ALSA: usb-audio: Disable low-latency playback for free-wheel mode

The free-wheel stream operation like dmix may not update the appl_ptr
appropriately, and it doesn't fit with the low-latency playback mode.
Disable the low-latency playback operation when the stream is set up
in such a mode.

Link: https://lore.kernel.org/r/20210929080844.11583-5-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/pcm.c

@@ -582,7 +582,8 @@ static int snd_usb_hw_free(struct snd_pcm_substream *substream)
 }
 
 /* check whether early start is needed for playback stream */
-static int lowlatency_playback_available(struct snd_usb_substream *subs)
+static int lowlatency_playback_available(struct snd_pcm_runtime *runtime,
+					 struct snd_usb_substream *subs)
 {
 	struct snd_usb_audio *chip = subs->stream->chip;
 
@@ -591,6 +592,9 @@ static int lowlatency_playback_available(struct snd_usb_substream *subs)
 	/* disabled via module option? */
 	if (!chip->lowlatency)
 		return false;
+	/* free-wheeling mode? (e.g. dmix) */
+	if (runtime->stop_threshold > runtime->buffer_size)
+		return false;
 	/* too short periods? */
 	if (subs->data_endpoint->nominal_queue_size >= subs->buffer_bytes)
 		return false;
@@ -630,7 +634,7 @@ static int snd_usb_pcm_prepare(struct snd_pcm_substream *substream)
 	subs->period_elapsed_pending = 0;
 	runtime->delay = 0;
 
-	subs->lowlatency_playback = lowlatency_playback_available(subs);
+	subs->lowlatency_playback = lowlatency_playback_available(runtime, subs);
 	if (!subs->lowlatency_playback)
 		ret = start_endpoints(subs);