Under certain situations (typically in the implicit feedback mode), USB-audio driver starts a playback stream already at PCM prepare call even before the actual PCM trigger-START call. For implicit feedback mode, this effectively starts two streams for data and sync endpoints, and if a coupled sync stream gets XRUN at this point, it results in an error -EPIPE. The problem is that currently we return -EPIPE error as is from the prepare. Then application tries to recover again via the prepare call, but it'll fail again because the sync-stop is missing. The sync-stop is missing because it's an internal trigger call (hence the PCM core isn't involved). Since we'll need to re-issue the prepare in anyway when trapped into this pitfall, this patch attempts to address it in a bit different way; namely, the driver tries to prepare once again after syncing the stop manually by itself -- so applications don't see the internal error. At the second failure, we report the error as is, but this shouldn't happen in normal situations. Reported-and-tested-by: Carl Hetherington <lists@xxxxxxxxx> Cc: <stable@xxxxxxxxxxxxxxx> Link: https://lore.kernel.org/r/b4e71631-4a94-613-27b2-fb595792630@xxxxxxxxx Signed-off-by: Takashi Iwai <tiwai@xxxxxxx> --- sound/usb/pcm.c | 13 +++++++++++-- 1 file changed, 11 insertions(+), 2 deletions(-) diff --git a/sound/usb/pcm.c b/sound/usb/pcm.c index 8ed165f036a0..9557bd4d1bbc 100644 --- a/sound/usb/pcm.c +++ b/sound/usb/pcm.c @@ -604,6 +604,7 @@ static int snd_usb_pcm_prepare(struct snd_pcm_substream *substream) struct snd_pcm_runtime *runtime = substream->runtime; struct snd_usb_substream *subs = runtime->private_data; struct snd_usb_audio *chip = subs->stream->chip; + int retry = 0; int ret; ret = snd_usb_lock_shutdown(chip); @@ -614,6 +615,7 @@ static int snd_usb_pcm_prepare(struct snd_pcm_substream *substream) goto unlock; } + again: if (subs->sync_endpoint) { ret = snd_usb_endpoint_prepare(chip, subs->sync_endpoint); if (ret < 0) @@ -638,9 +640,16 @@ static int snd_usb_pcm_prepare(struct snd_pcm_substream *substream) subs->lowlatency_playback = lowlatency_playback_available(runtime, subs); if (substream->stream == SNDRV_PCM_STREAM_PLAYBACK && - !subs->lowlatency_playback) + !subs->lowlatency_playback) { ret = start_endpoints(subs); - + /* if XRUN happens at starting streams (possibly with implicit + * fb case), restart again, but only try once. + */ + if (ret == -EPIPE && !retry++) { + sync_pending_stops(subs); + goto again; + } + } unlock: snd_usb_unlock_shutdown(chip); return ret; -- 2.35.3
