ALSA: usb-audio: Disable low-latency mode for implicit feedback sync

When a playback stream runs in the implicit feedback mode, its
operation is passive and won't start unless the capture packet is
received.  This behavior contradicts with the low-latency playback
mode, and we should turn off lowlatency_playback flag accordingly.

In theory, we may take the low-latency mode when the playback-first
quirk is set, but it still conflicts with the later operation with the
fixed packet numbers, so it's disabled all together for now.

Link: https://lore.kernel.org/r/20210929080844.11583-6-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>

diff --git a/sound/usb/pcm.c b/sound/usb/pcm.c
index 84b03a3..ec7eeb1 100644 (file)
--- a/sound/usb/pcm.c
+++ b/sound/usb/pcm.c
@@ -595,6 +595,9 @@ static int lowlatency_playback_available(struct snd_pcm_runtime *runtime,
        /* free-wheeling mode? (e.g. dmix) */
        if (runtime->stop_threshold > runtime->buffer_size)
                return false;
+       /* implicit feedback mode has own operation mode */
+       if (snd_usb_endpoint_implicit_feedback_sink(subs->data_endpoint))
+               return false;
        /* too short periods? */
        if (subs->data_endpoint->nominal_queue_size >= subs->buffer_bytes)
                return false;