hw_free > live ? hw_free - live : 0;
     frames_out_max = MIN(dead, hw_free);
     sw_max = st_rate_frames_in(sw->rate, frames_out_max);
-    fe_max = MIN(buf_len / sw->info.bytes_per_frame, sw->resample_buf.size);
+    fe_max = MIN(buf_len / sw->info.bytes_per_frame + sw->resample_buf.pos,
+                 sw->resample_buf.size);
     frames_in_max = MIN(sw_max, fe_max);
 
     if (!frames_in_max) {
         return 0;
     }
 
-    sw->conv(sw->resample_buf.buffer, buf, frames_in_max);
-    if (!sw->hw->pcm_ops->volume_out) {
-        mixeng_volume(sw->resample_buf.buffer, frames_in_max, &sw->vol);
+    if (frames_in_max > sw->resample_buf.pos) {
+        sw->conv(sw->resample_buf.buffer + sw->resample_buf.pos,
+                 buf, frames_in_max - sw->resample_buf.pos);
+        if (!sw->hw->pcm_ops->volume_out) {
+            mixeng_volume(sw->resample_buf.buffer + sw->resample_buf.pos,
+                          frames_in_max - sw->resample_buf.pos, &sw->vol);
+        }
     }
 
     audio_pcm_sw_resample_out(sw, frames_in_max, frames_out_max,
@@ -749,6 +754,22 @@ static size_t audio_pcm_sw_write(SWVoiceOut *sw, void *buf, size_t buf_len)
     sw->total_hw_samples_mixed += total_out;
     sw->empty = sw->total_hw_samples_mixed == 0;
 
+    /*
+     * Upsampling may leave one audio frame in the resample buffer. Decrement
+     * total_in by one if there was a leftover frame from the previous resample
+     * pass in the resample buffer. Increment total_in by one if the current
+     * resample pass left one frame in the resample buffer.
+     */
+    if (frames_in_max - total_in == 1) {
+        /* copy one leftover audio frame to the beginning of the buffer */
+        *sw->resample_buf.buffer = *(sw->resample_buf.buffer + total_in);
+        total_in += 1 - sw->resample_buf.pos;
+        sw->resample_buf.pos = 1;
+    } else if (total_in >= sw->resample_buf.pos) {
+        total_in -= sw->resample_buf.pos;
+        sw->resample_buf.pos = 0;
+    }
+
 #ifdef DEBUG_OUT
     dolog (
         "%s: write size %zu written %zu total mixed %zu\n",
@@ -1155,8 +1176,9 @@ static void audio_run_out (AudioState *s)
                 } else {
                     free = 0;
                 }
-                if (free > 0) {
-                    free = MIN(free, sw->resample_buf.size);
+                if (free > sw->resample_buf.pos) {
+                    free = MIN(free, sw->resample_buf.size)
+                           - sw->resample_buf.pos;
                     sw->callback.fn(sw->callback.opaque,
                                     free * sw->info.bytes_per_frame);
                 }

audio/audio_template.h

@@ -138,6 +138,12 @@ static int glue (audio_pcm_sw_alloc_resources_, TYPE) (SW *sw)
         return -1;
     }
 
+    /*
+     * Allocate one additional audio frame that is needed for upsampling
+     * if the resample buffer size is small. For large buffer sizes take
+     * care of overflows.
+     */
+    samples = samples < INT_MAX ? samples + 1 : INT_MAX;
     sw->resample_buf.buffer = g_new0(st_sample, samples);
     sw->resample_buf.size = samples;
     sw->resample_buf.pos = 0;