ALSA: usb-audio: Workaround for XRUN at prepare

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 <[email protected]>
Cc: <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Takashi Iwai <[email protected]>

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;