[PATCH 5.15 004/207] ALSA: usb-audio: Disable low-latency mode for implicit feedback sync

Greg Kroah-Hartman
2021-12-06T15:32:33Z

From: Takashi Iwai <tiwai@suse.de>

commit bceee75387554f682638e719d1ea60125ea78cea upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
sound/usb/pcm.c | 3 +++
1 file changed, 3 insertions(+)

--- a/sound/usb/pcm.c
+++ b/sound/usb/pcm.c
@@ -595,6 +595,9 @@ static int lowlatency_playback_available
/* 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;