The recent change for low latency playback works in most of test cases but it turned out still to hit errors on some use cases, most notably with JACK with small buffer sizes. This is because USB-audio driver fills up and submits full URBs at the beginning, while the URBs would return immediately and try to fill more -- that can easily trigger XRUN. It was more or less expected, but in the small buffer size, the problem became pretty obvious.

Fixing this behavior properly would require the change of the fundamental driver design, so it's no trivial task, unfortunately. Instead, here we work around the problem just by switching back to the old method when the given configuration is too fragile with the low latency stream handling. As a threshold, we calculate the total buffer bytes in all plus one URBs, and check whether it's beyond the PCM buffer bytes. The one extra URB is needed because XRUN happens at the next submission after the first round.

Fixes: 307cc9baac5c ("ALSA: usb-audio: Reduce latency at playback start, take#2") Cc: stable@vger.kernel.org Signed-off-by: Takashi Iwai tiwai@suse.de --- sound/usb/card.h | 2 ++ sound/usb/endpoint.c | 4 ++++ sound/usb/pcm.c | 13 +++++++++++-- 3 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/sound/usb/card.h b/sound/usb/card.h index 6c0a052a28f9..5b19901f305a 100644 --- a/sound/usb/card.h +++ b/sound/usb/card.h @@ -94,6 +94,7 @@ struct snd_usb_endpoint { struct list_head ready_playback_urbs; /* playback URB FIFO for implicit fb */

unsigned int nurbs; /* # urbs */ + unsigned int nominal_queue_size; /* total buffer sizes in URBs */ unsigned long active_mask; /* bitmask of active urbs */ unsigned long unlink_mask; /* bitmask of unlinked urbs */ char *syncbuf; /* sync buffer for all sync URBs */ @@ -187,6 +188,7 @@ struct snd_usb_substream { } dsd_dop;

bool trigger_tstamp_pending_update; /* trigger timestamp being updated from initial estimate */ + bool early_playback_start; /* early start needed for playback? */ struct media_ctl *media_ctl; };

diff --git a/sound/usb/endpoint.c b/sound/usb/endpoint.c index 8b1bec51c806..bf26c04cf471 100644 --- a/sound/usb/endpoint.c +++ b/sound/usb/endpoint.c @@ -1126,6 +1126,10 @@ static int data_ep_set_params(struct snd_usb_endpoint *ep) INIT_LIST_HEAD(&u->ready_list); }

+ /* total buffer bytes of all URBs plus the next queue; + * referred in pcm.c + */ + ep->nominal_queue_size = maxsize * urb_packs * (ep->nurbs + 1); return 0;

out_of_memory: diff --git a/sound/usb/pcm.c b/sound/usb/pcm.c index 4e5031a68064..f5cbf61ac366 100644 --- a/sound/usb/pcm.c +++ b/sound/usb/pcm.c @@ -614,6 +614,14 @@ static int snd_usb_pcm_prepare(struct snd_pcm_substream *substream) subs->period_elapsed_pending = 0; runtime->delay = 0;

+ /* check whether early start is needed for playback stream */ + subs->early_playback_start = + subs->direction == SNDRV_PCM_STREAM_PLAYBACK && + subs->data_endpoint->nominal_queue_size >= subs->buffer_bytes; + + if (subs->early_playback_start) + ret = start_endpoints(subs); + unlock: snd_usb_unlock_shutdown(chip); return ret; @@ -1394,7 +1402,7 @@ static void prepare_playback_urb(struct snd_usb_substream *subs, subs->trigger_tstamp_pending_update = false; }

- if (period_elapsed && !subs->running) { + if (period_elapsed && !subs->running && !subs->early_playback_start) { subs->period_elapsed_pending = 1; period_elapsed = 0; } @@ -1448,7 +1456,8 @@ static int snd_usb_substream_playback_trigger(struct snd_pcm_substream *substrea prepare_playback_urb, retire_playback_urb, subs); - if (cmd == SNDRV_PCM_TRIGGER_START) { + if (!subs->early_playback_start && + cmd == SNDRV_PCM_TRIGGER_START) { err = start_endpoints(subs); if (err < 0) { snd_usb_endpoint_set_callback(subs->data_endpoint,