asc: generate silence if FIFO empty but engine still running

MacOS (un)helpfully leaves the FIFO engine running even when all the samples have
been written to the hardware, and expects the FIFO status flags and IRQ to be
updated continuously.

There is an additional problem in that not all audio backends guarantee an
all-zero output when there is no FIFO data available, in particular the Windows
dsound backend which re-uses its internal circular buffer causing the last played
sound to loop indefinitely.

Whilst this is effectively a bug in the Windows dsound backend, work around it
for now using a simple heuristic: if the FIFO remains empty for half a cycle
(~23ms) then continuously fill the generated buffer with empty silence.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-ID: <20231004083806.757242-9-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>

diff --git a/hw/audio/asc.c b/hw/audio/asc.c
index 9084708eafabd5a5bc61130f7be29ea14377302d..0f36b4ce9b6f9617fc8f82fc236d2d203aef574d 100644 (file)
--- a/hw/audio/asc.c
+++ b/hw/audio/asc.c
@@ -341,6 +341,21 @@ static void asc_out_cb(void *opaque, int free_b)
     }
 
     if (!generated) {
+        /* Workaround for audio underflow bug on Windows dsound backend */
+        int64_t now = qemu_clock_get_ns(QEMU_CLOCK_VIRTUAL);
+        int silent_samples = muldiv64(now - s->fifo_empty_ns,
+                                      NANOSECONDS_PER_SECOND, ASC_FREQ);
+
+        if (silent_samples > ASC_FIFO_CYCLE_TIME / 2) {
+            /*
+             * No new FIFO data within half a cycle time (~23ms) so fill the
+             * entire available buffer with silence. This prevents an issue
+             * with the Windows dsound backend whereby the sound appears to
+             * loop because the FIFO has run out of data, and the driver
+             * reuses the stale content in its circular audio buffer.
+             */
+            AUD_write(s->voice, s->silentbuf, samples << s->shift);
+        }
         return;
     }
 
@@ -618,6 +633,7 @@ static void asc_unrealize(DeviceState *dev)
     ASCState *s = ASC(dev);
 
     g_free(s->mixbuf);
+    g_free(s->silentbuf);
 
     AUD_remove_card(&s->card);
 }
@@ -642,6 +658,9 @@ static void asc_realize(DeviceState *dev, Error **errp)
     s->samples = AUD_get_buffer_size_out(s->voice) >> s->shift;
     s->mixbuf = g_malloc0(s->samples << s->shift);
 
+    s->silentbuf = g_malloc0(s->samples << s->shift);
+    memset(s->silentbuf, 0x80, s->samples << s->shift);
+
     /* Add easc registers if required */
     if (s->type == ASC_TYPE_EASC) {
         memory_region_add_subregion(&s->asc, ASC_EXTREG_OFFSET,

diff --git a/include/hw/audio/asc.h b/include/hw/audio/asc.h
index d9412815c3248227e26a439847fb1989a68c280f..4741f92c461338dad0e35930c1aad82a67a74e9e 100644 (file)
--- a/include/hw/audio/asc.h
+++ b/include/hw/audio/asc.h
@@ -68,6 +68,8 @@ struct ASCState {
     int samples;
     int shift;
 
+    uint8_t *silentbuf;
+
     /* Time when we were last able to generate samples */
     int64_t fifo_empty_ns;