ALSA: usb-audio: Relax hw constraints for implicit fb sync

The fix commit the commit e4ea77f ("ALSA: usb-audio: Always apply
the hw constraints for implicit fb sync") tried to address the bug
where an incorrect PCM parameter is chosen when two (implicit fb)
streams are set up at the same time.  This change had, however, some
side effect: once when the sync endpoint is chosen and set up, this
restriction is applied at the next hw params unless it's freed via hw
free explicitly.

This patch is a workaround for the problem by relaxing the hw
constraints a bit for the implicit fb sync.  We still keep applying
the hw constraints for implicit fb sync, but only when the matching
sync EP is being used by other streams.

Fixes: e4ea77f ("ALSA: usb-audio: Always apply the hw constraints for implicit fb sync")
Reported-by: Ruud van Asseldonk <[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]>

Author: tiwai

sound/usb/pcm.c

@@ -937,8 +937,13 @@ get_sync_ep_from_substream(struct snd_usb_substream *subs)
 			continue;
 		/* for the implicit fb, check the sync ep as well */
 		ep = snd_usb_get_endpoint(chip, fp->sync_ep);
-		if (ep && ep->cur_audiofmt)
-			return ep;
+		if (ep && ep->cur_audiofmt) {
+			/* ditto, if the sync (data) ep is used by others,
+			 * this stream is restricted by the sync ep
+			 */
+			if (ep != subs->sync_endpoint || ep->opened > 1)
+				return ep;
+		}
 	}
 	return NULL;
 }