ALSA: usb-audio: Set and clear sync EP link properly

The sync EP setup isn't cleared at stopping the stream but expected to
be cleared at the next stream start.  This may leave the sync link
setup stale and can spoof wrongly when full duplex streams were
running in the implicit fb sync.  Let's initialize them properly at
start and end of the stream.
Tested-by: Keith Milner <kamilner@superlative.org>
Tested-by: Dylan Robinson <dylan_robinson@motu.com>
Link: https://lore.kernel.org/r/20201123085347.19667-7-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/pcm.c

@@ -238,6 +238,7 @@ static int start_endpoints(struct snd_usb_substream *subs)
 		err = snd_usb_endpoint_start(ep);
 		if (err < 0) {
 			clear_bit(SUBSTREAM_FLAG_SYNC_EP_STARTED, &subs->flags);
+			ep->sync_slave = NULL;
 			return err;
 		}
 	}
@@ -253,8 +254,10 @@ static void sync_pending_stops(struct snd_usb_substream *subs)
 
 static void stop_endpoints(struct snd_usb_substream *subs)
 {
-	if (test_and_clear_bit(SUBSTREAM_FLAG_SYNC_EP_STARTED, &subs->flags))
+	if (test_and_clear_bit(SUBSTREAM_FLAG_SYNC_EP_STARTED, &subs->flags)) {
 		snd_usb_endpoint_stop(subs->sync_endpoint);
+		subs->sync_endpoint->sync_slave = NULL;
+	}
 
 	if (test_and_clear_bit(SUBSTREAM_FLAG_DATA_EP_STARTED, &subs->flags))
 		snd_usb_endpoint_stop(subs->data_endpoint);
@@ -471,26 +474,10 @@ static int set_sync_endpoint(struct snd_usb_substream *subs,
 	bool implicit_fb;
 	int err;
 
-	/* we need a sync pipe in async OUT or adaptive IN mode */
-	/* check the number of EP, since some devices have broken
-	 * descriptors which fool us.  if it has only one EP,
-	 * assume it as adaptive-out or sync-in.
-	 */
 	attr = fmt->ep_attr & USB_ENDPOINT_SYNCTYPE;
 
-	if ((is_playback && (attr != USB_ENDPOINT_SYNC_ASYNC)) ||
-		(!is_playback && (attr != USB_ENDPOINT_SYNC_ADAPTIVE))) {
-
-		/*
-		 * In these modes the notion of sync_endpoint is irrelevant.
-		 * Reset pointers to avoid using stale data from previously
-		 * used settings, e.g. when configuration and endpoints were
-		 * changed
-		 */
-
-		subs->sync_endpoint = NULL;
-		subs->data_endpoint->sync_master = NULL;
-	}
+	subs->sync_endpoint = NULL;
+	subs->data_endpoint->sync_master = NULL;
 
 	err = set_sync_ep_implicit_fb_quirk(subs, dev, altsd, attr);
 	if (err < 0)
@@ -939,6 +926,11 @@ static int snd_usb_hw_free(struct snd_pcm_substream *substream)
 		sync_pending_stops(subs);
 		snd_usb_endpoint_deactivate(subs->sync_endpoint);
 		snd_usb_endpoint_deactivate(subs->data_endpoint);
+		if (subs->data_endpoint) {
+			subs->data_endpoint->sync_master = NULL;
+			subs->data_endpoint = NULL;
+		}
+		subs->sync_endpoint = NULL;
 		snd_usb_unlock_shutdown(subs->stream->chip);
 	}