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 <lists@carlh.net>
Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/b4e71631-4a94-613-27b2-fb595792630@carlh.net
Link: https://lore.kernel.org/r/20221205132124.11585-4-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>

diff --git a/sound/usb/pcm.c b/sound/usb/pcm.c
index 8ed165f..9557bd4 100644 (file)
--- 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;