usb-audio: do not count on avail bytes actually available

This assumption is no longer true when mixeng is turned off.
Signed-off-by: NKővágó, Zoltán <DirtY.iCE.hu@gmail.com>
Message-id: d63f4d39a0ee7a2e4e7e4a2eb005ba79120eaf1d.1570996490.git.DirtY.iCE.hu@gmail.com
Signed-off-by: NGerd Hoffmann <kraxel@redhat.com>

hw/usb/dev-audio.c

@@ -319,30 +319,31 @@ static int streambuf_put(struct streambuf *buf, USBPacket *p)
 {
     uint32_t free = buf->size - (buf->prod - buf->cons);
 
-    if (!free) {
+    if (free < USBAUDIO_PACKET_SIZE) {
         return 0;
     }
     if (p->iov.size != USBAUDIO_PACKET_SIZE) {
         return 0;
     }
-    assert(free >= USBAUDIO_PACKET_SIZE);
+
     usb_packet_copy(p, buf->data + (buf->prod % buf->size),
                     USBAUDIO_PACKET_SIZE);
     buf->prod += USBAUDIO_PACKET_SIZE;
     return USBAUDIO_PACKET_SIZE;
 }
 
-static uint8_t *streambuf_get(struct streambuf *buf)
+static uint8_t *streambuf_get(struct streambuf *buf, size_t *len)
 {
     uint32_t used = buf->prod - buf->cons;
     uint8_t *data;
 
     if (!used) {
+        *len = 0;
         return NULL;
     }
-    assert(used >= USBAUDIO_PACKET_SIZE);
     data = buf->data + (buf->cons % buf->size);
-    buf->cons += USBAUDIO_PACKET_SIZE;
+    *len = MIN(buf->prod - buf->cons,
+               buf->size - (buf->cons % buf->size));
     return data;
 }
 
@@ -374,16 +375,21 @@ static void output_callback(void *opaque, int avail)
     USBAudioState *s = opaque;
     uint8_t *data;
 
-    for (;;) {
-        if (avail < USBAUDIO_PACKET_SIZE) {
+    while (avail) {
+        size_t written, len;
+
+        data = streambuf_get(&s->out.buf, &len);
+        if (!data) {
             return;
         }
-        data = streambuf_get(&s->out.buf);
-        if (!data) {
+
+        written = AUD_write(s->out.voice, data, len);
+        avail -= written;
+        s->out.buf.cons += written;
+
+        if (written < len) {
             return;
         }
-        AUD_write(s->out.voice, data, USBAUDIO_PACKET_SIZE);
-        avail -= USBAUDIO_PACKET_SIZE;
     }
 }