[PATCH 5.15 003/207] ALSA: usb-audio: Disable low-latency playback for free-wheel mode

Greg Kroah-Hartman
2021-12-06T15:49:17Z

From: Takashi Iwai <tiwai@suse.de>

commit e581f1cec4f899f788f6c9477f805b1d5fef25e2 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
sound/usb/pcm.c | 8 ++++++--
1 file changed, 6 insertions(+), 2 deletions(-)

--- a/sound/usb/pcm.c
+++ b/sound/usb/pcm.c
@@ -582,7 +582,8 @@ static int snd_usb_hw_free(struct snd_pc
}

/* 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
/* 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 sn
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);