The recent regression report revealed that the judgment of the low-latency playback mode based on the runtime->stop_threshold cannot work reliably at the prepare stage, as sw_params call may happen at any time, and PCM dmix actually sets it up after the prepare call. This ended up with the stall of the stream as PCM ack won't be issued at all.

For addressing this, check the free-wheeling mode again at the PCM trigger right before starting the stream again, and allow switching to the non-LL mode at a late stage.

Fixes: d5f871f89e21 ("ALSA: usb-audio: Improved lowlatency playback support") Reported-and-tested-by: Kirill A. Shutemov kirill.shutemov@linux.intel.com Link: https://lore.kernel.org/r/20211117161855.m45mxcqszkfcetai@box.shutemov.name Signed-off-by: Takashi Iwai tiwai@suse.de --- sound/usb/pcm.c | 11 +++++++++-- 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/sound/usb/pcm.c b/sound/usb/pcm.c index 95ec8eec1bb0..57b046e73bfe 100644 --- a/sound/usb/pcm.c +++ b/sound/usb/pcm.c @@ -581,6 +581,12 @@ static int snd_usb_hw_free(struct snd_pcm_substream *substream) return 0; }

+/* free-wheeling mode? (e.g. dmix) */ +static int in_free_wheeling_mode(struct snd_pcm_runtime *runtime) +{ + return runtime->stop_threshold > runtime->buffer_size; +} + /* check whether early start is needed for playback stream */ static int lowlatency_playback_available(struct snd_pcm_runtime *runtime, struct snd_usb_substream *subs) @@ -592,8 +598,7 @@ static int lowlatency_playback_available(struct snd_pcm_runtime *runtime, /* disabled via module option? */ if (!chip->lowlatency) return false; - /* free-wheeling mode? (e.g. dmix) */ - if (runtime->stop_threshold > runtime->buffer_size) + if (in_free_wheeling_mode(runtime)) return false; /* implicit feedback mode has own operation mode */ if (snd_usb_endpoint_implicit_feedback_sink(subs->data_endpoint)) @@ -1552,6 +1557,8 @@ static int snd_usb_substream_playback_trigger(struct snd_pcm_substream *substrea subs); if (subs->lowlatency_playback && cmd == SNDRV_PCM_TRIGGER_START) { + if (in_free_wheeling_mode(substream->runtime)) + subs->lowlatency_playback = false; err = start_endpoints(subs); if (err < 0) { snd_usb_endpoint_set_callback(subs->data_endpoint,