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.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>

diff --git a/sound/usb/pcm.c b/sound/usb/pcm.c
index 4dd7f1c..84b03a3 100644 (file)
--- a/sound/usb/pcm.c
+++ b/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);